Technical articles
What Do Unnatural Amino Acids Really Change? Site-Specific Observation, Functional Engineering, and Property Optimization in Proteins and Peptides
What Do Unnatural Amino Acids Really Change? Site-Specific Observation, Functional Engineering, and Property Optimization in Proteins and Peptides
Introduction
The real change brought by unnatural amino acids is that they can now be designed around clearly defined goals. The 20 standard amino acids provide the default set of side chains used by biological systems; unnatural amino acids, by contrast, bring new functional groups, spectroscopic readouts, reactive handles, conformational constraints, and physicochemical properties directly into a sequence. As a result, a sequence is no longer merely the outcome of arranging natural residues, but becomes a chemical carrier in which observation, regulation, and optimization goals can be pre-positioned in advance. For proteins, this change is first reflected in site-specific observation and fine local functional tuning; for peptides, it is reflected more in the coordinated optimization of stability, conformation, selectivity, safety, and developability.
1. What exactly do unnatural amino acids refer to?
In the academic literature, noncanonical amino acids (ncAAs) are the more common core term. They do not belong to the 20 standard proteinogenic amino acids that are encoded by the default genetic code, but they are not all products of artificial synthesis. Reviews have pointed out that many ncAAs themselves occur naturally, or arise from post-translational modifications and specialized metabolic processes. By comparison, unnatural amino acids is a broader expression, typically encompassing artificially designed residues, residues outside the natural set, and a variety of nonstandard units introduced into proteins, peptides, or peptidomimetic systems. The main concern of this article is not whether these residues are “natural” in a literal sense, but whether they provide sequences with new functions that the 20 natural side chains do not intrinsically possess, such as new readout modes, reactivity, conformational control, or property-tuning space.
The table below summarizes several of the most common terms and those most easily confused with one another.
1.1 Basic distinctions among standard amino acids, noncanonical amino acids, and unnatural amino acids
Term | What it usually refers to | Key point of distinction |
Standard proteinogenic amino acids | The 20 standard amino acids that are encoded into proteins by the default genetic code | The benchmark set of natural side chains |
Noncanonical amino acids (ncAAs) | Amino acids outside the standard set of 20; they may occur naturally, be chemically synthesized, or be introduced into proteins through engineered translation systems | Emphasizes the introduction of new functions beyond the standard side chains |
Unnatural amino acids | A broader term that may include artificially designed residues, residues outside the natural set, and various nonstandard units introduced into proteins, peptides, or peptidomimetic systems | Emphasizes expanded units beyond the natural set |
Non-proteinogenic amino acids | Amino acids that do not belong to the set used by default in conventional translation; they may occur naturally or be synthesized artificially | Emphasizes that they are outside the conventional protein translation set |
2. How do unnatural amino acids enter proteins and peptides?
Unnatural amino acids bring new functions beyond the natural 20 side chains into sequences, but the way these units are introduced differs between proteins and peptides. On the protein side, the most representative route is genetic code expansion (GCE): researchers use mutually orthogonal aminoacyl-tRNA synthetase/tRNA pairs to write new residues site-specifically into proteins. Over the past decade or more, PylRS/tRNA^Pyl has become one of the most important and widely used tool systems in this field. It not only supports the incorporation of many types of noncanonical amino acids, but has also driven the development of multiple-codon strategies, multiple mutually orthogonal systems, and broader genetic code reprogramming.
In addition to site-specific incorporation, two other common protein-side approaches are cell-free protein synthesis (CFPS) and residue-specific global replacement. The former is better suited to improving tunability of reaction conditions and flexibility of incorporation in vitro; the latter is better suited to systematically rewriting the properties of a given class of residues at the whole-protein level, rather than modifying only a single site. By contrast, on the peptide side, the most common and most direct route is usually solid-phase peptide synthesis (SPPS), because it allows simultaneous control over sequence order, incorporation sites, protecting-group strategy, and combinations of multiple unnatural units.
