Nearly every research peptide under about 50 residues is made by Fmoc solid-phase peptide synthesis, in which the chain is built one residue at a time on an insoluble resin and released only when complete. The method decides which impurities a peptide can contain and why it arrives as a lyophilised trifluoroacetate salt. The chemistry below applies to the synthetic research peptides NuVion Health supplies as laboratory chemicals for in vitro research use only.
The solid-phase principle
Merrifield’s 1963 idea was to anchor the C-terminal amino acid covalently to a polymer bead so that every later reagent could be used in excess and washed away by filtration. The chain stays on the resin until the final cleavage. Because the anchor is through the carboxyl group, the chain grows from the C-terminus towards the N-terminus, the reverse of ribosomal synthesis. The resin is usually polystyrene cross-linked with about 1% divinylbenzene, or a polyethylene glycol-grafted equivalent. Its linker sets the C-terminal chemistry: a Wang linker releases a free carboxylic acid and a Rink amide linker releases a C-terminal amide.
Protecting groups and orthogonality
Every amino acid carries an amine and a carboxyl group, and many a reactive side chain, so unprotected residues would couple in every direction. The Fmoc strategy uses two classes of protection removed by different chemistry. The alpha-amine carries the base-labile 9-fluorenylmethoxycarbonyl (Fmoc) group, removed with 20% piperidine in dimethylformamide at each step. Side chains carry acid-labile groups that survive piperidine and come off only at the end in trifluoroacetic acid: tert-butyl on serine, threonine, tyrosine, aspartate and glutamate, Boc on lysine, trityl on asparagine, glutamine, histidine and cysteine, and Pbf on arginine. The two sets are orthogonal: base and acid do not interfere. Where one side chain must be unmasked on the resin, for lipidation of a lysine or a lactam bridge, a third orthogonal group such as Mtt, ivDde or Alloc is used.
The coupling step
Each round adds one residue in two reactions. Fmoc removal exposes the free amine of the growing chain. The incoming Fmoc-amino acid, added in three to five-fold excess, is activated with a coupling reagent, either a carbodiimide such as DIC with OxymaPure or an aminium salt such as HBTU or HATU with a tertiary base. Activation converts the carboxylic acid into an active ester, which the amine attacks to form the new peptide bond. A ninhydrin test on a few beads shows whether free amines remain; if so, the coupling is repeated or the unreacted chains are capped with acetic anhydride so they become easily separated truncated sequences.
Yield per step is the critical variable. At 99% completion per coupling, a 30-residue peptide reaches about 74% of the theoretical crude yield; at 98% it falls to about 55%. Sequences that aggregate on the resin through interchain hydrogen bonding are the usual cause of failed couplings, and the countermeasures are elevated temperature and pseudoproline dipeptides that break the hydrogen bonding pattern.
Side reactions and where impurities come from
Epimerisation at the alpha-carbon during activation, through an oxazolone intermediate, gives diastereomers; cysteine and histidine are the most prone, and additives such as OxymaPure suppress it. Aspartimide formation at Asp-Gly, Asp-Asn and Asp-Ser motifs occurs during repeated piperidine exposure and gives a mixture of alpha- and beta-linked products. Incomplete Fmoc removal or a failed coupling leaves a deletion sequence, the hardest impurity to separate because it differs from the product by a single residue.
Cleavage, purification and lyophilisation
When the sequence is complete the resin is washed and exposed to a cleavage mixture, usually 95% trifluoroacetic acid with triisopropylsilane and water as scavengers, which releases the peptide and strips the side-chain protecting groups in one step. The scavengers trap the carbocations that would otherwise alkylate tryptophan, methionine or cysteine. The crude peptide is precipitated in cold diethyl ether and purified by preparative reversed-phase HPLC on a C18 column with a water-acetonitrile gradient containing 0.1% trifluoroacetic acid. Fractions are checked by analytical HPLC, pooled and freeze-dried. The product is therefore a trifluoroacetate salt: every basic site carries a trifluoroacetate counter-ion, which adds to the mass in the vial.
Modifications built in during synthesis
Most catalogue peptides carry a deliberate modification introduced on the resin. A Rink amide linker gives the C-terminal amide of sermorelin. Acetic anhydride caps the N-terminus of thymosin alpha-1. D-amino acids, norleucine and 2-aminoisobutyric acid are added as ordinary Fmoc building blocks. The lipid chain of tirzepatide is built on a lysine side chain after selective removal of its orthogonal protecting group, by coupling a gamma-glutamate, two short PEG spacers and a C20 diacid. Lactam bridges are closed on the resin between an aspartate and a lysine bearing allyl-based protection, and disulfides, as in oxytocin, are formed after cleavage by oxidation in dilute solution. Chains beyond about 50 residues are made by native chemical ligation of synthetic fragments or by recombinant expression, which is how IGF-1 LR3 is produced.
Testing and supply from NuVion
The product is characterised by analytical RP-HPLC for purity and mass spectrometry for identity. NuVion Health has its peptides tested independently by Janoshik Analytical; most batches are tested, and the Certificate of Analysis for a tested batch is published on the product page and in the certificate library. Manufacture is GMP-audited, and the peptides are supplied lyophilised in sealed vials and dispatched from within Australia. The powder is reconstituted with bacteriostatic water, using the reconstitution calculator for concentration, and kept refrigerated per the product documentation.
Frequently asked questions
Why does solid-phase synthesis build the chain from the C-terminus?
The first residue is anchored through its carboxyl group, leaving the amine free to accept the next activated amino acid. The ribosome works the other way, adding residues to the C-terminus.
What limits the length of a synthetic peptide?
Cumulative yield. Even at 99% per coupling the crude yield of a 50-residue peptide is around 60%, and accumulated deletion sequences become harder to separate. Beyond that length, fragment ligation or recombinant expression is used.
Why is the peptide supplied as a trifluoroacetate salt?
Trifluoroacetic acid is used both for cleavage from the resin and as the ion-pairing agent in preparative HPLC, so the freeze-dried product carries trifluoroacetate on every protonated amine unless a salt exchange is carried out.
Browse the NuVion research peptide catalogue.
View productsResearch use only. This product is a laboratory chemical supplied for in vitro research. It is not included in the Australian Register of Therapeutic Goods and has not been assessed by the Therapeutic Goods Administration for quality, safety or efficacy. It is not for human or veterinary use, and nothing on this page is a representation about therapeutic use.
