Formation and Control of Carboxyl- and Hydroxyl-Related Impurities in Peptide Synthesis — Using Asp/Glu and Ser/Thr Residue Side Reactions as Examples
Formation and Control of Carboxyl- and Hydroxyl-Related Impurities in Peptide Synthesis — Using Asp/Glu and Ser/Thr Residue Side Reactions as Examples
1 Introduction: Peptide Impurities Often Arise from Misactivated Functional Groups
In peptide synthesis, impurities do not always originate from “incomplete coupling” or “incomplete deprotection.” Many quality issues arise because certain functional groups enter undesired reaction pathways under specific conditions. Carboxyl and hydroxyl groups are two typical examples. Carboxyl groups participate in peptide bond formation, while hydroxyl groups possess a certain degree of nucleophilicity. However, when acid–base conditions, solvent systems, protecting-group status, temperature, and processing time act together, these groups may shift from normal reaction sites to sources of side reactions.
The essence of such side reactions is not the loss of control in a single reaction step, but rather a mismatch between functional-group reactivity and process conditions. This article focuses on four common residues—aspartic acid Asp, glutamic acid Glu, serine Ser, and threonine Thr—to analyze the chemical logic behind carboxyl- and hydroxyl-related side reactions, and to provide a practical decision-making framework for route design, purification selection, and impurity troubleshooting.
2 To Assess Side-Reaction Risk, Start with Five Variables
Carboxyl- and hydroxyl-related side reactions in peptide synthesis are usually not determined by a single factor, but by the combined effect of multiple variables. The triggering logic can be understood as follows:
Triggering logic of carboxyl/hydroxyl side reactions
Peptide sequence containing Asp/Glu/Ser/Thr → functional groups become exposed or activated → acid–base conditions + solvent system + temperature/time + protecting-group status → undesired reaction pathways are opened → esterification, migration, elimination, cyclization, cleavage, and other side reactions → decreased purity, reduced yield, and a more complex impurity profile
In practical research and development, five variables should be considered when evaluating whether a peptide chain is prone to carboxyl- or hydroxyl-related side reactions.
Assessment Variable | Key Question to Consider | Impact on Side Reactions |
Functional-group type | Does the sequence contain Asp/Glu/Ser/Thr? | Determines potential sources of side reactions |
Protecting status | Are the side chains protected, and are the protecting groups stable? | Determines whether functional groups are exposed or misactivated |
Acid–base conditions | Does the process involve strong acid, strong base, or prolonged base treatment? | Affects esterification, migration, elimination, and cyclization |
Solvent system | Are methanol, acidic aqueous alcohol systems, or highly reactive reagents used? | Affects carboxyl esterification and hydroxyl acylation |
Temperature and time | Are heating, concentration, or prolonged processing involved? | Amplifies the cumulative effect of slow side reactions |
3 Carboxyl Side Reactions: Why Asp/Glu Can Easily Become Sources of Impurities
3.1 The Core Risk of Carboxyl Groups Is Conversion in Acidic Alcohol Systems
The side chains of both Asp and Glu contain carboxyl groups, and the C-terminus of a peptide chain may also contain a free carboxyl group. Carboxyl groups are fundamental structural units for amide-bond formation in peptide synthesis, but under certain conditions they may also undergo esterification, transesterification, or resin-related side reactions.
The core logic of carboxyl side reactions is that when a carboxyl group is exposed to an acidic alcohol system, a base-catalyzed substitution system, or prolonged concentration conditions, a site that should remain in the carboxylic acid form may be converted into an ester impurity. One relatively common and easily overlooked issue is methyl esterification in acidic alcohol systems. For Asp-containing sequences, aspartimide formation should also be considered. This is one of the important Asp-related side reactions in Fmoc solid-phase peptide synthesis.
Schematic illustration of Asp/Glu carboxyl methyl esterification
Peptide-COOH + CH3OH
│Acidic conditions: TFA, formic acid, etc.
│Risk factors: methanol residue, acidic eluate, concentration with heating
↓
Peptide-COOCH3 + H2O
TFA (trifluoroacetic acid) or formic acid is commonly used for cleavage, deprotection, or as an acidic modifier in reversed-phase liquid chromatography. When methanol is also present in the system, the side-chain carboxyl groups of Asp/Glu or the C-terminal carboxyl group of the peptide may form methyl esters. These by-products are structurally similar to the target peptide, may increase the difficulty of downstream separation, and can directly affect product purity.
