Decision Logic for Peptide Synthesis Routes: Applicability and Criteria for Choosing among SPPS, LPPS, and Recombinant Expression

Introduction

The core issue in peptide synthesis is no longer simply “what methods are available to link amino acids together,” but rather how to choose a more suitable preparation route for peptides of different lengths, sequence characteristics, modification requirements, and scale-up goals, while anticipating the major bottlenecks that may arise. As the applications of therapeutic peptides and functional peptides continue to expand, synthetic efficiency, purity control, manufacturing scale-up, and environmental burden are increasingly influencing route selection more directly. Current reviews generally regard Fmoc solid-phase peptide synthesis (Fmoc solid-phase peptide synthesis, Fmoc-SPPS) as one of the most widely used and most mature chemical routes for peptide preparation today, and it is often the starting point for most projects; however, it is not automatically suitable for every task.

Liquid-phase peptide synthesis (liquid-phase peptide synthesis, LPPS) has regained attention in recent years, mainly because it has demonstrated clearer methodological value in larger-scale preparation, process organization, and reducing the use of excess reagents and solvents. Recombinant expression, by contrast, offers advantages distinct from chemical synthesis for the large-scale production of certain natural or near-natural sequence peptides. For longer, more difficult, or more complex targets, route selection should not be limited to the three options of “SPPS, LPPS, and recombinant expression” alone; segmented construction followed by subsequent ligation or connection may also be an important strategy that should be evaluated early.

1. Before Choosing a Route, First Identify the Main Synthetic Challenges of the Target Peptide

The starting point for selecting a peptide synthesis route is not to compare different methods first, but to identify where the principal challenges of the target peptide actually lie. Factors that are truly meaningful for experimental design usually include the following: how long the sequence is; whether it is prone to aggregation during synthesis; whether it contains sites susceptible to epimerization or other side reactions; whether non-natural amino acids, terminal modifications, or post-modification mimics need to be introduced; whether purification difficulty will rise markedly with increasing length and complexity; and whether the project goal is milligram-scale exploration or more oriented toward scale-up production.

Together, these factors determine the focus of route selection. When the goal is to obtain a sample with a defined sequence and clearly specified modifications as quickly as possible, SPPS is often the most direct choice. When the problem shifts toward long peptides, complex peptides, scale-up production, resource consumption, or manufacturing cost, however, route selection must be adjusted accordingly. For targets that have already entered the range of long peptides, difficult sequences, or protein fragments, it is also important to assess early whether the problem is no longer suitable for a single linear route and instead requires segmented construction, fragment condensation, or ligation-based assembly.

1.1 Primary Factors That Determine the Choice of Peptide Synthesis Route

Evaluation Dimension	Experimental Questions to Ask	Impact on Route Selection
Sequence length	Is it a short peptide, a medium-length peptide, or already approaching the range of long peptides/protein fragments?	As length increases, cumulative failures in stepwise coupling and purification burden become more pronounced, and the advantage of a single route becomes weaker.
Sequence properties	Is it rich in hydrophobic residues, β-sheet-prone residues, repetitive sequences, or “difficult coupling” segments?	If aggregation readily occurs on the resin, the efficiency and purity of SPPS may decline substantially.
Structural complexity	Does it require the introduction of non-natural amino acids, cyclization, tags, lipidation, fluorophores, or site-specific modifications?	The greater the required modification flexibility, the more advantageous chemical methods usually are.
Stereochemical and side-reaction risks	Are there motifs prone to epimerization, aspartimide formation, or acid-/base-sensitive segments?	Greater emphasis is needed on coupling conditions, protecting-group strategy, and segmented design.
Scale target	Is the goal milligram-scale screening, gram-scale preparation, or even larger-scale production?	As scale increases, solvent use, reagent equivalents, purification burden, and waste generation play a greater role in route selection.

2. Why Fmoc-SPPS Remains the Common Starting Point for Most Projects

Fmoc solid-phase peptide synthesis (Fmoc solid-phase peptide synthesis, Fmoc-SPPS) is commonly used as the starting point of peptide projects mainly because it is not only one of the most widely used and most mature chemical routes for peptide preparation today, but also particularly well suited to the early R&D task of “rapidly obtaining the target sequence and completing iterative validation.” Current reviews generally regard Fmoc-SPPS as one of the most widely used methods in peptide synthesis today. Its main advantages arise from stepwise chain elongation on a resin support, its high compatibility with automation, and the rich and well-established Fmoc building-block platform. For projects requiring rapid synthesis of a series of analogs, the introduction of terminal or side-chain modifications, or the validation of preliminary structure–activity relationships, Fmoc-SPPS remains the most direct and most mature entry point; in the preparation of many therapeutic peptides, it also remains one of the major chemical production routes.

