Molecular weight is the number that turns a mass into a count of molecules, and it is the one most often taken from the wrong place. Every conversion from mg/mL to molarity divides by it. Summing the residues gives a figure that is wrong for any modified peptide; using the salt's weight gives another. The table below lists approximate free-base masses for the catalogue, with one caveat over all of them: the observed mass on the lot certificate is the number to calculate from.
The table
Masses are approximate, average, for the free peptide including its named modifications and excluding counter-ion and water. The last column is the molar concentration of a 1 mg/mL solution of pure peptide — before the correction for net content that every real stock needs.
| Compound | Residues | Approx. mass (Da) | 1 mg/mL ≈ |
|---|---|---|---|
| Vilon | 2 | 275 | 3,640 µM |
| Glutathione (reduced) | 3 | 307 | 3,260 µM |
| GHK (peptide only) | 3 | 340 | 2,940 µM |
| KPV | 3 | 342 | 2,920 µM |
| Epithalon | 4 | 390 | 2,560 µM |
| Dihexa | 2 + caps | 504 | 1,980 µM |
| SS-31 | 4 | 640 | 1,560 µM |
| Ipamorelin | 5 | 712 | 1,400 µM |
| Selank | 7 | 751 | 1,330 µM |
| Semax | 7 | 813 | 1,230 µM |
| DSIP | 9 | 849 | 1,180 µM |
| GHRP-6 | 6 | 873 | 1,150 µM |
| TB-500 | 7 | 890 | 1,120 µM |
| Melanotan-2 | 7 | 1,024 | 977 µM |
| PT-141 | 7 | 1,025 | 975 µM |
| Kisspeptin-10 | 10 | 1,302 | 768 µM |
| BPC-157 | 15 | 1,419 | 705 µM |
| Melanotan-1 | 13 | 1,647 | 607 µM |
| AOD-9604 | 16 | 1,815 | 551 µM |
| MOTS-c | 16 | 2,174 | 460 µM |
| Thymosin α1 | 28 | 3,108 | 322 µM |
| Sermorelin | 29 | 3,358 | 298 µM |
| Semaglutide | 31 | 4,114 | 243 µM |
| Retatrutide | 39 | 4,731 | 211 µM |
| Tirzepatide | 39 | 4,814 | 208 µM |
| Tesamorelin | 44 | 5,136 | 195 µM |
| IGF-1 LR3 | 83 | 9,111 | 110 µM |
The range is thirty-fold. Prepared at the same mg/mL, Vilon is present at thirty times the molar concentration of IGF-1 LR3, which is the whole reason mass concentration and molarity disagree.
Why do these differ from the sum of the residues?
Because most are modified, and each modification changes the mass. C-terminal amidation removes about one unit. N-terminal acetylation adds 42. An acyl chain and its linker add several hundred — semaglutide's C18 diacid and spacer account for roughly 500 daltons of its total. A lactam ring removes 18; a disulphide removes 2. Non-natural residues have their own masses: Aib is 14 heavier than alanine, norleucine is 18 lighter than methionine. The notation around a sequence lists the modifications; the mass follows from them.
Why does the certificate's mass sometimes differ from this table?
For one of three reasons, each of which is checkable. The certificate may report monoisotopic mass where this table is average, a difference of one to three daltons for peptides of this size — the method line says which. The batch may carry a modification state this table assumes differently, most often amidation. Or the observed mass may genuinely differ, which is the finding that matters: a mass sixteen units high is an oxidised methionine, one unit high is a deamidation, several hundred low is a missing acyl chain. The table is a reference for what the molecule should weigh; the certificate is what it does.
Which mass should a molarity calculation use?
The free peptide's mass, applied to the peptide's mass in the vial. Not the salt's molecular weight, which includes the counter-ions, and not the label mass, which includes counter-ion and water. A 5 mg vial of a trifluoroacetate salt at 80% net peptide content holds 4 mg of peptide; that 4 mg divided by the free-base mass gives the moles. Using 5 mg overstates the moles by a quarter; using the salt's weight understates them by a similar amount, and the two errors do not reliably cancel. Net content is where the correction lives.
Why is GHK listed without its copper?
Because the copper adds a defined mass that depends on the complex's stoichiometry, and the figure reported on a certificate for GHK-Cu may describe the peptide, the complex, or both. The tripeptide alone is 340; with one copper(II) ion, less the two protons it displaces, the complex is about 402. A molarity calculation for the complex should use the complex's mass, and a certificate that reports only the peptide mass has left that step to you.
How accurate are these figures?
To within a few daltons for the unmodified sequences and to within the uncertainty of the modification state for the rest. They are working figures for planning a stock, and they are not a substitute for the identity panel. For any calculation that will be reported, the number on the lot certificate replaces the number here, because the certificate describes the molecule that was actually made and this table describes the one that was intended.
How is the molar concentration read from the table?
Divide 1,000 by the mass in daltons to get millimolar at 1 mg/mL, or use the last column directly in micromolar. A 1 mg/mL solution of BPC-157 at 1,419 daltons is 0.705 mM, or 705 µM. For any other concentration the figure scales linearly: 0.1 mg/mL is a tenth of the last-column value. What does not scale is the correction for net content, which has to be applied to the mass before the division and is the step most often skipped.
Why are the masses given as averages rather than exact?
Because a peptide of this size is a population of molecules differing in their isotopes, and "the mass" is a summary of that population. The average mass weights every isotope by its abundance and is what a balance would report if it could weigh a single molecule many times. The monoisotopic mass is the lightest member of the population and is what a high-resolution mass spectrometer reports. For molarity the average is the correct choice, because a weighed sample contains the whole population; for matching a certificate the method line decides which one to compare against.
All products referenced here are supplied for laboratory and research use only. They are not drugs, foods, supplements or cosmetics, and are not for human or veterinary use.