3.2 Methanol Is Not Merely a Solvent; It May Also Participate in the Reaction
In peptide purification, methanol is often used as the organic phase. However, for peptides containing Asp/Glu with an evident tendency toward esterification, methanol should not be viewed only as an elution solvent. In particular, when the following conditions occur simultaneously, methanol can become a reactant in carboxyl methyl esterification:
① The peptide contains free carboxyl groups or acid-sensitive carboxyl-containing structures;
② Acidic modifiers such as TFA or formic acid are present in the system;
③ The collected fractions require subsequent concentration;
④ Concentration is accompanied by elevated temperature or prolonged residence time.
Therefore, for esterification-sensitive peptides, the feasibility of using acetonitrile instead of methanol should be evaluated first during preparative reversed-phase purification. If the process must use methanol, the acid concentration, concentration temperature, and residence time should be controlled, and methyl esterification impurities should be monitored by LC-MS (liquid chromatography–mass spectrometry).
3.3 Carboxyl Transesterification Should Be Evaluated under Alcoholic Solvent and Basic Treatment Conditions
In Boc/Bzl or other ester-type side-chain protecting systems, if the peptide resin or protected amino acid is exposed to alcoholic solvents, strong bases, alkoxides, or prolonged basic treatment, the possibility of transesterification of side-chain ester protecting groups should be evaluated. For example, in certain basic alcoholic systems, the Asp/Glu side-chain benzyl ester could theoretically undergo transesterification, generating the corresponding alkyl ester impurity.
Schematic illustration of Asp/Glu side-chain transesterification
Asp/Glu-COOBn + CH3OH
│ Base-catalyzed loading or treatment
↓
Asp/Glu-COOCH3 + BnOH
Here, Bn represents benzyl. This process indicates that the resin, base, and solvent are not neutral background conditions; they can alter the stability of side-chain protecting groups. For peptide chains containing Asp/Glu, route design should take resin type, side-chain protecting groups, and loading conditions into account at the same time.
3.4 Key Control Points for Carboxyl Side Reactions
The prevention of carboxyl side reactions should focus on “reducing the simultaneous exposure of carboxyl groups to alcohols, acids, and heat.”
Risk Scenario | Possible Issue | Control Strategy |
Resin washing with methanol | Methanol residue enters the cleavage system | Avoid methanol washing for esterification-sensitive peptides, or strengthen drying control |
Acidic methanol purification system | Methyl esterification of Asp/Glu or C-terminal carboxyl groups | Preferentially evaluate acetonitrile systems and reduce exposure to acidic methanol |
Excessively long concentration time | Accumulation of slow esterification | Lower the temperature, shorten residence time, and avoid over-concentration |
Base-catalyzed resin treatment | Side-chain transesterification | Select appropriate resin, protecting groups, and basic treatment conditions |
Impurity peak close to the target peak | Increased purification difficulty | Use LC-MS to confirm whether +14 Da methyl esterification-related impurities are present |
4 Hydroxyl Side Reactions: Ser/Thr Are Not Simply Inert Side Chains
4.1 The β-Hydroxyl Groups of Ser/Thr May Become Competitive Reaction Sites
The side chains of both Ser and Thr contain β-hydroxyl groups. Compared with amino groups, hydroxyl groups are generally less nucleophilic, so they are not necessarily the main reaction sites under routine coupling conditions. However, when amino-group reaction becomes slow, acylating reagents are used in excess, basicity increases, or the hydroxyl group is activated by specific reagents, the β-hydroxyl group may participate in side reactions.
The risks associated with Ser/Thr can be summarized into three categories:
β-Hydroxyl group is incorrectly modified → alkylation, acylation
β-Hydroxyl group participates in migration → O→N acyl migration
β-Hydroxyl group induces structural change → β-elimination, cyclization, cleavage
Ser/Thr are not merely hydrophilic residues, nor are they completely inert spectators. Their side reactions are often associated with neighboring peptide bonds, N-terminal protecting status, the strength of basic treatment, and the type of coupling reagent used.