However, treating Fmoc-SPPS as the default starting point does not mean that it retains the same advantages at every stage. As sequences become longer, hydrophobic segments increase, difficult coupling sites accumulate, and purity requirements rise, deletion peptides, side reactions, and overall resource consumption become more prominent. When the research objective shifts from “obtaining the target sequence as quickly as possible” to “improving the quality of long peptides, optimizing scale-up production, or reducing process burden,” route selection often needs to be reconsidered.

2.1 Applicability, Advantages, and Limitations of Fmoc-SPPS as a Starting Route

Applicable Situation	Main Advantages	Main Limitations
Early-stage sequence exploration, rapid analog synthesis, introduction of non-natural amino acids or site-specific modifications	Mature automation, abundant building blocks, rapid iteration of sequences and modifications	For long sequences, difficult sequences, and high-purity targets, side reactions, deletion peptides, and purification burden increase significantly
Need to rapidly obtain milligram-scale to pre-pilot samples	Mature operational workflow and broad laboratory accessibility	High consumption of solvents, reagents, and resin; process burden becomes more pronounced upon scale-up

3. When Should One Move from SPPS to Other Routes?

When the research question begins to shift from “how to obtain the target sequence as quickly as possible” to “how to prepare it more rationally,” route selection should no longer remain focused on SPPS alone. At that point, the key comparison is whether the next bottleneck is more related to chemical process engineering or to biological production. If the bottleneck is first manifested in scale-up, resource consumption, and process management, LPPS is often more worthy of priority evaluation. If the bottleneck is more related to obtaining certain natural or near-natural sequence peptides on a larger scale, recombinant expression should be seriously assessed.

3.1 Under What Circumstances Should LPPS Be Evaluated?

Liquid-phase peptide synthesis (liquid-phase peptide synthesis, LPPS) has regained attention in recent years mainly because its value in scale-up preparation, process organization, and resource utilization has become clearer. Relevant reviews regard LPPS as a category of peptide preparation route that has once again attracted attention after classical liquid-phase synthesis and solid-phase synthesis. Its characteristic features are the elongation of peptide chains in solution and the temporary introduction of support groups that facilitate separation and recovery, together with corresponding intermediate-handling strategies, thereby improving process controllability while retaining the flexibility of chemical modification. For projects in which the target sequence has already been defined and attention has begun to shift toward reagent equivalents, solvent burden, separation steps, and scale-up, the significance of LPPS lies in whether it can accomplish the synthesis with more rational process logic.

Therefore, when a project has moved from the discovery stage into the preparation stage, and flexibility in chemical modification remains important but purification cost, process complexity, and resource consumption are becoming the dominant factors, LPPS should enter the range of routes to be prioritized for evaluation. It is suitable for tasks that no longer focus only on “obtaining the target peptide as quickly as possible,” but rather on “preparing the target peptide in a more controllable manner.”

Note: As used here, LPPS mainly refers to modern liquid-phase routes that have regained attention in recent years and emphasize the use of separable support groups or tag-assisted strategies together with corresponding intermediate-management methods. In a broader sense, liquid-phase peptide synthesis can also include classical stepwise solution-phase synthesis and fragment condensation strategies.

3.2 Under What Circumstances Should Recombinant Expression Be Evaluated?

When the core task of a project is no longer highly flexible molecular modification, but rather the acquisition of certain natural or near-natural sequence peptides on a relatively large scale, recombinant expression is usually more worthy of evaluation. Its principal advantage lies in biological production capacity rather than flexibility in chemical modification. For some small peptides, labile peptides, antimicrobial peptides, or peptides that are toxic to the host, recombinant expression often also requires fusion expression to improve stability, reduce host toxicity, and facilitate downstream purification and cleavage release.

Therefore, recombinant expression is better suited as a preparation route centered on host adaptation, fusion expression, expression control, cleavage release, and downstream purification. It is usually more attractive for tasks that place greater emphasis on obtaining natural sequences, increasing yield, and achieving large-scale biological production. For projects that depend on multi-site modification, the introduction of non-natural amino acids, or complex chemical modification design, however, chemical synthesis is generally still the more direct approach.

3.3 Applicability, Advantages, and Limitations of LPPS and Recombinant Expression

Route	Better Suited Tasks	Main Advantages	Main Limitations
Liquid-phase peptide synthesis (LPPS)	Chemical preparation in which the target is already defined and attention is shifting toward scale-up, process burden, and resource utilization	Better suited to organizing the synthetic workflow from a process perspective; greater potential for scale-up and greener manufacturing	More complex route design, intermediate management, and separation strategy
Recombinant expression	Targets biased toward natural or near-natural peptides, with the project focus shifting to large-scale biological production	Suitable for larger-scale preparation, with potential advantages in unit cost and production capacity	Lower modification flexibility; often requires fusion expression, expression optimization, and complex downstream purification

4. Division of Roles among SPPS, LPPS, and Recombinant Expression

Solid-phase peptide synthesis (solid-phase peptide synthesis, SPPS), liquid-phase peptide synthesis (liquid-phase peptide synthesis, LPPS), and recombinant expression are not simple substitutes for one another in addressing the same problem; rather, each corresponds to a different task priority. The main advantage of SPPS lies in rapid synthesis and iterative optimization at the early research stage, making it suitable for sequence exploration, analog preparation, and the introduction of complex modifications. LPPS is better suited to re-optimizing the chemical route from the perspectives of process organization, scale-up preparation, and resource utilization once the target sequence has been defined. Recombinant expression, by contrast, is better suited to tasks centered on the large-scale production of natural or near-natural sequence peptides. In current reviews, the main focus of discussion of these three types of routes is not on determining which one can replace the others, but on selecting the more suitable path according to the nature of the target peptide, the modification requirements, and the production scale.