2.1 Major routes by which unnatural amino acids enter protein and peptide systems (distinguished between the protein side and peptide side)
The table below lists these common routes separately for protein and peptide systems. Some methods emphasize site-level functional writing in proteins, some are better suited to global property rewriting, while the introduction of unnatural units into peptides relies mainly on chemical synthesis routes.
Target system | Route | Main target | Main features | Problems commonly addressed |
Protein side | Site-specific incorporation (GCE) | Recombinant proteins, intracellularly expressed proteins | Introduces noncanonical amino acids at designated codons and defined positions | Whether a particular site needs a new local readout, reactivity, or regulatory unit |
Protein side | PylRS/tRNA^Pyl-mediated incorporation | Multiple biological expression systems | One of the most central and widely used implementation systems for site-specific incorporation; supports many classes of noncanonical amino acids and drives the development of multiple-codon and multiple mutually orthogonal systems | How to write new residues into proteins site-specifically in a broader and more robust manner |
Protein side | Cell-free protein synthesis (CFPS) | In vitro protein expression systems, difficult-to-express proteins, multi-site incorporation systems | Reaction conditions are easier to tune; suitable for improving control and combinatorial flexibility of incorporation in vitro | How to improve flexibility and controllability of incorporation outside the cellular environment |
Protein side | Residue-specific global replacement | Protein materials, spectroscopic labeling systems, systems for global property rewriting | Replaces an entire class of natural residues throughout a protein, rather than modifying only a single site | How to systematically rewrite the properties of a given class of residues at the whole-protein level |
Peptide side | Direct incorporation by solid-phase peptide synthesis (SPPS) | Synthetic peptides, short peptides, therapeutic peptides | The most direct control over site, sequence order, and combinations of multiple unnatural units | How to coordinately optimize stability, conformation, membrane interaction, selectivity, and developability |
3. How do unnatural amino acids change the way proteins are observed?
3.1 Observation signals can be placed directly inside proteins
Once unnatural amino acids are introduced into proteins, what changes first is often not protein activity itself, but the way researchers obtain information. In the past, much structural and functional information had to be inferred through externally attached probes, post-labeling steps, or indirect signals. After probe-type noncanonical amino acids are introduced, changes in local environment, conformational changes, and binding events can be observed directly at preselected positions. In this way, researchers can decide not only what to observe, but also where to observe it already at the stage of protein sequence design. Recent reviews of GCE likewise regard this as one of the major values of the technology in biophysical and cellular studies.
3.2 Representative example: site-specific incorporation of a ^19F probe
The 2007 work by Jackson and co-workers is a classic example in this direction. The researchers introduced trifluoromethyl-phenylalanine site-specifically into proteins and used it as a ^19F NMR probe. According to the abstract, this approach enabled the incorporation of fluorinated residues in vivo with high translational efficiency and fidelity, and allowed conformational changes induced by substrate, inhibitor, and cofactor binding to be monitored through ^19F chemical shifts. The study also detected remote environmental changes approximately 25 Å from the active site. This shows that unnatural amino acids can do more than provide a readout at a designated position; they can also help identify local changes and long-range responses within proteins.
3.3 Why this type of method is more information-rich
The value of this design lies in placing the observational capability directly inside the protein. Compared with bulky labels attached in a later step, small-molecule probe residues introduced site-specifically are generally more likely to preserve positional information and may reduce perturbation arising from additional labeling procedures, although their effects on local protein structure and function still need to be verified case by case.
Many questions that once had to rely on indirect characterization—such as local binding, long-range conformational propagation, changes in membrane environment, and protein interaction states—can more readily be converted into analyzable chemical changes at defined sites. Recent reviews also show that such methods have already been widely applied in cell imaging, membrane protein studies, and protein interaction analysis, indicating that they constitute a class of site-specific observation methods with sustained utility in protein research.