4.2 Deprotection of Allyl-Type Protecting Groups: Insufficient Scavenging May Lead to Allyl-Transfer By-Products
Allyl-type protecting groups are commonly used in orthogonal protection strategies for peptide synthesis, including Alloc (allyloxycarbonyl), allyl esters, and OAll (O-allyl ether) forms. Among them, Alloc is commonly used for amino-group protection, whereas Ser/Thr hydroxyl groups are more commonly protected as OAll or related forms. During Pd(0)-catalyzed deprotection, insufficient allyl scavenger or mismatched reaction conditions may lead to allyl transfer, generating O-allylated or N-allylated by-products, and may also result in incomplete deprotection.
Transfer risk during deprotection of allyl-type protecting groups
Ser/Thr-OAll or other allyl-type protected structure
│ Pd(0) deprotection
│ insufficient allyl scavenging or mismatched conditions
↓
Allyl residue, O-allylated or N-allylated related by-products
This reaction suggests that, in orthogonal protection strategies, deprotection conditions themselves may introduce new modification risks. The key to controlling this type of side reaction is to select an appropriate allyl scavenger and control the reagent ratio, processing time, and reaction completeness of the Pd(0)-catalyzed system. If the hydroxyl or amino group is not fully restored after deprotection, LC-MS should be used to investigate allyl residues or allyl-transfer by-products.
4.3 Acylation and O→N Migration: When Amino-Group Reaction Becomes Slow, Hydroxyl Groups May Be Incorrectly Acylated
During normal peptide-bond formation, the amino group is the primary nucleophilic site. However, if the target amino group reacts slowly, or if acylating reagents are present in excess, the β-hydroxyl group of Ser/Thr may be acylated to form an O-acyl intermediate. Subsequently, under deprotection or basic treatment conditions, or in the presence of a neighboring amino group capable of participating in migration, the O-acyl structure may undergo O→N or N→O acyl migration, forming ester/amide bond positional isomers or incorrectly acylated products, thereby complicating the impurity profile.
Schematic illustration of Ser/Thr hydroxyl acylation and O→N migration
Ser/Thr-OH
│ Excess acylating reagent or slow amino coupling
↓
Ser/Thr-O-acyl
│ Basic treatment or deprotection conditions
↓
Ser/Thr-N-acyl by-product
The key point of this side reaction is that, in a competitive reaction, reduced amino-group reaction efficiency increases the opportunity for the hydroxyl group to participate. Therefore, for sequences containing Ser/Thr, difficult coupling should not be addressed only by indefinitely increasing the amount of acylating reagent or extending the reaction time; the risk of hydroxyl acylation should also be evaluated.
4.4 β-Elimination: Base, Electron-Withdrawing Groups, and Activated Hydroxyl Groups Can Jointly Amplify the Risk
β-Elimination is one of the important side reactions associated with Ser/Thr. When the Ser/Thr side-chain hydroxyl group is connected to electron-withdrawing groups such as Ts (tosyl) or Ms (mesyl), elimination is more likely to occur under basic conditions, generating dehydroamino acid structures. For example, Ser can form dehydroalanine, while Thr can form dehydrobutyrine-related structures.
Schematic illustration of Ser/Thr β-elimination
Ser/Thr-CH(OH)-CH-
│ Basic conditions, electron-withdrawing leaving group, increased temperature/time
↓
Dehydroamino acid structure + leaving group
The β-elimination risk of Ser/Thr increases significantly mainly when the hydroxyl group is converted into a better leaving group, such as a phosphate ester, sulfate ester, sulfonate ester, halide, or certain activated carbonate intermediates. Under conditions where the Ser/Thr hydroxyl group is unactivated, β-elimination is usually not the main side reaction; however, strong base, high temperature, prolonged treatment, and additional hydroxyl activation may still amplify the elimination risk in special sequences.
The key to controlling β-elimination is to avoid the simultaneous presence of strong base, prolonged processing, and high temperature. For sensitive sequences, the Fmoc (9-fluorenylmethoxycarbonyl) deprotection time should be shortened, the base concentration should be optimized, and additional activation of the Ser/Thr hydroxyl group should be minimized.
4.5 Cyclization By-Products: β-Hydroxyl Groups May Attack Neighboring Carbonyl Groups to Form Five-Membered Ring Structures
The β-hydroxyl group of Ser/Thr is adjacent to amide, carbamate, or peptide-bond structures. Under specific conditions, it may undergo intramolecular attack to form five-membered ring by-products. This type of reaction is commonly observed in two situations.
① N-terminal Ser/Thr under Boc (tert-butoxycarbonyl) or Cbz/Z (benzyloxycarbonyl) carbamate protection conditions: the β-hydroxyl group attacks the carbonyl associated with the protecting group, forming an oxazolidinone by-product.