4.1 Applicability, Advantages, and Limitations of SPPS, LPPS, and Recombinant Expression

Route	Better Suited Tasks	Main Advantages	Main Limitations
Solid-phase peptide synthesis (SPPS)	Discovery-stage work, rapid analog synthesis, introduction of complex modifications	High degree of automation, abundant building-block resources, high flexibility in modification	Side reactions accumulate more readily in long and difficult sequences; solvent and reagent consumption is high
Liquid-phase peptide synthesis (LPPS)	Chemical routes for scale-up preparation, process optimization, and reduced resource consumption	Combines the ability for chemical modification with good process manageability; more suitable for large-scale preparation	More complex route design, intermediate management, and separation strategy; not necessarily an appropriate starting point for every project
Recombinant expression	Large-scale production of natural or near-natural sequence peptides	Strong scale-up potential; suitable for preparation tasks centered on biological production	Lower flexibility in modification; often requires fusion expression, expression optimization, and more complex downstream purification workflows

5. Key Operational Issues That Determine the Success or Failure of Peptide Experiments

Even when the overall route has been judged correctly, the experimental work may still fail to proceed smoothly. In peptide projects, the more common problem is often not that the wrong broad method category was chosen, but that the operational steps most likely to go out of control within that route were not identified in advance. For solid-phase peptide synthesis (SPPS), the core issues typically include the accumulation of deletion peptides caused by difficult couplings, epimerization, aspartimide formation, on-resin aggregation, and sample loss during cleavage and purification. Relevant reviews place these issues at the center of modern Fmoc-SPPS improvement, indicating that the focus of peptide chemistry optimization is not merely to make coupling occur, but to keep it controllable even in complex sequences.

For liquid-phase peptide synthesis (LPPS), the main challenges are more often concentrated in intermediate management, separation strategy, and overall process balance. For recombinant expression, the major risks more often arise in expression toxicity, fusion expression, cleavage release, folding state, and downstream purification. Because the principal risks differ from one route to another, experimental design cannot stop at the method name alone; it must also identify the specific steps most likely to limit yield, purity, and scalability.

5.1 Key Issues That Should Be Prioritized for Monitoring in Peptide Experiments

Key Issue	Commonly Seen in Which Route	Why It Matters	What Should Be Prioritized Experimentally
Incomplete coupling and deletion peptides	Mainly seen in SPPS, but can also affect segmented chemical synthesis routes	In stepwise synthesis, a single failed coupling can be propagated through subsequent elongation, ultimately affecting crude purity and target peptide yield	Identification of difficult segments, evaluation of coupling efficiency, and process monitoring
Epimerization	Mainly seen in chemical coupling routes	Directly affects stereochemical purity and interferes with later evaluation of activity, structure–activity relationships, and analytical results	Activation conditions, substrate sensitivity, and the coupling time window
Aspartimide formation and related side reactions	Mainly seen in Fmoc-SPPS	Increase impurity complexity, reduce the proportion of target product, and significantly raise purification difficulty	Residue combinations near Asp sites, protecting-group strategy, and deprotection conditions
On-resin or intermediate aggregation	Common in difficult sequences, especially in SPPS	Reduces coupling and deprotection efficiency and promotes impurity accumulation	Sequence-feature analysis, segmented design, and adjustment of synthesis pacing
Downstream purification and scale-up burden	Can occur in all three routes	Determines whether the sample can be obtained reproducibly and whether subsequent scale-up can proceed smoothly	Incorporating purification difficulty, process complexity, and resource consumption into route selection at an early stage

6. Priority Route Selection under Different Experimental Objectives

Different experimental objectives correspond to different priority routes. For projects primarily aimed at sequence exploration, site-specific modification, and rapid sample acquisition, Fmoc solid-phase peptide synthesis (Fmoc-SPPS) is usually the first choice. When the target sequence has already been defined and the emphasis shifts toward scale-up preparation and process cost control, liquid-phase peptide synthesis (LPPS) deserves to be brought into priority evaluation. For projects involving peptides closer to natural sequences, where the core task is large-scale biological production, recombinant expression should be assessed more seriously. For long peptides, difficult sequences, or more complex protein-like targets, one should also determine early whether segmented construction followed by subsequent ligation is required, rather than treating these options merely as remedial measures after the failure of a single-route strategy. The key to route selection is not to decide in advance which method is “more advanced,” but to determine whether the current task first needs to solve rapid iteration, process scale-up, biological production, or segmented assembly and overall workflow reorganization.

