Contemporary methods for peptide and protein synthesis

This review describes current methods for peptide and protein syntheses, largely from a strategic point of view. The solid-phase method is useful for the rapid preparation of peptides. Two major synthetic strategies have been adopted by this method, namely, the Boc and Fmoc strategies. At the final stage of the Boc solid-phase method, a protected peptide resin is treated with a strong acid to obtain a free peptide. On the other hand, in the Fmoc solid-phase method, a free peptide is obtained by treating a protected peptide resin with a weak acid. Both solid phase methods are quite useful for the preparation of peptides with molecular weights in the vicinity of five thousand. Ligation methods were developed to overcome the molecular weight barrier existing in a solid phase method. Building blocks used for ligation are prepared by the solid phase method, or more recently by biological methods. All the current ligation methods that produce a native peptide bond use peptide C-terminal thiocarboxylic acids or thioesters as building blocks. Blake et al. developed a selective activation method of the C-terminal carbonyl group by the combination of thiocarboxylic acid and silver ions. Based on this approach, a thioester method was developed, in which partially protected peptide thioesters are used as building blocks. Subsequently, a new ligation method was developed using peptide thioesters, in which protecting group is no longer necessary. The discovery of protein splicing phenomenon added a biological route to the preparation of peptide thioesters. A partially protected peptides segment can be also derived from an expressed peptide segment. Polypeptides with a molecular weight of more than 10 thousand can be routinely synthesized.