The "quirks" of carboxyl and hydroxyl groups in peptide synthesis

In peptide synthesis, carboxyl and hydroxyl groups are two common functional groups. The carboxyl group is the basic structural unit for building peptide bonds, while the hydroxyl group, due to its nucleophilicity, frequently participates in side chain protection and deprotection reactions. Both play crucial roles in peptide synthesis. However, due to their high reactivity, they are prone to various unexpected side reactions during synthesis, directly affecting the purity and yield of the target product. Below, we summarize the common side reactions of these two groups to provide a reference for practical operations.

I. Carboxyl side reactions of aspartic acid (Asp)/glutamic acid (Glu)

Carboxyl side reactions mainly occur on the backbone and side chains of aspartic acid (Asp)/glutamic acid (Glu), and mainly include the following categories:

1. Alkali-catalyzed transesterification

In solid-phase synthesis, when using chloromethyl resin (Merrifield resin), amino acids are fixed on the resin as ester bonds via base-catalyzed substitution reactions. Commonly used bases include cesium salts, triethylamine, and tetramethylammonium hydroxide (TMAH), with TMAH showing superior catalytic performance. However, a problem arises: this process is accompanied by transesterification of the benzyl ester on the Asp/Glu side chain, as shown in Figure 1. In the methanol system, the benzyl ester is converted to a methyl ester, and in the subsequent HF-mediated deprotection step, it is difficult to achieve side chain deshielding.

Figure 1. Transesterification side reaction of Asp/Glu side chain

However, this side reaction, if utilized, can also be an effective synthetic strategy. For example, by treating peptide-Merrifield resin with TMAH/tert-butanol at 70°C, benzyl ester of the Asp/Glu side chain can be converted into tert-butyl ester, while simultaneously cleaving the peptide chain from the solid support to directly obtain the corresponding C-terminal tert-butyl ester.

2. Methionization side reaction during pyrolysis and purification process

During peptide resin cleavage or global deprotection of side chains, residual methanol (e.g., introduced during washing) can modify the carboxyl groups in the peptide under acidic conditions, initiating a methyl esterification side reaction. Therefore, for esterification-sensitive peptides, methanol washing of the resin should be avoided, or the drying process should be strictly controlled.

also, In reversed-phase liquid chromatography (RP-HPLC) purification, methanol is widely used as an organic eluent due to its cost advantage. However, methanol/water systems typically require the addition of acidic modifiers such as TFA or formic acid, resulting in an acidic peptide solution. During subsequent concentration, the carboxyl groups of the Asp/Glu side chains or the peptide backbone are highly susceptible to methyl esterification, affecting peptide quality. If the tendency for methyl esterification is severe, it is recommended to use acetonitrile as the organic eluent.

II. Hydroxyl side reactions of serine (Ser)/threonine ( Thr )

Serine (Ser)/threonine ( Thr ) are prone to various side reactions in peptide synthesis due to the nucleophilicity of their β -hydroxyl groups. Simultaneously, the acyl protecting group on the hydroxyl group can also undergo side reactions such as O→N migration and β -elimination under specific conditions.

1. Alkylation side reactions

The unique orthogonality of the alloc protecting group makes it commonly used for the selective protection of hydroxyl groups. However, when O- alloc- protected serine residues are removed under Pd(0) catalysis, the alloc- protected hydroxyl group may undergo an O-allylation side reaction, generating an O-allyl-protected serine byproduct (Figure 2). This byproduct is chemically stable to Pd(0) and the free β-hydroxyl group cannot be regenerated by extending the treatment time . However, this side reaction can be effectively controlled in the presence of an effective acrylic acid scavenger.

Figure 2. Alloc deprotection-induced allylation side reaction

2. Acylation and O→N migration side reactions

During amino acid coupling, the use of excessive acylation reagents or slow reaction of the target amino acid may reduce the nucleophilicity difference between the hydroxyl and amino groups, leading to acylation of the β-hydroxyl group in Ser/ Thre . Furthermore, during deprotection of the amino protecting group and subsequent alkali treatment, the acylated hydroxyl group undergoes acyl O→N migration, resulting in unexpected peptide chain termination (Figure 3 ).

