Synthesis Resin
A synthesis resin is the insoluble polymer bead that anchors a growing peptide during solid-phase synthesis, its linker chemistry determining the C-terminal group of the finished chain.
The resin is the insoluble support a peptide is built on. The bead itself is usually polystyrene lightly crosslinked with divinylbenzene, or a polyethylene-glycol-based matrix chosen for better solvation of difficult sequences. Attached to it is a linker, and the linker dictates what the C-terminus becomes on cleavage: Wang resin releases a free carboxylic acid, Rink amide releases a C-terminal amide, and 2-chlorotrityl releases a still-protected fragment under mild acid.
Loading, quoted in millimoles of attachment site per gram, is the other design variable and typically sits between roughly 0.2 and 1.0. High loading crowds neighbouring chains, encourages on-resin aggregation and secondary structure, and raises deletion rates on long or hydrophobic sequences; low loading gives cleaner crude material at the cost of throughput. The bead must also swell properly in the reaction solvent, because reagents reach most chains inside the polymer rather than on its surface.
Linker choice is a structural decision with pharmacological consequences, not process bookkeeping. Many bioactive peptides require a C-terminal amide for receptor recognition, and building the same residue sequence on an acid-releasing resin yields a different molecule that may be substantially less active. Fragment-condensation routes depend on trityl-type linkers for the opposite reason, because the protected fragment has to come off intact.
Where this bites is in reading specifications. A sequence written without indicating C-terminal amidation is ambiguous, and the resin named on a synthesis record is what resolves it. Resin-derived impurities are real too: linker fragments and scavenger adducts appear in crude material and are removed by purification rather than by never forming, so a certificate with no impurity profile leaves them unaccounted for.