3.4 What kinds of protein questions are probe-type unnatural amino acids better suited to answer?
Research task | Common mode of incorporation | Information obtained |
Observing local environmental changes at a specific site | Introduce a spectroscopic probe-type unnatural amino acid at the target site | Allows direct observation of environmental changes near that site, rather than only an overall averaged result |
Determining whether ligand binding triggers a local response | Introduce a probe residue capable of reporting changes near the binding site | Makes it easier to distinguish whether the local microenvironment changes before and after binding |
Identifying remote responses outside the active site | Introduce a reporter residue outside the functional site | Helps reveal conformational propagation or allostery-related changes |
Preserving positional information in cells or complex samples | Write the probe into the protein site-specifically through cotranslational incorporation | Makes it easier than post hoc bulky labeling to determine exactly where the change occurs |
4. How are unnatural amino acids used for functional engineering in proteins?
4.1 Protein engineering is no longer limited to substitutions within the natural residue set
When the research goal shifts from observing structural and binding changes to directly altering protein function, the value of unnatural amino acids becomes even more evident. Recent reviews in biocatalysis have pointed out that genetic code reprogramming can now introduce new local electronic effects, specific functional-group substitutions, and new regulatory units into enzyme studies. Such designs not only help clarify mechanisms, but have also been used to improve activity, selectivity, and stability, and to construct artificial regulatory elements that respond to external stimuli.
4.2 Representative example: a single-site introduction of an unnatural residue increased activity by 8–11 fold
In 2011, Ugwumba et al. replaced Tyr309 in bacterial phosphotriesterase with the coumarin-type unnatural amino acid L-(7-hydroxycoumarin-4-yl)ethylglycine. According to the abstract, this single-site design further increased the already high native activity of the enzyme by 8–11 fold. The authors related this enhancement to electrostatic repulsion of the negatively charged product during the product-release step. The abstract also notes that this result stood in contrast to large-scale screening based solely on natural amino acid substitutions, which had still failed to further improve the native activity.
4.3 What kinds of protein-engineering problems are unnatural amino acids better suited to solve?
When the research question has already been narrowed to a specific level—for example, the local electronic environment, substrate or product release, special interactions near the active site, or responsiveness to external stimuli—unnatural amino acids are often more targeted than simply continuing to expand the mutation range among natural residues. For this reason, high-value ncAA-based protein engineering usually does not begin by blindly enlarging a mutation library; rather, it begins by clarifying the mechanistic question and then determining whether new functions beyond the natural side chains are actually needed.
4.4 Different roles of natural residue scanning and unnatural amino acid design in protein engineering
Research task | Common first-choice approach | Situations suitable for introducing unnatural amino acids |
Basic activity evaluation | Natural residue substitution or conventional mutational scanning | In general, there is no need to introduce unnatural amino acids too early |
Verification of local mechanism | Mechanism-driven single-site design | When it is necessary to test electronic effects or local reactivity not available from natural side chains |
Simultaneous adjustment of readout and function | Joint design of probe residues and functional residues | When both observation of change and direct functional alteration are required |
Construction of new functions | Conventional natural residues are often insufficient | When new reactivity, stimulus responsiveness, or artificial regulatory units are needed |
5. How are unnatural amino acids used for structural optimization and developability-oriented modification of peptides?
5.1 Peptide design is more concerned with stability, selectivity, and downstream development conditions
In peptide design, the question is often not merely whether a sequence can be synthesized, but whether the molecule can continue toward practical application. Recent reviews and medicinal chemistry studies have pointed out that common limitations of peptides include susceptibility to proteolytic degradation, unstable secondary structure, difficulty balancing membrane permeability with toxicity, and unsatisfactory exposure and drug-like properties. Unnatural amino acids are important because they can influence several of these interrelated properties at the same time.
