Semaglutide

Semaglutide is a synthetic 31-amino-acid GLP-1 receptor agonist. The backbone is derived from native GLP-1(7-37) with two engineered modifications:

• Position 2: α-aminoisobutyric acid (Aib) replaces alanine. This non-natural residue blocks dipeptidyl peptidase-4 (DPP-4) cleavage and dramatically extends in-vivo stability.
• Position 26 (lysine): conjugated through a γ-glutamic acid spacer to a C18 fatty diacid (octadecanedioic acid). The lipid promotes reversible albumin binding, which is what gives semaglutide its once-weekly pharmacokinetics.

Molecular formula C187H291N45O59; average molecular weight approximately 4113.58 Da. CAS 910463-68-2.

Semaglutide is built by SPPS, then site-selectively acylated at Lys26 with the activated γGlu-C18-diacid linker. Each step has a characteristic failure mode:

• Backbone deletions and truncations — 31 residues is long for SPPS, and even >99% per-cycle coupling efficiency accumulates a measurable deletion-sequence burden.
• Aib-position diastereomers — Aib is achiral, but adjacent residues can racemize during coupling.
• Mis-acylation — the lipid linker is intended for Lys26 only; competing acylation at Lys20 or the N-terminus produces positional isomers that share the same intact mass.
• Free linker — unconjugated C18 diacid from incomplete coupling.
• Oxidized methionine and Trp byproducts from storage.
• Aggregates and dimers — the lipid promotes micelle formation, which encourages aggregation.

Intact-mass LC-MS is necessary but not sufficient for a lipidated peptide — positional isomers of the linker share the same mass. The CertikLabs panel adds peptide mapping to confirm the linker is on the right lysine:

• RP-HPLC purity with wide-pore C4/C8 column (TFA / MeCN, UV 214 nm)
• LC-HRMS identity (ESI-Q-TOF, deconvoluted average mass)
• Peptide mapping by tryptic digest + LC-MS/MS to confirm sequence and lipid attachment
• Quantitative HPLC vs reference standard for content
• Size-exclusion HPLC for aggregates

Method conditions are reported in full on the Certificate of Analysis. See characterizing a peptide for what each method measures and what it cannot.

Selective GLP-1 receptor agonist. Activation increases glucose-dependent insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite via central GLP-1R signaling. Extensive published clinical and preclinical literature; no clinical guidance offered here.

Primary sources:

• Lau J. et al., Discovery of the once-weekly GLP-1 analogue semaglutide.(PMID 26308095)
• Knudsen L.B., Lau J., The discovery and development of liraglutide and semaglutide.(PMID 31031702)

CertikLabs does not publish dosing, administration routes, or clinical recommendations for semaglutide.

The fatty-diacid linker makes semaglutide prone to micelle formation and surface adsorption in aqueous solution. Lyophilized material is generally stable refrigerated and protected from light; reconstituted material is sensitive to oxidation, aggregation, and container interaction.