② The β-hydroxyl group of internal Ser/Thr in the peptide chain attacks a neighboring peptide-bond carbonyl group, forming a five-membered ring intermediate. This may then undergo N→O acyl migration, dehydration, or oxidation, generating peptide ester isomers, oxazoline, or oxazole by-products.
Schematic illustration of Ser/Thr-induced cyclization and oxazole-type by-product formation
N-terminal Ser/Thr + carbamate protecting group
│ Intramolecular attack by β-hydroxyl group
↓
Oxazolidinone by-product
Internal Ser/Thr + neighboring peptide-bond carbonyl
│ Intramolecular attack by β-hydroxyl group
↓
Five-membered ring intermediate
├─ N→O acyl migration → peptide ester isomer
└─ dehydration/oxidation → oxazoline/oxazole by-products
5 From Side Reactions to Process Judgment: How to Identify Risks in Advance
The key to controlling carboxyl and hydroxyl side reactions is to identify triggering conditions before route design. For peptides containing Asp/Glu/Ser/Thr, risks can be evaluated in the following order.
5.1 First Look at the Sequence: Which Residues May Become Risk Points?
If the sequence contains Asp/Glu, attention should be paid to carboxyl esterification, transesterification, and methyl esterification risks in acidic alcohol systems. If the number of Asp/Glu residues is high, the C-terminus is a free carboxyl group, or the target peptide requires prolonged acidic purification and concentration afterward, the risk increases further.
If the sequence contains Ser/Thr, attention should be paid to hydroxyl acylation, O→N migration, β-elimination, cyclization, and cleavage risks. If Ser/Thr is located at the N-terminus, adjacent to a slow-reacting residue, or associated with special protecting groups, basic treatment, or activating reagents, the risk is higher.
5.2 Then Look at the Steps: Which Operations Open Undesired Pathways?
Carboxyl side reactions often occur in the following steps:
Resin loading/fixation → washing → cleavage/deprotection → reversed-phase purification → concentration/drying
Among these, methanol washing, acidic methanol elution, concentration of collected fractions, and heated drying are key steps that should be investigated for carboxyl methyl esterification.
Hydroxyl side reactions often occur in the following steps:
Side-chain protection/deprotection → coupling → Fmoc deprotection → basic treatment → post-treatment
Among these, excess acylating reagent, overly long coupling time, Pd(0) deprotection, relatively strong basic treatment, and hydroxyl activation are key risk sources for Ser/Thr side reactions.
5.3 Finally Look at Detection: Abnormal Peaks Should Be Interpreted Mechanistically
When abnormal peaks appear in liquid chromatography, they should not simply be treated as “more impurities.” Instead, the source should be inferred by combining mass differences, sequence residues, and process steps.
Observation | Possible Direction | Key Points to Investigate |
+14 Da related impurity appears | Methyl esterification risk | Was methanol used? Was the system acidic? Was concentration performed with heating? |
Abnormal acylation peaks appear in a Ser/Thr-containing sequence | Hydroxyl acylation or migration | Were acylating reagents used in excess? Was amino-group reaction slow? |
Product amount decreases and dehydration-related or deprotection-abnormal signals appear | β-Elimination, cyclization, or acyl migration | Are activated hydroxyl groups, strong base, high temperature, or prolonged treatment present? |
By-products are obvious in an N-terminal Ser/Thr sequence | Oxazolidinone risk | Do the N-terminal protecting group and basic treatment conditions match? |
Functional groups are not fully restored after allyl-type deprotection | Allyl residue or allyl-transfer by-product | Are the Pd(0) deprotection system, scavenger, and reaction completeness appropriate? |
Isomeric peaks or multiple difficult-to-separate peaks appear in an Asp-containing sequence | Aspartimide formation and ring-opening isomerization risk | Was prolonged Fmoc deprotection performed? Is Asp followed by a small, low-steric-hindrance residue? Was relatively strong basic treatment used? |
6 Representative Chemical Product Classification Tables Related to the Control of Carboxyl and Hydroxyl Side Reactions in Peptide Synthesis
Table 1 Products Related to Acidic Cleavage, Deprotection, and Cleavage Scavenging
