Molecular Weight
Retatrutide’s molecular weight is reported at approximately 4.7 kDa, consistent with a lipidated 39-residue peptide. The figure matters mainly for molar calculations and for confirming identity by mass spectrometry.
Key concepts
- Approximately 4.7 kDa for the lipidated 39-residue peptide
- Salt form and net peptide content change the working figure
- Molecular weight is required for molar concentration
- Mass spectrometry confirms identity; HPLC confirms purity
- Batch-specific figures should always come from the certificate of analysis
Reported value and what it reflects
Published and developer sources place retatrutide’s molecular weight in the region of 4.7 kDa. The exact figure depends on whether the free peptide or a salt form is being described, so any calculation should state which value was used.
The mass reflects three contributions: the 39-residue backbone, the non-standard residues, and the fatty diacid conjugate. Removing the lipid conjugate from the calculation therefore produces a noticeably lower and incorrect value.
| Property | Reported value |
|---|---|
| Development code | LY3437943 |
| Peptide length | 39 amino acids |
| Approximate molecular weight | ~4.7 kDa |
| Physical form supplied | Lyophilised powder |
| Identity confirmation | Mass spectrometry |
Using it in molar calculations
Converting a mass concentration to a molar concentration requires the molecular weight of the exact material in hand. For a nominal 10 mg vial reconstituted in 2 mL, a mass concentration of 5 mg/mL corresponds to roughly 1.06 mM at 4.7 kDa — a figure to recompute against the certificate rather than to copy.
Peptide content and salt content both affect the true peptide mass in a vial. Where a certificate reports net peptide content, that is the figure to use.
Confirming mass analytically
Mass spectrometry compares measured mass against the expected value and is the standard identity check for a synthetic peptide. HPLC reports chromatographic purity but not molecular identity; the two tests answer different questions and are normally reported together.
A mass that differs materially from the expected value indicates a different molecule, a modification, or degradation — not a rounding difference.
Why the figure should be treated as approximate
Reported molecular weight figures in secondary sources vary slightly depending on rounding conventions, whether a salt or counter-ion is included, and which isotopic convention is used for the mass calculation. None of these variations are large enough to change a general understanding of the molecule, but they are large enough to matter in a precise molar calculation.
For that reason, any calculation feeding into a research protocol should cite the specific molecular weight value used and its source, whether that is the primary characterisation paper or the batch certificate of analysis for the material in hand, rather than a rounded figure from an unspecified secondary reference.
Common calculation errors
A frequent error is to calculate molar concentration from the total vial mass printed on a label rather than from the net peptide content stated on the certificate of analysis, which is typically lower once salt content, residual moisture and counter-ions are accounted for. Using the label mass overstates the true molar concentration.
Another common error is to reuse a molecular weight figure from a related but distinct peptide, on the assumption that similar peptides in this class share a molecular weight. Chain length, residue substitutions and the specific lipid conjugate all affect mass, so figures should not be transferred between compounds.
Where to source a verified figure
The primary characterisation paper by Coskun and colleagues is the appropriate reference for the molecular weight of the peptide as originally described, and the batch certificate of analysis is the appropriate reference for a specific supplied lot, since manufacturing and salt form can differ between suppliers.
Retatrutide research material supplied by Biovanta Research is intended strictly for laboratory research use, and any molecular weight or reconstitution figure used in a protocol should be checked against the documentation supplied with that specific batch rather than assumed from memory or from a general web reference.
Reconstitution consequences of the lipid conjugate
Because the fatty diacid conjugate makes the molecule more lipophilic than an unmodified peptide of similar length, reconstitution technique affects how reliably the labelled mass translates into an accurate working concentration. Adding solvent slowly down the side of the vial and swirling gently, rather than shaking or pipetting forcefully, reduces the risk of foaming or aggregation that can trap peptide out of solution and cause the true dissolved concentration to fall below the value calculated from the vial’s nominal content.
Where a molar concentration is critical to an experiment, confirming the actual concentration of a reconstituted stock by an independent method, such as UV absorbance at an appropriate wavelength or an analytical HPLC injection against a standard, is a more defensible basis for downstream calculations than relying solely on the vial label and the nominal molecular weight.
Open questions and where to check current figures
Because retatrutide remains an investigational compound, its exact molecular weight to further decimal precision, along with any salt-form-specific variants, should be treated as a detail that can be refined as further characterisation is published, rather than as a permanently fixed figure. The approximate value in current use is well supported, but researchers requiring high-precision mass figures for a specific application should request the analytical documentation for their exact batch rather than relying on a general reference figure.
For the most current authoritative figures, the primary characterisation paper by Coskun and colleagues and the batch-specific certificate of analysis remain the two sources to check first, ahead of any calculator, forum post or general summary that may not specify which form of the molecule its figure describes.
Strength of the current evidence
The approximate mass is consistently reported. Batch-specific figures should always be taken from the certificate of analysis rather than from a general reference.
References
- Coskun T et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist. Cell Metabolism, 2022 (PubMed)
- Eli Lilly clinical development pipeline (Eli Lilly)
Related Retatrutide research
- Molecular Structure — Sequence features, lipidation and structural modifications that define the retatrutide molecule.
- Amino Acid Sequence — What is publicly documented about retatrutide’s 39-residue sequence, its GIP-derived backbone and its lipidation site.
- Pharmacology — Receptor pharmacology, reported potency framework and pharmacokinetic design features of retatrutide as an investigational peptide.
Back to the Retatrutide research hub · Retatrutide research material · Certificates of analysis
Research use only. Everything on this page describes published laboratory and preclinical research into a compound supplied strictly for in-vitro research by qualified persons. It is not medical advice, it does not describe human or veterinary use, and no dosing guidance is given.