5.2 Unnatural amino acids are mainly used to adjust multiple key parameters simultaneously
A 2024 review by Sharma et al. pointed out that unnatural amino acids can modulate the physicochemical and pharmacological characteristics of peptides and peptidomimetic molecules by altering functional groups, reactivity, lipophilicity, hydrophilicity, charge, and enzymatic stability. This type of modification is not simply about replacing one residue with a new one; rather, it aims to address multiple issues at once, including activity, selectivity, stability, and developability. Accordingly, the introduction of unnatural amino acids into peptide design is usually not for single-parameter optimization, but for tuning overall molecular performance.
5.3 Representative example: modification of antimicrobial peptides must consider activity, structure, and safety together
A 2022 review by Du et al. pointed out that introducing noncanonical amino acids into antimicrobial peptides can increase their physicochemical and pharmacological diversity. The 2017 Cbf-14-2 example reported by Kang et al. provides more specific evidence: the Orn-containing mutant showed minimum inhibitory concentrations of 4–32 μg/mL against multiple drug-resistant strains, while also displaying low hemolysis and negligible cytotoxicity toward mouse splenocytes. Circular dichroism results further showed a higher α-helical content in SDS. This case illustrates that unnatural amino acid modification in antimicrobial peptides usually has to be judged by considering activity, conformation, and safety together, rather than by looking only at whether antimicrobial potency has improved.
5.4 To judge whether unnatural amino acids are truly valuable, several parameters need to be considered together
In peptide design, it is often not enough to look only at whether “activity has improved.” A more informative evaluation should consider at least four aspects simultaneously: whether proteolytic stability has increased, whether secondary structure has become more controllable, whether membrane interactions have become more selective, and whether improved activity is accompanied by increased hemolysis or cytotoxicity. Once development moves into later stages, exposure, half-life, bioavailability, and dosing feasibility should also be examined further; however, whether these parameters improve must still be verified case by case in a specific system.
5.5 Problems for which unnatural amino acids are more commonly used in peptide design
Optimization goal | Common modification direction | Parameters that must be considered simultaneously |
Improve proteolytic stability | D-amino acids, N-substituted residues, conformationally constrained residues | Stability, activity, conformation, metabolic behavior |
Stabilize secondary structure | α,α-Disubstituted residues, cyclization, or constrained backbone units | Helical or folding propensity, target binding, selectivity |
Adjust membrane interactions | Rewrite hydrophobic/cationic distribution and introduce unnatural side chains | Activity, hemolysis, cytotoxicity, selectivity |
Improve downstream developability | Adjust charge, lipophilicity, hydrogen-bond donors/acceptors, and backbone flexibility | Permeability, exposure, half-life, bioavailability, and dosing feasibility |
6. When is it worth prioritizing the introduction of unnatural amino acids?
Unnatural amino acids are better suited to tasks that already clearly require new readouts, new reactivity, or new property-tuning capabilities beyond the 20 natural side chains. In protein research, such situations typically include site-specific observation, validation of local mechanisms, and construction of new functions; in peptide research, they are more commonly relevant when stability, structure, selectivity, and developability already need to be optimized together.