Category | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Mild acidic conditioning reagent | 64-19-7 | Glacial acetic acid | GR, ≥99.5% | Used for acidic treatment of peptides, adjustment of system acidity, and screening of mild post-treatment conditions | |
Strong acid cleavage reagent | 7664-39-3 | H116232 | Hydrofluoric acid | GR, ≥40% | Used for hydrofluoric-acid-related strong-acid treatment and mechanistic studies of traditional Boc/Bzl deprotection; associated with removal of benzyl ester protecting groups from aspartic acid and glutamic acid. Traditional HF cleavage requires differentiation between anhydrous HF and aqueous hydrofluoric acid systems. |
Acidic cleavage and deprotection reagent | 76-05-1 | Trifluoroacetic acid solution (TFA) | For protein sequencing, 25% solution in water | Used for acidic treatment, screening of mild deprotection conditions, and analytical/sample pretreatment studies; when coexisting with methanol and exposed to prolonged residence time or concentration with heating, the risk of carboxyl methyl esterification should be evaluated | |
Acidic mobile-phase modifier | 64-18-6 | Formic acid (FA) | Pharmaceutical grade, PharmPure™, ≥98% | Used as an acidic mobile-phase modifier in peptide analysis and purification; associated with evaluation of carboxyl esterification risk in acidic alcohol systems | |
Strong acid cleavage scavenger | 100-66-3 | Anisole | Anhydrous grade, ≥99.7% | Used to capture reactive cations in strong acid cleavage systems and reduce secondary modifications during protecting-group removal | |
Strong acid deprotection reagent | 1493-13-6 | Trifluoromethanesulfonic acid (TFMSA) | ≥99.5% | Used for strong acid deprotection and benzyl protecting-group cleavage studies; associated with screening of carboxyl protecting-group removal conditions | |
Strong Lewis acid deprotection reagent | 27607-77-8 | Trimethylsilyl trifluoromethanesulfonate (TMSOTf) | ≥99% | Used for removal of special protecting groups and strongly acidic silylation treatment; associated with studies on deprotection conditions for sensitive side chains | |
Thioether-type cleavage scavenger | 100-68-5 | Methyl phenyl sulfide | ≥99% | Used to capture electrophilic intermediates in strong acid cleavage systems and reduce side reactions related to aromatic side chains and protecting-group cleavage | |
Silane-type cleavage scavenger | 6485-79-6 | Triisopropylsilane (TIPS) | ≥98.5% | Used for cation scavenging in acidic cleavage systems and to reduce non-target modifications of peptide side chains during deprotection | |
Thiol-type cleavage scavenger | 540-63-6 | 1,2-Ethanedithiol | ≥97% | Used to capture highly electrophilic intermediates in acidic cleavage systems; suitable for deprotection studies of peptides containing sensitive side chains |
Table 2 Reaction Solvents, Purification Solvents, and Resin-Processing Media
Category | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Halogenated reaction solvent | 67-66-3 | C1506275 | Chloroform (regulated precursor chemical) | AR, ≥99.0% | Used in hydrophobic reaction systems and development of palladium-catalyzed deprotection conditions; associated with studies on allyl protecting-group removal |
Alcohol purification solvent | 67-56-1 | Methanol | For protein sequencing, ≥99.9% | Used for reversed-phase purification and washing of peptides; under acidic conditions, it may participate in methyl esterification of aspartic acid and glutamic acid side-chain carboxyl groups | |
Reversed-phase purification solvent | 75-05-8 | Acetonitrile (ACN) | Anhydrous grade, ≥99.8%, H2O ≤0.001% | Used for reversed-phase analysis and preparative purification of peptides; can be used to reduce the risk of carboxyl esterification caused by acidic methanol systems | |
Resin swelling and washing solvent | 75-09-2 | D433565 | Dichloromethane | Anhydrous grade, ≥99.8%, contains 40–150 ppm pentene as stabilizer | Used for resin swelling, washing, and protecting-group treatment; associated with resin-phase reaction efficiency in solid-phase peptide synthesis |
Amide-type solid-phase synthesis solvent | 68-12-2 | N,N-Dimethylformamide (DMF) | Anhydrous grade, ≥99.8% | Used for solid-phase peptide coupling, deprotection, and resin swelling; associated with peptide-bond formation efficiency after carboxyl activation | |