6.1 Choosing the Priority Route According to the Research or Experimental Objective

Current Research or Experimental Objective	Recommended Priority Route	Reason	Additional Routes or Factors to Consider
Rapidly obtain the target peptide first and verify activity or structure–activity relationships	Fmoc-SPPS	Fast iteration and flexible modification make it most suitable for early-stage exploration	Later evaluate whether a transition to LPPS or a segmented synthesis strategy is needed
Introduce multiple non-natural amino acids or site-specific modifications	Fmoc-SPPS	Fmoc building-block resources are abundant, and the chemical modification space is more mature	At the same time, pay attention to side-reaction control and purification cost
The project has entered the stage of scale-up and process optimization	LPPS	Better suited for route optimization from the perspectives of resource utilization, separation workflow, and process organization	Also evaluate purification burden and overall manufacturing cost
The target is closer to a natural peptide and larger-scale biological production is desired	Recombinant expression	Greater biological production potential, making it more suitable for tasks focused on yield and scale-up	Focus on evaluating fusion expression, host adaptation, cleavage release, and downstream purification
Long peptides, difficult sequences, or protein fragments continue to fail in a single route, or the risk of linear elongation is expected to be high from the outset	Segmented or combined routes	Such tasks are often not solved simply by “switching to another broad method,” but by reorganizing the synthetic workflow through fragment construction, fragment condensation, or ligation-based assembly	Reassess sequence difficulties, fragment boundaries, ligation-site design, scale-up goals, and acceptable process complexity

7. Product Selection Guide for Peptide Synthesis Research and Experimental Tasks (Tables 1–5)

Research or Experimental Objective	Which Table to Consult First	Why This Table Should Be Consulted First	Which Table to Consult in Addition	Guidance
To establish a basic Fmoc solid-phase peptide synthesis (Fmoc-SPPS) route, first determine which type of resin the peptide chain should be attached to and whether the final product should be a C-terminal carboxylic acid or a C-terminal amide	Table 1	Table 1 is centered on solid supports and linker resins; the first decision is how the peptide chain is loaded, how it is cleaved, and what C-terminal form is ultimately obtained	Then see Tables 3 and 4	Once the correct resin and linker are chosen, the coupling reagents, bases, solvents, and cleavage system can be selected more rationally, making the experimental route more robust. The resin/linker itself is one of the fundamental variables determining SPPS success or failure.
To carry out routine linear peptide synthesis, with emphasis on establishing stepwise coupling, activation, and low-epimerization control	Table 3	Table 3 contains carbodiimide, uronium, and phosphonium coupling reagents, as well as activation/additive systems such as HOBt, HOAt, Oxyma, and NHS, which are central to amide-bond construction	Then see Tables 4 and 1	First determine the coupling system to use, then fine-tune the resin, deprotection, and cleavage conditions. This is most suitable for the initial stage of routine Fmoc-SPPS and solution-phase coupling. The reason Fmoc-SPPS is so widely used is that resin systems and coupling systems can be combined rapidly.
The target sequence is already known, but difficult coupling, sterically hindered substrates, poorly nucleophilic amines, or reduced epimerization risk are major concerns	Table 3	Table 3 includes HATU, COMU, PyBOP, HOAt, Oxyma, and other components more suitable for difficult coupling and activation control	Then see Table 4	In such tasks, the first response is usually not to change the resin, but to adjust the activation mode and the combination of base and additives; cleavage and post-processing conditions can then be fine-tuned in Table 4.
To clarify N-terminal protection, protecting-group switching, or the preparation of solution-phase amino acid/peptide-fragment precursors before entering the coupling stage	Table 2	Table 2 focuses on the common N-terminal protecting-group introduction reagents Fmoc, Boc, and Cbz, making it suitable for establishing the precursor protection strategy first	Then see Tables 3 and 4	When the experimental focus is still on how to prepare monomers or fragments, Table 2 should be consulted first; once the precursors are defined, coupling in Table 3 and deprotection/cleavage conditions in Table 4 can be addressed.
To establish the operational workflow for cleavage, deprotection, capping, resin washing, and crude peptide release	Table 4	Table 4 contains core operational components such as TFA, HF, piperidine, DIPEA, DMF, NMP, DCM, TIPS, EDT, and acetic anhydride, covering the key operational windows of both Fmoc-SPPS and Boc-SPPS	Then see Tables 1 and 3	If coupling is already basically feasible but the crude product contains many impurities, recovery after cleavage is unstable, or deletion peptides are excessive, troubleshooting should usually begin with Table 4, then return to Tables 1 and 3.
To prepare peptide fragments with side-chain protection retained, fragment-condensation precursors, or systems more sensitive to strong acid	Table 1	The 2-chlorotrityl, Sieber, and Rink resins/linkers in Table 1 directly determine whether peptide fragments can be released under relatively mild conditions	Then see Tables 2, 3, and 4	For such tasks, the key first question is not “which coupling reagent is stronger,” but whether the linkage mode supports subsequent fragment-based operations; once the resin is chosen correctly, the coupling and cleavage systems can then be added.
The research focus is shifting from “rapid sample acquisition” to “liquid-phase routes, fragment precursors, or more controllable process organization”	Tables 2 and 3	Table 2 covers protecting groups and precursor preparation, while Table 3 covers carboxylic acid activation and amide-bond construction in solution; together they form the basis of LPPS and fragment-based strategies	Then see Table 4	The core of LPPS is not merely changing the solvent, but reorganizing precursor protection, activation mode, and process steps; therefore, Tables 2 and 3 usually need to be consulted together.
To obtain peptide precursors or fusion proteins by recombinant expression, or to handle inclusion bodies, His-tag purification, refolding, and related bioproduction tasks	Table 5	Table 5 contains common reagents for recombinant expression and purification, such as IPTG, antibiotics, imidazole, guanidine hydrochloride, urea, Tris, EDTA, DTT, and 2-mercaptoethanol	If chemical modification is needed, then return to Tables 3 and 4	Once the route has shifted to biological expression, the core issues become induction, screening, lysis, denaturation, affinity purification, and refolding, rather than resins and coupling reagents. Many peptides, especially antimicrobial peptides, are often produced via fusion expression to improve yield and stability.
It is still unclear whether to use chemical synthesis or recombinant expression, and the immediate goal is simply to determine whether the project is primarily a “molecular design problem” or a “biological production problem”	For chemical routes, first see Tables 1 and 3; for biological routes, first see Table 5	Tables 1 + 3 best represent the core variables of chemical routes, whereas Table 5 best represents the core variables of the expression and purification platform	For chemical routes, then see Table 4; for biological routes, supplement with Tables 3 and 4 as needed	When high-flexibility modification, non-natural amino acids, or site-specific alteration are required, it is usually more appropriate to assess the chemical route first; when the need is to obtain natural or near-natural sequences on a larger scale, the expression route is generally the better starting point.