Research situation | Is it worth prioritizing? | Basis for judgment |
A need to directly observe conformation, binding, or local environmental changes at a specific site | Worth prioritizing | Natural side chains usually cannot directly provide this kind of site-resolved readout |
A need to test local electronic effects, special reactivity, or new functional units | Worth prioritizing | The requirement already exceeds the conventional functional scope of the natural residue set |
Peptide optimization has entered a stage requiring balance among multiple parameters such as stability, structure, selectivity, and safety | Worth prioritizing | Unnatural amino acids are better suited to adjusting multiple related parameters simultaneously |
Only basic activity profiling or general mutational scanning is being performed | Not necessary to prioritize | Scanning with natural residues first is usually more efficient and easier to interpret |
The research objective is still unclear, and the intention is only to try adding several new residues first | Usually not recommended as a priority | Before the key question has been sufficiently narrowed, introducing unnatural amino acids often adds variables and experimental complexity first, without necessarily improving information quality or decision-making efficiency at the same time |
7. Product Navigation Table for Unnatural Amino Acid Design and Functional Studies (Tables 1–4)
Current research or experimental goal | Recommended table(s) to consult first | Why this table should be consulted first | Recommended related table(s) | Navigation notes |
To first establish a basic selection framework for unnatural amino acids in research, and determine whether the current task is more oriented toward “site-specific functional writing in proteins” or “property modification of peptide sequences” | Table 1, Table 3 | Table 1 focuses on site-specific incorporation, click chemistry, photocrosslinking, photocontrol, and spectroscopic probe residues; Table 3 focuses on peptide conformation, stability, hydrophobic side chains, and protease-resistance-related residues. Together, they are the most suitable starting point for deciding whether the research object is a protein or a peptide | Then see Table 2, Table 4 | If the research object is a recombinant protein, enzyme, receptor, or protein interaction system, it is usually best to start with Table 1; if the research object is a functional peptide, antimicrobial peptide, cell-penetrating peptide, or peptide drug optimization project, it is usually best to start with Table 3 |
To introduce new chemical handles, fluorescent/spectroscopic probes, or photocrosslinking sites into proteins site-specifically for labeling, imaging, binding-site capture, or conformational analysis | Table 1 | Table 1 brings together azide-, alkyne-, ketone-, cyano-, fluorinated aromatic, benzophenone, and caged tyrosine residues, making it the most suitable for tasks involving site-specific labeling and functional writing | Then see Table 2 | First use Table 1 to determine whether the goal is click conjugation, photocrosslinking, fluorinated/cyano probes, or light-triggered release; if the study also involves mimicking a phosphorylation state or a lysine modification state, it is more appropriate to consult Table 2 as well |
To study how post-translational modifications such as phosphorylation, acetylation, and methylation affect protein structure, recognition, or signal transduction | Table 2 | Table 2 focuses on O-phosphoserine, O-phosphothreonine, O-phosphotyrosine, Nε-acetyllysine, and trimethyllysine, and is suitable for directly establishing experimental systems around modification states | Then see Table 1 | If the subsequent experiment will also incorporate site-specific probes, photocontrol triggering, or binding capture, Table 1 can then be used in combination; if the goal is simply to compare how different modification states affect function, Table 2 should be prioritized |
To improve peptide conformational stability, enhance α-helical propensity, improve resistance to proteolytic degradation, or systematically compare L/D configuration and N-methylation effects | Table 3 | Table 3 focuses on Aib, Sar, N-methyl-L-alanine, D-alanine, D-phenylalanine, β-alanine, trans-4-hydroxy-L-proline, and related residues, making it the most suitable for optimization centered on peptide backbone and secondary structure | Then see Table 4 | It is generally more consistent with common peptide-optimization logic to first clarify peptide backbone conformation, flexibility, and stability, and then decide whether further tuning of cationic side-chain length and membrane interactions is needed |
