Solid-phase peptide synthesis (revision 10)

Solid-phase peptide synthesis (SPPS) assembles a peptide on an insoluble polymeric support. Because the growing chain is anchored, excess reagents and soluble by-products are removed simply by washing, which is what makes stepwise synthesis of long sequences practical.^[1]

The cycle is: remove the temporary N-terminal protecting group, wash, couple the next protected amino acid in excess, wash. Excess reagent drives each coupling towards completion, and the washing that removes it is the operation solution-phase synthesis cannot perform cheaply.^[2]

Its principal limitation follows from the same design. Intermediates cannot be purified — everything anchored to the resin travels together — so a failed coupling generates a deletion sequence that persists to the end and must be separated at the purification stage.^[2]

The resin is typically cross-linked polystyrene or a polyethylene glycol–polystyrene composite, functionalised with a linker that attaches the first residue and defines how the finished peptide is released.^[2]

The linker determines the C-terminal functionality. Wang and related linkers give a free acid on cleavage; Rink amide linkers give a C-terminal amide. Since several therapeutic peptides are C-terminally amidated, linker choice is a specification-level decision rather than a convenience.

Resin loading — millimoles of site per gram — sets the scale and also affects chain crowding. High loading gives more material per batch but increases the chance of interchain aggregation on the support, which is the main cause of difficult couplings. See Peptide aggregation for the solution-phase analogue of the same phenomenon.^[1]