Lactam-type solid-phase synthesis solvent | 872-50-4 | N-Methylpyrrolidone (NMP) | Anhydrous grade, ≥99.5% | Used for resin swelling, coupling reactions, and organic base systems; suitable for screening synthesis conditions for difficult sequences |
Table 3 Products Related to Basic Treatment and Orthogonal Deprotection
Category | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Tertiary amine coupling base | 109-02-4 | N-Methylmorpholine | For protein sequencing, ≥99.8% (GC) | Used for acid capture and acid–base adjustment in peptide coupling; associated with control of condensation efficiency after carboxyl activation | |
Fmoc deprotection base | 110-89-4 | P1506348 | Piperidine (regulated precursor chemical) | Suitable for peptide synthesis, 20% in DMF | Used for removal of the Fmoc protecting group; for sequences containing serine or threonine, treatment time should be controlled to reduce the risk of β-elimination and acyl migration |
Hindered tertiary amine coupling base | 7087-68-5 | N-Ethyldiisopropylamine solution | Suitable for peptide synthesis, ~2 M in 1-methyl-2-pyrrolidinone | Used for acid capture in uronium and phosphonium salt coupling systems, regulating carboxyl activation and peptide-bond formation | |
Palladium-catalyzed orthogonal deprotection reagent | 14221-01-3 | Tetrakis(triphenylphosphine)palladium(0) | Pd ≥8.9% | Used for removal of allyl and allyloxycarbonyl protecting groups; associated with orthogonal protection strategies for serine hydroxyl groups | |
Palladium-catalyzed deprotection ligand | 603-35-0 | Triphenylphosphine | ≥99% (GC) | Used in palladium-catalyzed deprotection and phosphine ligand systems; associated with regulation of allyl protecting-group removal conditions | |
Allyl scavenger | 769-42-6 | 1,3-Dimethylbarbituric acid | ≥99% | Used to capture allyl intermediates during palladium-catalyzed deprotection and reduce hydroxyl allylation by-products | |
Strong basic treatment reagent | 6674-22-2 | 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) | ≥99% | Used in strong basic deprotection and reaction-promoting systems; associated with studies on β-elimination and acyl migration risks of serine and threonine | |
Palladium-catalyzed deprotection auxiliary reagent | 694-53-1 | Phenylsilane | ≥97% (GC) | Used in reduction and scavenging systems during palladium-catalyzed deprotection; associated with allyl protecting-group removal reactions |
Table 4 Solid-Phase Peptide Synthesis Resins
Category | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Acid-sensitive chlorotrityl resin | 42074-68-0 | Polymer-bound 2-chlorotrityl chloride | 200–400 mesh, labeling range: 1.0–1.5 mmol/g Cl loading, 1% cross-linked | Used for preparation of protected peptide fragments and synthesis of carboxylic-acid-terminated peptides; associated with mild cleavage and side-reaction control for sensitive sequences | |
Carboxylic-acid-terminated peptide resin | 201058-08-4 | Wang resin | 100–200 mesh | Used for synthesis of carboxylic-acid-terminated peptides; associated with acidic cleavage, side-chain deprotection, and control of C-terminal carboxyl structures | |
Acid-sensitive chlorotrityl resin | 934816-82-7 | 2-Chlorotrityl resin | 100–200 mesh, 1% DVB, 0.4–3.0 mmol/g | Used for release of protected peptide fragments and synthesis of acid-sensitive sequences; associated with mild treatment of aspartic acid, glutamic acid, serine, and threonine residues | |
Amide-terminated peptide resin | 183599-10-2 | Rink Amide-AM resin | 0.3–0.8 mmol/g, 100–200 mesh, 1% DVB | Used for synthesis of amide-terminated peptides; associated with solid-phase cleavage, side-chain deprotection, and control of terminal amide structures |
Table 5 Products Related to Carboxyl Activation, Coupling, and Hydroxyl Activation
Category | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Carbodiimide-type carboxyl activation reagent | 538-75-0 | N,N′-Dicyclohexylcarbodiimide (DCC) | ≥99% | Used for carboxyl activation and peptide-bond formation; hydroxyl competitive acylation should be considered for sequences containing serine or threonine | |
Carbodiimide-type carboxyl activation reagent | 693-13-0 | N,N′-Diisopropylcarbodiimide (DIC) | ≥98.5% | Used for solid-phase peptide coupling and carboxyl activation; associated with evaluation of hydroxyl activation, acyl migration, and β-elimination risks | |