To optimize antimicrobial peptides, membrane-active peptides, or cell-penetrating peptides, with emphasis on comparing the relationship among positive-charge distribution, side-chain length, and membrane selectivity | Table 4 | Table 4 focuses on lysine-replacement-type basic residues such as Orn, Dab, and Dap, and is the most suitable for fine comparisons of cationic density, side-chain length, and membrane interaction strength | Then see Table 3 | If the goal is not only to tune charge but also to improve stability, control the hydrophobic face, or adjust secondary structure at the same time, Table 3 should be consulted together; Table 4 is the “entry point for tuning charge and membrane interactions,” whereas Table 3 is the “entry point for backbone and conformational optimization” |
To optimize hydrophobic side-chain volume, aromatic environment, or local electronic properties, and compare how different unnatural residues affect binding, catalysis, or sequence–activity relationships | Table 3, Table 1 | In Table 3, norleucine, norvaline, tert-leucine, and cyclohexylalanine are more suitable for scanning hydrophobic bulk and side-chain length; in Table 1, fluorinated, cyano, ketone, and catechol-containing residues are more suitable for tuning aromatic environment and electronic properties | Then see Table 4 | If the core issue is adjustment of the hydrophobic/aromatic surface, first consult Tables 3 and 1; if these changes are later to be combined with cationic residues to assess membrane activity or cellular selectivity, then bring in Table 4 as well |
To conduct combination experiments involving “site-specific functional writing + modification-state mimicking,” for example, mimicking phosphorylation at a specific site together with probe readout or photocontrol triggering | Table 2 | Table 2 first helps determine which type of modification state is being mimicked, avoiding the confusion of “modification mimicry” and “probe incorporation” from the very beginning | Then see Table 1 | First define the modification question clearly, then use Table 1 to add site-specific labeling, spectroscopic readout, photocrosslinking, or photocontrol methods, which is more conducive to building a clearly layered experimental system |
To perform “multi-parameter coordinated tuning of peptide properties,” considering activity, stability, conformation, hydrophobicity, and cellular selectivity together rather than changing only one parameter | Table 3 | Table 3 is the most suitable first-round entry point for peptide-sequence optimization because it addresses backbone, conformation, hydrophobic surface, and protease-resistance issues first | Then see Table 4 | Once Table 3 has largely tuned conformation and stability into place, Table 4 can then be used to fine-tune cationic side chains such as Orn/Dab/Dap; this usually makes it easier to identify the source of variables and is also closer to actual peptide-optimization workflows |
Brief usage summary:
1. If the research starting point is site-specific incorporation into proteins, imaging, probes, crosslinking, or photocontrol, consult Table 1 first.
2. If the research starting point is the post-translational modification state itself, consult Table 2 first.
3. If the research starting point is optimization of peptide conformation, stability, hydrophobicity, or resistance to degradation, consult Table 3 first.
4. If the research starting point is tuning cationic side-chain length, membrane interactions, and antimicrobial peptide selectivity, consult Table 4 first.
Table 1 | Site-Specific Incorporation, Spectroscopic Probes, Bioorthogonal Handles, and Photocontrollable Residues
Classification | CAS No. | Aladdin Catalog No. | Name | Grade or Purity | Product Features and Applications |
Catechol-functional side-chain residue | 59-92-7 | L-dopa | Moligand™, ≥99% | Contains a catechol side chain; can be used to construct adhesive peptides, redox-responsive systems, and metal-coordination studies, and is also commonly used to simulate how catechol groups influence interfacial binding and crosslinking behavior. | |
