Solid-phase peptide synthesis

Solid-phase peptide synthesis builds a chain one residue at a time on an insoluble resin, from the C terminus toward the N terminus. Each cycle deprotects the growing chain, couples the next protected amino acid, and washes the excess away. Because the product stays on the resin, purification between steps is a wash rather than a chromatography run, which is what makes chains of thirty or forty residues practical at all.

Fmoc is the default, and the reason matters

Two protecting group strategies exist. Boc chemistry uses acid for each deprotection and hydrogen fluoride for final cleavage, which needs specialized apparatus. Fmoc chemistry uses a mild base, usually piperidine, for deprotection and trifluoroacetic acid for cleavage. Fmoc is the standard for that reason, and it is what almost any supplier means when they quote a synthesis.

The tradeoff shows up with base-sensitive sequences. Repeated piperidine treatment drives aspartimide formation at aspartate residues, particularly in Asp-Gly and Asp-Asn motifs, which scrambles the backbone and produces a family of closely related impurities that are difficult to separate. Where the sequence contains those motifs, a backbone amide protecting group on the following residue suppresses it, and it is worth specifying up front rather than diagnosing later.

Which sequences fight back

Yield does not fall evenly across a sequence. Chains aggregate on the resin when the growing peptide forms beta sheet with its neighbors, which buries the reactive N terminus and stalls coupling. The symptom is a synthesis that runs cleanly for fifteen residues and then produces deletion sequences at every subsequent step.

The sequences that do this are predictable. Long runs of beta-branched residues, valine, isoleucine and threonine, are the usual culprits, along with polyalanine and hydrophobic stretches generally. Glutamine and asparagine rich regions do it as well.

Three interventions help. Lowering resin loading reduces how close the chains sit to each other and is the simplest fix. Pseudoproline dipeptides inserted at serine or threonine positions break the sheet and are the most effective single trick available. Elevated temperature, either microwave or simple heating, improves diffusion into the aggregated region.

Coupling reagents and the racemization question

The carbodiimide reagents with an additive, most often DIC with Oxyma, are cheap and suitable for routine positions. The uronium and phosphonium reagents, HATU, HBTU and PyBOP, couple faster and are worth reserving for hindered positions and for the residues after a difficult stretch.

Racemization is the reason the choice is not purely about speed. Cysteine and histidine epimerize readily under strongly basic activation, and the resulting D isomer is a stereochemical impurity with nearly identical mass and retention. Where cysteine appears, use a milder base and a lower temperature for that coupling. The mass spectrum will not tell you this happened, which is why the decision has to be made before the run rather than diagnosed afterward.

Cleavage and what comes off the resin

Final cleavage with trifluoroacetic acid removes the peptide from the resin and strips the side chain protecting groups at the same time. The protecting groups leave as reactive cations that will alkylate tryptophan, methionine, tyrosine and cysteine unless something else absorbs them. That is the entire job of a scavenger cocktail, and the composition follows the sequence: triisopropylsilane and water for most peptides, with a thiol scavenger added where cysteine or methionine are present.

Cleavage time is a balance. Too short leaves arginine protecting groups attached, and too long increases side reactions, so two to three hours is the usual window and longer runs need a reason.

Expect the crude material to be a mixture. A forty-residue peptide at ninety-nine percent efficiency per step is about sixty-seven percent pure before purification, and real efficiencies are lower. The synthesis decides what is in the crude; purification only decides what you keep.