Methods for the chemical synthesis of peptides include solution-phase peptide synthesis and solid-phase peptide synthesis. Solution-phase peptide synthesis involves the coupling of amino acids in solution; it typically proceeds in a C-terminal to N-terminal direction and is suitable for the synthesis of short peptides (e.g., those containing fewer than 10 amino acids). Commonly employed protection strategies include the BOC (tert-butyloxycarbonyl) and Z (benzyloxycarbonyl) methods. This approach offers advantages such as lower cost and ease of scale-up, though it necessitates the purification of intermediate products. The primary coupling reagents utilized are of the carbodiimide type (e.g., DCC); however, these may induce racemization side reactions, which can be suppressed through the addition of additives such as HOBt.
The fundamental principle of solid-phase peptide synthesis involves immobilizing the C-terminal amino acid of the peptide chain onto a solid support (such as a resin). The peptide chain is then progressively elongated through a series of repetitive steps involving deprotection, activation, and coupling; the direction of synthesis generally proceeds from the C-terminus to the N-terminus. The principal protection strategies employed are the BOC (tert-butyloxycarbonyl) and Fmoc (9-fluorenylmethyloxycarbonyl) methods, with the Fmoc method having emerged as the dominant approach due to its mild reaction conditions. Commonly used coupling reagents include both carbodiimide-type reagents (e.g., DCC) and onium salts (e.g., HBTU). Following synthesis, the peptide must be cleaved-typically using reagents such as TFA or HF-to elute it from the resin support.
The selection of a peptide synthesis method is contingent upon the length of the peptide chain and the specific characteristics of its sequence: solid-phase synthesis offers high efficiency and is readily amenable to automation, making it well-suited for the synthesis of medium-length peptides (e.g., approximately 30 amino acids); conversely, solution-phase synthesis is better suited for the production of short peptides. For "difficult" peptides-such as sequences prone to aggregation or those containing specialized amino acids-alternative strategies, such as the pseudoproline method or *ortho*-hydroxybenzyl-type protection methods, may be employed.