Basic principles of peptide bond formation in customized peptide synthesis

Basic principles of peptide bond formation in customized peptide synthesis

Customized peptide synthesis

The formation of a peptide bond (producing a dipeptide) appears to be a simple chemical process, which refers to the connection of two amino acid components through a peptide bond (amide bond) while removing water

Under mild reaction conditions, the formation of peptide bonds is achieved by activating the carboxyl portion of one amino acid (A), while the second amino acid (B) nucleophilically attacks the activated carboxyl portion to form dipeptides (A-B). If the amino group of carboxyl component (A) is not protected, the formation of peptide bonds is uncontrollable and may result in byproducts such as linear and cyclic peptides, mixed with the target compound A-B. Therefore, in the process of peptide synthesis, all functional groups that do not participate in peptide bond formation must be protected in a temporarily reversible manner.

Therefore, peptide synthesis - the formation of each peptide bond - involves three steps of polymerization.

The first step is to prepare partially protected amino acids, where the zwitterionic structure of amino acids no longer exists;

In the second step of the two-step reaction to form peptide bonds, the carboxyl group of the N-protected amino acid must first be activated into the active intermediate, followed by the formation of peptide bonds. This coupling reaction can be carried out as either a one-step reaction or as two consecutive reactions.

The third step is to selectively or completely remove the protective group. Although complete removal can only be carried out after the peptide chain is fully assembled, in order to proceed??? Selective removal of protective groups is also necessary for peptide synthesis.

Due to the presence of side chain functional groups that require selective protection in 10 amino acids (Ser, Thr, Tyr, Asp, Glu, Lys, Arg, His, Sec, and Cys), peptide synthesis becomes more complex. Due to different requirements for selectivity, it is necessary to distinguish between temporary and semi permanent protective groups. Temporary protective groups are used to temporarily protect the amino or carboxyl functional groups of the amino acids to be reacted in the next step. They are removed only when they do not interfere with the semi permanent protective groups of the formed peptide bonds or amino acid side chains, and sometimes also during the synthesis process.

In an ideal state, the activation of carboxyl components and subsequent peptide bond formation (coupling reaction) should be a rapid reaction without racemization or byproduct formation, and equimolar reactants should be applied to achieve high yields. Unfortunately, there is currently no chemical coupling method that can meet these requirements, and there are few methods suitable for practical synthesis.

In the process of peptide synthesis, functional groups involved in various reactions are often linked to a chiral center (glycine is the only exception), which poses a potential risk of racemization.