peptides are FAQ

Use orthogonal protecting groups on each cysteine pair. Trt (triphenylmethyl) for one pair, Acm (acetamidomethyl) for another. So disulfide bonds form sequentially rather than randomly. After cleavage from the resin, oxidize the Trt-protected pair first using iodine in methanol, then remove Acm groups with iodine in TFA to form the second disulfide. Random oxidation yields 15 possible disulfide isomers for a four-cysteine peptide; only one is biologically active. Controlled stepwise oxidation guarantees correct pairing, which is why peptides like Thymalin with native disulfide bridges require method development beyond standard SPPS protocols.

Use orthogonal protecting groups (Acm, Trt) on cysteine side chains to control which cysteines form disulfide bonds during oxidative folding. Cysteines form disulfide bridges spontaneously in aqueous solution under oxidising conditions, and without selective protection, you'll get scrambled disulfides. Misfolded peptides with no biological activity. Controlled disulfide formation requires sequential deprotection: remove Trt groups first with mild acid, allow those cysteines to oxidise and form their intended bridge, then remove Acm groups with iodine to form the second bridge. This is why cyclic peptides and toxin-derived sequences often require custom synthesis protocols. One-size-fits-all SPPS doesn't handle complex disulfide topologies reliably.

The peptide likely contains enantiomeric impurities (D-amino acids instead of L-), aggregates masking the active site, or misfolded structure despite correct composition. Run circular dichroism (CD) spectroscopy to confirm secondary structure. Alpha-helix, beta-sheet, or random coil. Matches the expected fold. Re-run HPLC with a chiral column to detect D-amino acid contamination, which standard reverse-phase HPLC cannot resolve. Aggregation shows up as high-molecular-weight shoulders on size-exclusion chromatography (SEC); disaggregation protocols using 10–20% DMSO or gentle heating (37°C, 10 minutes) before reconstitution often restore activity.

Switch from sterile water to bacteriostatic water containing 0.9% benzyl alcohol, which inhibits bacterial growth, or add 10mM acetic acid to drop pH to 4–5, slowing hydrolysis of ester bonds and reducing aggregation of hydrophobic sequences. Store reconstituted peptides at 2–8°C and aliquot into single-use vials to avoid repeated freeze-thaw cycles, which denature peptides through ice crystal formation. For peptides prone to oxidation like Selank Amidate Peptide, reconstitute under argon or nitrogen atmosphere and store in amber glass vials to block UV-catalyzed degradation. Some peptides remain stable for 28 days refrigerated; others degrade 15–20% within 72 hours. Degradation kinetics are sequence-specific, not universal.

Run the sample on a different HPLC column chemistry (phenyl-hexyl instead of C18) or use mass spectrometry to identify each peak. Close-eluting peaks often represent diastereomers (sequences with a single D-amino acid instead of L), deletion sequences missing one residue, or peptides with incomplete side-chain deprotection. C18 columns separate based on hydrophobicity, but switching to phenyl-hexyl or C4 columns changes selectivity and may resolve peaks that co-elute on C18. Mass spec identifies each peak by molecular weight. A deletion sequence will be 50–150 Da lighter than the target, while a diastereomer has identical mass but different retention time. If MS confirms the major peak is correct mass and the minor peaks are impurities, you're fine. If MS shows the target peak is a minor component, the batch failed synthesis and should be rejected.

Extend coupling times from 30 minutes to 2 hours and double the molar excess of incoming amino acid from 3× to 6×, especially for sterically hindered sequences where the growing chain's C-terminus is blocked by bulky side chains. Pre-activate the amino acid with coupling reagents for 2–5 minutes before adding to the resin to ensure full conversion to the reactive ester. For particularly difficult couplings. Proline following a bulky residue, or consecutive valines. Use microwave-assisted SPPS, which heats the reaction vessel to 50–75°C in controlled pulses, increasing coupling efficiency to 99.5%+ in sequences where conventional room-temperature SPPS stalls at 95%. Low crude purity is rarely random; it signals predictable coupling failures that method optimization corrects.

Switch to a more polar resin or incorporate pseudoproline dipeptides at aggregation-prone positions to disrupt chain aggregation. Hydrophobic peptides aggregate on the resin during synthesis, forming β-sheet structures that prevent incoming amino acids from accessing the growing chain terminus. This causes coupling failures that don't show up until purification, when you discover your target peptide is a minor component in a mixture of truncated sequences. Pseudoprolines are temporary dipeptide replacements (serine-serine or threonine-threonine) that introduce kinks in the chain, preventing aggregation during synthesis. After cleavage, the pseudoproline reverts to the native sequence. You get the correct peptide without the aggregation problem.