Figure 3 Acyl O→N migration reaction

3. β-elimination side reactions

β-elimination is one of the common side reactions in Ser/ Thr . When electron-withdrawing groups (such as Ts, Ms , etc.) are attached to the hydroxyl groups on the side chains of Ser/ Thr , β- elimination reactions readily occur under basic conditions (Figure 4).

β- elimination reaction catalyzed by base

Even with unprotected side chains, Ser/ Thre does not completely prevent such reactions. β- elimination reactions can still occur under the influence of factors such as base concentration, temperature, reaction time, and peptide sequence . Furthermore, some coupling agents can also induce this side reaction; for example, DSC, CDI, and carbodiimide coupling agents can activate the β- hydroxyl groups of Ser/ Thre , thereby initiating β- elimination reactions (Figure 5).

Figure 5. CDI-induced β- elimination reaction

4. Formation of oxazole byproducts

The formation of oxazole byproducts mainly occurs in two ways: First, when the N-terminus is protected with a carbamate (Z, Boc, etc.), the unprotected Ser/ Thr side chain may attack the carbamate backbone under alkaline conditions , forming an oxazolidinone byproduct (Figure 6) . Therefore, protecting the β-hydroxyl group of the N-terminal Ser/ Thr during alkaline treatment of the peptide , or using milder conditions, can effectively avoid the occurrence of side reactions .

Figure 6. Mechanism of oxazolidinone formation

Secondly, the Ser/ Thre residues within the peptide chain can also attack neighboring peptide bonds via β-hydroxyl groups , generating a five-membered ring intermediate. This intermediate can be converted into an ester derivative via acyl N→O migration , or it can be converted into an oxazoline or even an oxazole product via dehydration and oxidation (Figure 7) . Similar processes are also observed with cysteine ( Cys ) and β-aminoalanine, generating corresponding thiazoline/thiazolium and imidazoline/imidazolium derivatives , respectively .

Figure 7. Mechanism of isomeric peptide ester/oxazolyl(line) formation

5. Reverse aldol condensation cleavage

the Ser/ Thre side chain forms a β-hydroxycarbonyl structure with the adjacent carbonyl group— essentially the structure of a post-aldol condensation product. Therefore, reverse aldol condensation cleavage can occur under both acidic and alkaline conditions. Under alkaline conditions, this process leads to the cleavage of the Cα-Cβ bond of the Ser/ Thre residues, generating formaldehyde/acetaldehyde and the corresponding ketone compounds ( Figure 8 ). Although such side reactions are uncommon, they should still be considered during peptide synthesis or other related processes .

Figure 8 Reverse aldol condensation cleavage

III . Summary

The above summarizes some of the more common and typical side reaction types . In actual synthesis, carboxyl and hydroxyl-related side reactions are diverse in form and complex in mechanism, but most can be avoided through reasonable selection of protecting groups, strict control of process parameters, and optimization of appropriate reaction conditions. A thorough understanding of the patterns of these side reactions is an important foundation for improving the quality and yield of peptide drugs .

Company Introduction

Suzhou Haofan Biotech Co., Ltd. (Stock Code: 301393.SZ), founded in 2003 and headquartered in Suzhou High-tech Zone, is a national high-tech enterprise providing specialty raw materials to pharmaceutical R&D and manufacturing companies worldwide. Its products are mainly used in the synthesis of peptides, nucleotides, and pharmaceuticals, covering a wide range of products including condensing agents for specialty amide bonds, protective agents, linking agents, protein cross-linking agents for antibody-drug conjugates, molecular building blocks, liposomes, and phosphorus reagents. To date, it has cumulatively developed and produced over 1,500 different products.

After more than two decades of unremitting efforts and accumulation, Haofan Biotech has continuously cultivated its expertise in the global peptide synthesis reagent field. It has now developed into a leading enterprise with extensive customized product coverage and significant advantages in large-scale production, capable of meeting the specific needs of various customers. We sincerely invite customers interested in this product to contact us to learn more about product details and explore cooperation opportunities.

References:

[1] Side Reactions in Peptide Synthesis. Yi Yang