Cyano-aromatic probe residue | 167479-78-9 | H-Phe(4-CN)-OH | ≥98% | The para-cyano group can serve as an environment-sensitive spectroscopic probe for examining local polarity, conformational changes, and binding events in proteins or peptides. | |
Azide bioorthogonal handle residue | 33173-53-4 | 4-Azido-L-phenylalanine | ≥98% | A classic azide-handle residue suitable for site-specific labeling, click conjugation, attachment of imaging probes, and studies of protein chemical modification. | |
Cyano-aromatic probe residue | 104531-20-6 | 4-Cyano-L-phenylalaine hydrochloride | ≥98% | Like H-Phe(4-CN)-OH, this is a cyano-aromatic probe residue suitable for analysis of protein conformational changes, local microenvironments, and ligand binding. | |
Photocaged residue | 207727-86-4 | NB-caged Tyrosine hydrochloride | ≥97% (HPLC) | Can be used to construct light-triggered activation systems, masking tyrosine-site function before irradiation; suitable for studying spatiotemporally controlled enzyme activity, signal transduction, or protein interactions. | |
Fluorinated tyrosine mechanistic probe residue | 73246-30-7 | (S)-2-Amino-3-(3，5-difluoro-4-hydroxyphenyl)propanoic acid | ≥97% | A fluorinated tyrosine analog that can be used to tune the electronic properties of aromatic side chains and to study enzyme mechanisms, hydrogen-bond networks, and proton-transfer-related questions. | |
Carbonyl bioorthogonal handle residue | 122555-04-8 | 4-Acetyl-L-phenylalanine | ≥97% | Contains a ketone side chain and can serve as a site-specific conjugation entry point for protein labeling, probe attachment, spin labeling, or grafting of functional molecules. | |
Azide bioorthogonal handle residue | 942518-29-8 | L-Azidohomoalanine hydrochloride | ≥97% | A methionine-replacement-type azide residue commonly used in metabolic labeling of newly synthesized proteins, whole-protein tracking, and click chemistry conjugation; it can also serve as a common handle in residue-specific incorporation routes. | |
Fluorinated aromatic probe residue | 114926-38-4 | 4-(Trifluoromethyl)-L-phenylalanine | ≥96% | A trifluoromethyl-containing aromatic residue suitable for hydrophobicity tuning, fluorine-based spectroscopic studies, and analysis of local protein environmental responses. | |
Alkyne bioorthogonal handle residue | 610794-20-2 | (S)-2-Amino-3-[4-(prop-2-yn-1-yloxy)phenyl]propanoic Acid | ≥95% | Contains a terminal alkyne and can serve as a click-conjugation site for site-specific incorporation of fluorophores, biotin, or functional molecules. | |
Photocrosslinking residue | 104504-45-2 | 4-Benzoyl-L-phenylalanine | ≥95% | A classic photocrosslinking residue suitable for capturing transient protein–protein or protein–ligand interactions and for mapping binding sites. |
Table 2 | Residues for Introducing Post-Translational Modifications and Studying Modification States
Classification | CAS No. | Aladdin Catalog No. | Name | Grade or Purity | Product Features and Applications |
Phosphoserine-mimicking residue | 407-41-0 | L-O-Phosphoserine (L-SOP) | Moligand™, ≥98% (T) | Suitable for mimicking serine-site phosphorylation and for studying protein regulation, kinase/phosphatase recognition, and phosphorylation-dependent structural changes. | |
Acetyllysine-mimicking residue | 692-04-6 | Nε-Acetyl-L-lysine | ≥98% (T) | Commonly used to mimic lysine acetylation and is suitable for studying epigenetic modifications, protein interactions, and the functional consequences of acetylation. | |
Trimethyllysine-mimicking residue | 55528-53-5 | Nε,Nε,Nε-Trimethyllysine hydrochloride | ≥97% (TLC) | Can be used to mimic lysine trimethylation and is commonly applied in histone-modification studies, methylation recognition, and analysis of epigenetic binding interfaces. | |
Phosphotyrosine-mimicking residue | 21820-51-9 | O-Phospho-L-tyrosine | ≥95% (HPLC) | Suitable for mimicking tyrosine-site phosphorylation in studies of receptor signal transduction, SH2 recognition, and phosphorylation-dependent interactions. | |
Phosphothreonine-mimicking residue | 1114-81-4 | O-Phospho-L-threonine | ≥95% | Suitable for mimicking threonine-site phosphorylation in studies of kinase substrate recognition, regulatory-site function, and phosphorylation-induced conformational effects. |
Table 3 | Residues for Regulating Peptide Conformation, Stability, and Hydrophobic Side Chains