Coupling activation additive | 39968-33-7 | 1-Hydroxy-7-azabenzotriazole (HOAt) | ≥99% | Used to improve coupling efficiency and control racemization; suitable for controlling hydroxyl competitive acylation risk in difficult couplings | |
Coupling activation additive | 2592-95-2 | 1-Hydroxybenzotriazole (HOBt) | ≥99% | Used for formation of active esters in carbodiimide coupling systems; associated with peptide-bond formation efficiency and racemization control | |
Carbonylation and hydroxyl activation reagent | 530-62-1 | N,N′-Carbonyldiimidazole (CDI) | ≥99% | Used for carbonylation and hydroxyl activation studies; associated with β-hydroxyl activation, carbonate formation, and subsequent migration/elimination risk evaluation for serine and threonine | |
Uronium salt coupling reagent | 148893-10-1 | O-(7-Azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) | ≥99% | Used for efficient carboxyl activation and difficult peptide-bond formation, reducing the opportunity for hydroxyl competitive acylation caused by insufficient coupling | |
Uronium salt coupling reagent | 94790-37-1 | Benzotriazol-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HBTU) | ≥99% | Used for routine solid-phase peptide coupling; associated with carboxyl activation, peptide-bond formation, and racemization control | |
Water-soluble carbodiimide-type carboxyl activation reagent | 25952-53-8 | 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride | ≥98% | Used for solution-phase carboxyl activation, coupling, and crosslinking reactions; suitable for studies on carboxyl reactivity | |
Phosphonium salt coupling reagent | 128625-52-5 | Benzotriazol-1-yloxytripyrrolidinophosphonium hexafluorophosphate | ≥98% | Used for peptide-bond formation, fragment condensation, and difficult coupling; associated with carboxyl activation efficiency and side-reaction control | |
Oxime-type coupling additive | 3849-21-6 | Ethyl 2-cyano-2-(hydroxyimino)acetate | ≥98% | Used to form active esters in coupling systems; associated with racemization control and peptide-bond formation efficiency | |
Carbonate-forming and hydroxyl activation reagent | 74124-79-1 | N,N′-Disuccinimidyl carbonate (DSC) | ≥98% | Used for hydroxyl activation and carbonate formation studies; associated with β-hydroxyl activation, carbonate formation, and subsequent migration/elimination risk evaluation for serine and threonine | |
Uronium salt coupling reagent | 125700-67-6 | O-Benzotriazol-N,N,N′,N′-tetramethyluronium tetrafluoroborate | ≥98% | Used for carboxyl activation and solid-phase peptide coupling; associated with coupling efficiency, racemization control, and side-reaction screening |
Table 6 Basic Amino Acids and Protected Amino Acid Raw Materials
Category | CAS No. | Aladdin Catalog No. | Name | Specification or Purity | Product Features and Applications |
Basic amino acid with hydroxyl residue | 56-45-1 | L-Serine | Animal-free, USP, Moligand™, European Pharmacopoeia (Ph. Eur.), ≥98.5% | Used as a raw material and reference for hydroxyl-residue-containing peptides; associated with analysis of hydroxyl acylation, acyl migration, cyclization, and β-elimination under special conditions | |
Basic amino acid with carboxyl residue | 56-86-0 | L-Glutamic acid | Ultrapure grade, ≥99.5% (NT) | Used as a raw material and reference for side-chain-carboxyl-containing peptides; associated with identification of carboxyl esterification, transesterification, and methyl esterification impurities | |
Basic amino acid with carboxyl residue | 56-84-8 | L-Aspartic acid | UltraBio™, ultrapure grade, ≥99.5% (T) | Used as a raw material and reference for side-chain-carboxyl-containing peptides; associated with analysis of aspartic acid side-chain esterification and carboxyl side reactions | |
Basic amino acid with hydroxyl residue | 72-19-5 | L-Threonine | UltraBio™, ultrapure grade, ≥99.5% (NT) | Used as a raw material and reference for β-hydroxyl-residue-containing peptides; associated with analysis of hydroxyl acylation, acyl migration, cyclization, and β-elimination under special conditions | |
Boc-protected threonine tert-butyl amino acid | 13734-40-2 | Boc-O-tert-butyl-L-threonine | ≥99% | Used for introduction of tert-butyl-protected threonine; associated with hydroxyl protection, acidic deprotection, and control of β-hydroxyl side reactions | |