Table 1 | Solid-Phase Peptide Synthesis Supports and Linker Resins

Classification	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Chloromethyl starting resin / solid support	55844-94-5	C684314	Chloromethyl polystyrene resin	2% DVB crosslinked (100–200 mesh) (0.8–1.2 mmol/g)	A classic chloromethyl-type solid support that can first be loaded with the initial amino acid and then extended stepwise; suitable for constructing resin-linkage systems other than acid-cleavage conditions, and also commonly used in early Merrifield-type solid-phase routes.
2-Chlorotrityl linker resin	42074-68-0	C432714	2-Chlorotrityl chloride, polymer-bound	200–400 mesh, loading range: 1.0–1.5 mmol/g Cl loading, 1% crosslinked	The linker is acid-sensitive and allows relatively mild loading conditions, making it suitable for loading C-terminal amino acids that are prone to epimerization or otherwise sensitive; commonly used to obtain C-terminal carboxylic acid peptides while reducing side reactions during the loading stage.
Rink amide resin / C-terminal amide peptide support	431041-83-7	R589109	Rink Amide MBHA resin	100–200 mesh, 1% DVB, 0.3–0.8 mmol/g	A commonly used amide-type resin for Fmoc-SPPS that directly affords C-terminal amide peptides after cleavage; suitable for mimicking naturally amidated peptide termini or improving the stability and activity of certain peptide segments.
Sieber amide resin / mild-acid cleavage support	915706-90-0	F347706	9-Fmoc-aminoxanthen-3-yloxy polystyrene resin	100–200 mesh, 1% DVB, 0.1–2.8 mmol/g	Enables peptide-chain release under relatively mild acidic conditions; suitable for the preparation of peptide fragments with side-chain protection retained, the acquisition of fragment-condensation precursors, and the handling of systems more sensitive to strong acid.
Rink amide resin / highly general-purpose support	183599-10-2	R118279	Rink Amide AM resin	0.3–0.8 mmol/g, 100–200 mesh, 1% DVB	One of the classic general-purpose resins for Fmoc-SPPS; affords C-terminal amide peptides after cleavage; suitable for routine linear peptide synthesis, preparation of peptide drug analogs, and subsequent purification evaluation.