Classification | CAS No. | Aladdin Catalog No. | Name | Grade or Purity | Product Features and Applications |
N-methyl backbone-regulating residue | 107-97-1 | Sarcosine | UltraBio™, ultrapure grade | As N-methylglycine, it can be used to regulate peptide-backbone flexibility, improve resistance to enzymatic degradation, and study how backbone methylation affects conformation and pharmacokinetic properties. | |
Hydroxyproline conformational-regulating residue | 51-35-4 | trans-4-Hydroxy-L-proline | Moligand™, for cell culture, ≥98.5% | Commonly used in collagen-related peptides and in studies of proline-induced turns and stability; also suitable for examining how hydroxylation affects peptide conformation and hydrogen-bond networks. | |
β-Amino-acid backbone-extension residue | 107-95-9 | β-Alanine | Moligand™, ≥99% | As the most basic β-amino acid, it can be used for backbone extension, flexible spacer-arm design, and studies aimed at improving peptide tolerance to proteolytic degradation. | |
D-Amino-acid stability-regulating residue | 338-69-2 | D-Alanine | Moligand™, ≥98% | Commonly used to improve peptide resistance to proteases, alter secondary-structure preferences, and compare how L/D configurational substitution affects activity and selectivity. | |
D-Amino-acid stability-regulating residue | 673-06-3 | D-Phenylalanine | Moligand™, ≥98% | Commonly used to enhance peptide stability, adjust hydrophobic-surface distribution, and examine how inversion of aromatic side-chain configuration affects activity and receptor recognition. | |
Sterically restricted hydrophobic residue | 20859-02-3 | L-tert-Leucine | ≥99% | Has a highly branched side chain and is suitable for increasing hydrophobic bulk occupancy, tuning conformational restriction, and optimizing the steric environment of peptides or small-molecule fragments. | |
Linear hydrophobic side-chain replacement residue | 327-57-1 | L-Norleucine | ≥99% | Can serve as a hydrophobic replacement for methionine or leucine, helping reduce oxidation sensitivity and adjust hydrophobic interactions. | |
Linear hydrophobic side-chain replacement residue | 6600-40-4 | L-Norvaline | ≥99% | Suitable for fine-tuning side-chain length and hydrophobicity, and commonly used in peptide-sequence scanning and structure–activity relationship comparisons. | |
N-methyl backbone-regulating residue | 3913-67-5 | N-Methyl-L-alanine | ≥98% | Can be used to study how backbone N-methylation affects peptide conformation, membrane permeability, and resistance to enzymatic degradation. | |
α,α-Disubstituted conformationally restricted residue | 62-57-7 | α-Aminoisobutyric acid | ≥98% | A classic helix-inducing residue commonly used to enhance α-helical propensity, improve conformational stability, and increase resistance to enzymatic degradation. | |
Strongly hydrophobic bulky residue | 27527-05-5 | (S)-2-Amino-3-cyclohexylpropanoic acid | ≥96% | Has a bulky hydrophobic side chain and is suitable for strengthening peptide interactions with hydrophobic interfaces or membrane environments, as well as for structure–activity optimization. |
Table 4 | Residues for Basic Side-Chain Replacement and Regulation of Membrane Interactions
Classification | CAS No. | Aladdin Catalog No. | Name | Grade or Purity | Product Features and Applications |
Basic side-chain replacement residue | 3184-13-2 | L-Ornithine monohydrochloride | BioReagent Plus, ≥99% | Commonly used as a lysine substitute to tune side-chain length and positive-charge distribution; suitable for studies of antimicrobial peptides, cell-penetrating peptides, and membrane interactions. | |
Short-chain basic residue | 1883-09-6 | L-2,4-Diaminobutyric acid hydrochloride | ≥98% | Has a shorter side chain than lysine and is suitable for comparing how cationic spacing influences membrane selectivity, activity, and cytotoxicity. | |
Even shorter-chain basic residue | 1482-97-9 | (S)-(+)-2,3-Diaminopropionic Acid Hydrochloride | ≥97% | Can be used to further shorten the length of the cationic side chain and is suitable for studying charge density, peptide–membrane binding modes, and fine sequence optimization. |
Note: The products listed above are representative Aladdin products. For more product specifications, please refer to the product list at the end of the article or search the Aladdin website using the product name/CAS number/catalog number.
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