Fmoc-protected aspartic acid benzyl ester amino acid | 86060-84-6 | Fmoc-L-aspartic acid β-benzyl ester | ≥99% | Used for introduction of benzyl ester-protected aspartic acid; associated with side-chain carboxyl protection, transesterification, and strong acid deprotection studies | |
Fmoc-protected glutamic acid benzyl ester amino acid | 123639-61-2 | Fmoc-L-glutamic acid γ-benzyl ester | ≥98% (HPLC) | Used for introduction of benzyl ester-protected glutamic acid; associated with side-chain carboxyl protection and strong acid deprotection systems | |
Boc-protected aspartic acid benzyl ester amino acid | 7536-58-5 | Boc-L-aspartic acid 4-benzyl ester | ≥98% | Used for introduction of benzyl ester-protected aspartic acid; associated with carboxyl protection, transesterification, and acid cleavage studies | |
Boc-protected threonine benzyl amino acid | 15260-10-3 | Boc-O-benzyl-L-threonine | ≥98% | Used for introduction of benzyl-protected threonine; associated with hydroxyl protection and strong acid deprotection condition studies | |
Fmoc-protected serine tert-butyl amino acid | 71989-33-8 | Fmoc-O-tert-butyl-L-serine | ≥98% | Used for introduction of tert-butyl-protected serine; associated with hydroxyl acylation, cyclization, and acidic deprotection risk control | |
Fmoc-protected aspartic acid tert-butyl ester amino acid | 71989-14-5 | Fmoc-L-aspartic acid β-tert-butyl ester | ≥98% | Used for introduction of tert-butyl ester-protected aspartic acid; associated with side-chain carboxyl protection and methyl esterification risk identification | |
Fmoc-protected threonine trityl amino acid | 133180-01-5 | Fmoc-O-trityl-L-threonine | ≥98% | Used for introduction of trityl-protected threonine; associated with selective hydroxyl protection and synthesis of sensitive peptide fragments | |
Fmoc-protected threonine tert-butyl amino acid | 71989-35-0 | Fmoc-O-tert-butyl-L-threonine | ≥98% | Used for introduction of tert-butyl-protected threonine; associated with β-hydroxyl protection, acidic deprotection, and side-reaction control | |
Fmoc-protected glutamic acid tert-butyl ester amino acid | 71989-18-9 | Fmoc-O-tert-butyl-L-glutamic acid | ≥98% | Used for introduction of tert-butyl ester-protected glutamic acid; associated with side-chain carboxyl protection and esterification risk control | |
Fmoc-protected serine benzyl amino acid | 83792-48-7 | Fmoc-O-benzyl-L-serine | ≥98% | Used for introduction of benzyl-protected serine; associated with hydroxyl protection and strong acid deprotection condition studies | |
Boc-protected serine tert-butyl amino acid | 13734-38-8 | N-Boc-O-tert-butyl-L-serine | ≥98% | Used for introduction of tert-butyl-protected serine; associated with hydroxyl protection and acidic deprotection systems | |
Boc-protected serine benzyl amino acid | 23680-31-1 | N-Boc-O-benzyl-L-serine | ≥98% | Used for introduction of benzyl-protected serine; associated with traditional protection strategies and hydroxyl side-reaction studies | |
Fmoc-protected threonine benzyl amino acid | 117872-75-0 | Fmoc-O-benzyl-L-threonine | ≥98% | Used for introduction of benzyl-protected threonine; associated with hydroxyl protection, acid cleavage, and β-hydroxyl side-reaction studies | |
Fmoc-protected serine allyl amino acid | 704910-17-8 | (2S)-3-Allyloxy-2-(9H-fluoren-9-ylmethoxycarbonylamino)propanoic acid | ≥97% | Used for introduction of allyl-protected serine; associated with palladium-catalyzed orthogonal deprotection and hydroxyl allylation by-product studies | |
Fmoc-protected serine trityl amino acid | 111061-56-4 | Fmoc-O-trityl-L-serine | ≥97% | Used for introduction of trityl-protected serine; associated with mild deprotection and protection strategies for sensitive hydroxyl residues | |
Boc-protected glutamic acid benzyl ester amino acid | 13574-13-5 | N-Boc-L-glutamic acid 5-benzyl ester | ≥97% | Used for introduction of benzyl ester-protected glutamic acid; associated with carboxyl protection, transesterification, and strong acid cleavage systems |
Note: The products listed above are representative Aladdin products related to scientific research and formulation studies. For more product specifications, grades, and COA information, please search by “product name/CAS/catalog number” on the Aladdin official website.
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