Table 2 | N-Terminal Protection and Protecting-Group Introduction Reagents

Classification	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Fmoc protecting-group introduction reagent	28920-43-6	F106534	Fmoc chloride	For HPLC derivatization, ≥99% (HPLC)	Used for Fmoc protection of amino groups, introducing a base-labile N-terminal protecting group; suitable for Fmoc amino acid derivatization, pre-protection of amine substrates, and peptide precursor construction.
Boc protecting-group introduction reagent	24424-99-5	D106159	Di-tert-butyl dicarbonate	≥99%	A commonly used Boc protecting reagent, suitable for temporary protection of amino groups or orthogonal protection design in fragment synthesis; applicable to solution-phase amino acids, peptide fragments, and amine intermediates.
Cbz protecting-group introduction reagent	501-53-1	B105737	Benzyl chloroformate	≥96%, contains 0.1% sodium carbonate as stabilizer	Used to introduce the Cbz (Z) protecting group; suitable for amino acids or peptide intermediates that require a hydrogenolysis-based deprotection route; commonly used in liquid-phase peptide synthesis and in protecting-group strategy switching.

Table 3 | Coupling Reagents, Activating Reagents, Coupling Additives, and Related Bases

Classification	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Strong organic base / Fmoc deprotection accelerator	6674-22-2	D106478	1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)	≥99%	A strongly basic organic base that can be used together with piperidine to accelerate Fmoc deprotection, and can also serve as a base in certain activation or cyclization steps; suitable for difficult deprotection or short-time processing workflows.
Coupling additive / low-epimerization promoter	39968-33-7	H109328	1-Hydroxy-7-azabenzotriazole	≥99%	An efficient coupling additive that can improve difficult coupling efficiency and reduce the risk of epimerization during the activation stage for certain sensitive substrates; often used together with carbodiimide, uronium, or phosphonium coupling reagents.
Coupling additive / activation aid	2592-95-2	H684271	1-Hydroxybenzotriazole (HOBT)	≥99%	A classic coupling additive that can improve the properties of activated intermediates and reduce certain side reactions; suitable for use together with carbodiimide-type coupling reagents.
Carbodiimide coupling reagent	538-75-0	D106074	N,N′-Dicyclohexylcarbodiimide	≥99%	A classic carboxylic acid activating reagent suitable for solution-phase peptide coupling and active ester construction; the byproduct DCU is insoluble, making it convenient to remove by filtration in certain systems.
Carbonyl activating reagent / imidazolization reagent	530-62-1	C109315	N,N'-Carbonyldiimidazole (CDI)	≥99%	Can convert carboxylic acids into acyl imidazole intermediates and can also be used for amino acid ester activation and fragment pretreatment; suitable for mild activation routes without added uronium salts.
Uronium coupling reagent	148893-10-1	H109327	HATU	≥99%	A highly active coupling reagent suitable for amide-bond formation involving sterically hindered substrates, difficult couplings, and poorly nucleophilic amines; commonly used in high-efficiency Fmoc-SPPS and solution-phase coupling.
Uronium coupling reagent	94790-37-1	H106174	HBTU	≥99%	A widely used general-purpose coupling reagent suitable for routine stepwise amino acid coupling; broadly applied under standard SPPS conditions and familiar to most operators.
Carbodiimide coupling reagent	693-13-0	N420184	N,N'-Diisopropylcarbodiimide	≥98.5%	A commonly used carbodiimide coupling reagent for SPPS, often used in combination with additives such as HOBt, HOAt, Oxyma, or NHS; suitable for on-resin coupling and in situ generation of active esters.
Water-soluble carbodiimide coupling reagent	25952-53-8	E106172	N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride	≥98%	A water-soluble carboxylic acid activating reagent suitable for peptide coupling in aqueous or water-containing systems, NHS ester formation, and pretreatment for biomolecule conjugation.
Phosphonium coupling reagent	128625-52-5	P109336	1H-Benzotriazol-1-yloxytripyrrolidinophosphonium Hexafluorophosphate	≥98%	A PyBOP-type phosphonium coupling reagent suitable for both solution-phase and solid-phase peptide coupling; shows good activation efficiency with certain substrates.
Oxyma-type additive	3849-21-6	E138773	Ethyl (hydroxyimino)cyanoacetate	≥98%	Commonly used as a low-epimerization additive in carbodiimide-based coupling systems; suitable for improving coupling efficiency and the purity of certain sequences; it is also often regarded as an alternative with lower explosion risk relative to HOBt and HOAt.
Oxyma-type integrated coupling reagent	1075198-30-9	C340003	COMU	≥98%	An Oxyma-derived uronium coupling reagent that combines relatively high activity with a lower tendency toward epimerization; suitable for routine to moderately difficult peptide couplings.
NHS active ester construction component	6066-82-6	H109330	N-Hydroxysuccinimide (NHS)	≥98%	Used to prepare NHS active esters, facilitating the separation, storage, and subsequent amine coupling of carboxylic acid activation intermediates; commonly used in peptide fragments, side-chain modification, and pretreatment for bioconjugation.
Uronium coupling reagent	125700-67-6	T109338	TBTU	≥98%	A commonly used peptide coupling reagent suitable for routine SPPS and solution-phase coupling; in combination with organic bases, it enables stable and efficient carboxylic acid activation.

Table 4 | Bases, Solvents, Deprotection, Cleavage, and Capping Components

Classification	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Strong-acid cleavage reagent / dedicated final cleavage component for Boc-SPPS	7664-39-3	H116232	Hydrofluoric acid	Guaranteed reagent grade, ≥40%	A classic strong-acid component for final cleavage and global deprotection in Boc-SPPS, and can also be used to remove certain special acid-stable protecting groups; this is a high-risk, highly specialized cleavage system suitable only for settings with dedicated equipment and strict safety controls, and should not be understood as an ordinary laboratory-general operation comparable to routine Fmoc-SPPS cleavage conditions.
Purification and precipitation solvent	67-56-1	M116128	Methanol	For protein sequencing, ≥99.9%	Commonly used for resin washing, redissolution/precipitation adjustment of certain intermediates, and sample handling before HPLC; can also be used for solvent exchange after solid-phase operations.
Organic base / acid scavenger for coupling	109-02-4	M104644	N-Methyl morpholine	For protein sequencing, ≥99.8% (GC)	A commonly used organic base and acid scavenger suitable for condensation reactions, activation steps, and certain protecting-group introduction reactions; widely used in liquid-phase peptide synthesis.
Acid cleavage reagent and HPLC mobile-phase additive	76-05-1	T103293	Trifluoroacetic acid	For protein sequencing, ≥99%	The most commonly used acid for peptide-chain cleavage and side-chain deprotection in Fmoc-SPPS, and also used as an ion-pairing additive in reversed-phase HPLC mobile phases; suitable for crude peptide release and analytical purification.
Resin swelling / washing solvent	75-09-2	D433565	Dichloromethane	Anhydrous grade, ≥99.8%, contains 40–150 ppm pentene as stabilizer	Commonly used for resin swelling, washing, solvent switching before and after cleavage, and certain protection/deprotection operations; highly compatible with solid-phase workflows.
Coupling and analytical solvent	75-05-8	A119012	anhydrous Acetonitrile (ACN)	Anhydrous grade, ≥99.8%, H2O ≤ 0.003%	Commonly used for peptide sample preparation, reversed-phase HPLC mobile phases, and certain coupling/activation systems; suitable for analytical purification and operations under low-water conditions.
Main solvent for solid-phase synthesis	68-12-2	D119450	N,N-Dimethylformamide (DMF)	Anhydrous grade, ≥99.8%	One of the most commonly used solvents in Fmoc-SPPS, suitable for resin swelling, amino acid coupling, deprotection, and additive dissolution; compatible with most coupling reagents.
Main solvent for solid-phase synthesis	872-50-4	M119668	1-Methyl-2-pyrrolidinone (NMP)	Anhydrous grade, ≥99.5%	Commonly used as a substitute for or in combination with DMF for resin swelling and coupling; suitable for certain poorly soluble substrates or high-loading resin systems.
Fmoc deprotection base	110-89-4	P1506346	Piperidine solution	Biotechnology grade, ≥99.5%	The standard deprotection base for Fmoc-SPPS, commonly used in DMF solution to rapidly remove the Fmoc protecting group; suitable for cyclic on-resin elongation workflows.
Capping reagent	108-24-7	A1506320	Acetic anhydride	European Pharmacopoeia (Ph.Eur.), puriss. p.a., ISO, ACS, ≥99% (GC)	Used to cap unreacted amine sites and reduce the continued elongation of deletion sequences; a common capping component in solid-phase peptide synthesis.
Organic base / acid scavenger for coupling	7087-68-5	D109322	N,N-Diisopropylethylamine	Distilled grade, ≥99.5%	A commonly used non-nucleophilic base suitable for carboxylic acid activation and coupling with HATU, HBTU, TBTU, COMU, PyBOP, and related reagents; also frequently used in protecting-group introduction.
Cleavage scavenger	100-68-5	T107511	Methyl phenyl sulfide	≥99%	Thioanisole; commonly used in TFA or HF cleavage systems to trap carbocation-type side-reaction intermediates, protect sensitive side chains, and improve crude product quality.
Cleavage scavenger	6485-79-6	T420182	Triisopropylsilane (TIPS)	≥98.5%	A commonly used scavenger in TFA cleavage systems that can reduce re-alkylation and related side reactions following tert-butyl deprotection; suitable for routine Fmoc peptide cleavage systems.
Cleavage scavenger	540-63-6	E106222	1,2-Ethanedithiol	≥97%	A powerful sulfur-containing scavenger suitable for cleavage systems containing larger numbers of sensitive residues such as Cys, Met, and Trp; helps reduce oxidation and side reactions.

Table 5 | Components Related to Recombinant Expression, Denaturing Purification, and Protein-Level Peptide Preparation

Classification	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Denaturant / inclusion body solubilizer	50-01-1	G108675	Guanidine Hydrochloride (GACl)	For protein analysis, ≥99.5%	A strong denaturant suitable for inclusion body solubilization, denaturing purification, and intermediate steps before refolding after recombinant expression; can also be used to unfold poorly soluble fusion proteins.
Antibiotic selection agent	25389-94-0	K742567	Kanamycin sulfate	Potency ≥750 IU/MG	Used for selection and maintenance of engineered strains carrying Kan-resistant vectors; suitable for cultivation of recombinant peptide/fusion-protein expression systems.
Metal-affinity purification buffer component	288-32-4	I432539	Imidazole	Anhydrous grade, ACS, ≥99%	Commonly used as an elution or gradient-competition component in His-tag protein purification, and can also be used for optimization of equilibration buffers; suitable for purification of recombinant peptide precursors or fusion proteins.
Denaturant / protein unfolding agent	57-13-6	U432960	Urea	Crystalline, pharmaceutical grade, Ph. Eur., BP, ChP, JP, USP	Commonly used for mild denaturation, solubilization, and refolding workflow design for proteins and fusion peptides; suitable for screening lower-intensity denaturing conditions.
Buffering agent	77-86-1	T110601	Tris(hydroxymethyl)aminomethane (Tris base)	Molecular biology grade, ≥99.9% (T)	A core component of commonly used buffer systems, suitable for multi-stage operations including expression, lysis, affinity purification, and refolding.
Reducing agent	3483-12-3	D104860	DL-Dithiothreitol (DTT)	Molecular biology grade, ≥99%	Commonly used to maintain thiols in the reduced state and prevent disulfide mispairing and aggregation; suitable for handling Cys-containing peptide precursors and fusion proteins, and for protection before refolding.
Metal-ion chelating agent	60-00-4	E112485	Ethylenediaminetetraacetic acid	Ultrapure, anhydrous grade, ≥99.5% (T)	Used to chelate divalent metal ions, inhibit metalloprotease activity, and stabilize lysis buffers; suitable for sample protection and downstream processing.
Expression inducer	367-93-1	I274316	IPTG (isopropyl-β-D-thiogalactopyranoside)	Ultrapure, ≥99%	A commonly used inducer for Lac/T7 and related systems, used to initiate expression of recombinant peptide precursors or fusion proteins; suitable for controlled induction culture.
Reducing agent	60-24-2	M755744	2-Mercaptoethanol	UltraBio™, molecular biology grade, ≥99% (GC)	Commonly used for thiol protection in lysis buffers, purification buffers, and sample processing; helps maintain the solubility of protein/peptide precursors and reduce oxidation.
Antibiotic selection agent	69-52-3	A105483	Ampicillin Na	PharmPure™, USP	Used for screening and culture maintenance of Amp-resistant expression plasmids; suitable for routine Escherichia coli recombinant expression systems.

Note: The above are representative Aladdin products. For more product specifications, please refer to the product list at the end of the article or search on the Aladdin website using the “product name/CAS/catalog number.”

References

[1] Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. J Am Chem Soc. 1963;85(14):2149-2154. doi:10.1021/ja00897a025.

[2] Behrendt R, White P, Offer J. Advances in Fmoc solid-phase peptide synthesis. J Pept Sci. 2016;22(1):4-27. doi:10.1002/psc.2836.

[3] Sharma A, Kumar A, de la Torre BG, Albericio F. Liquid-phase peptide synthesis (LPPS): a third wave for the preparation of peptides. Chem Rev. 2022;122(16):13516-13546. doi:10.1021/acs.chemrev.2c00132.

[4] Wang L, Wang N, Zhang W, Cheng X, Yan Z, Shao G, Wang X, Wang R, Fu C. Therapeutic peptides: current applications and future directions. Signal Transduct Target Ther. 2022;7(1):48. doi:10.1038/s41392-022-00904-4.

[5] Isidro-Llobet A, Kenworthy MN, Mukherjee S, Kopach ME, Wegner K, Gallou F, Smith AG, Roschangar F. Sustainability challenges in peptide synthesis and purification: from R&D to production. J Org Chem. 2019;84(8):4615-4628. doi:10.1021/acs.joc.8b03001.

[6] Li Y. Carrier proteins for fusion expression of antimicrobial peptides in Escherichia coli. Biotechnol Appl Biochem. 2009;54(1):1-9. doi:10.1042/BA20090087.

[7] Li Y. Recombinant production of antimicrobial peptides in Escherichia coli: a review. Protein Expr Purif. 2011;80(2):260-267. doi:10.1016/j.pep.2011.08.001.

[8] Jadhav KB, Woolcock KJ, Muttenthaler M. Anhydrous hydrogen fluoride cleavage in Boc solid phase peptide synthesis. In: Hussein WM, Skwarczynski M, Toth I, eds. Peptide Synthesis: Methods and Protocols. Methods in Molecular Biology. Vol 2103. New York, NY: Humana Press; 2020:41-57. doi:10.1007/978-1-0716-0227-0_4.

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Silicon Reagent-Assisted Peptide Synthesis: Research Progress and Methodological Value of Peptide Bond Formation from Unprotected Substrates