Moles, Molarity and mg/mL: The Arithmetic Behind Peptide Concentrations

Everything here rests on three relationships: moles equal mass divided by molar mass, molarity being moles per litre, and C₁V₁ = C₂V₂ when diluting. Since a mass in daltons and a molar mass in grams per mole are numerically identical, each conversion is one division and nothing more. The difficulty is that syringes and vial labels speak in mg/mL, whereas enzyme kinetics, binding studies and cell-culture protocols are written in molar units. Translating between them is what stops two peptides described as being "at the same concentration" from actually differing several-fold in the number of molecules present. What follows works each conversion through with real masses from the catalogue, covers the net-peptide-content correction that almost everyone omits, and finishes with a µmol-per-mg lookup table.

The three equations

1. Mass to moles: mol = mass (g) ÷ MW (g/mol). Rewritten in the units a peptide lab actually uses, that becomes µmol = mg ÷ MW × 1,000 — or, more conveniently, µmol per mg = 1,000 ÷ MW.
2. Concentration to molarity: since a concentration in mg/mL is the same number in g/L, mol/L = (mg/mL) ÷ MW. Multiply by 1,000 to express it as mM, or 1,000,000 as µM.
3. Dilutions: C₁V₁ = C₂V₂, with any consistent set of units; solving for how much stock to take gives V₁ = (C₂ × V₂) ÷ C₁.

Those three lines are the entire toolkit. The molarity calculator implements all of them, but it remains worth being able to do them longhand — that is how you notice when a calculator has been given the wrong units.

Worked example 1: from vial to molarity

Take a 5 mg BPC-157 vial; its molar mass is 1,419.55 g/mol.

1. Reconstitute: 2 mL of diluent added to 5 mg gives 2.5 mg/mL (5 ÷ 2).
2. Convert: 2.5 mg/mL is the same as 2.5 g/L, and 2.5 ÷ 1,419.55 = 0.001761 mol/L — i.e. 1.76 mM.
3. If your protocol prefers micromolar, that is 1,761 µM.
4. Cross-check by the other route: 1,000 ÷ 1,419.55 = 0.704 µmol per mg, so a 5 mg vial holds 3.52 µmol; dissolved in 0.002 L that is 1,761 µM — the same number reached from a different direction.

Worked example 2: from molarity to mass

Now imagine a cell-culture protocol calling for 10 mL of working solution at 100 µM.

1. Moles required: 100 µM equals 1 × 10⁻⁴ mol/L, and multiplying by 0.010 L gives 1 × 10⁻⁶ mol — one micromole.
2. Mass required: one micromole of a 1,419.55 g/mol peptide is 1,419.55 µg, i.e. 1.42 mg.
3. Or get there by dilution instead. From the 1.76 mM stock prepared above, V₁ = (100 µM × 10 mL) ÷ 1,761 µM = 0.568 mL, made up to a final 10 mL.

The dilution approach almost always wins: measuring out 0.568 mL by pipette is far more accurate than weighing 1.42 mg, and it uses less peptide.

Worked example 3: identical masses, different molar amounts

Avoiding precisely this error is the reason the conversion matters.

• GHK-Cu, MW 403.93: 1,000 ÷ 403.93 = 2.476 µmol in every milligram.
• Semaglutide, MW 4,113.58: 1,000 ÷ 4,113.58 = 0.243 µmol in every milligram.

So on a per-milligram basis the copper tripeptide supplies 10.2 times the number of molecules that semaglutide does. Comparing two peptides at matched mg/mL therefore means comparing unequal quantities of molecules wherever receptor occupancy is what you care about. Exactly the same issue is built into any blend formulated at equal masses, which is why blends versus vials of single peptides matters for stoichiometric work.

Worked example 4: correcting for net peptide content

Because synthesis and purification yield salts, the "5 mg" on a vial is 5 mg of solid consisting of the peptide plus its counter-ion, residual solvent and water. The peptide's own share is typically 70 to 90%, and the certificate ought to report it.

1. Uncorrected: 2.5 mg/mL from 5 mg in 2 mL, i.e. 1.761 mM, exactly as above.
2. Corrected at 85%: the vial really holds 4.25 mg (0.85 × 5), giving 2.125 mg/mL in 2 mL, which divided by 1,419.55 is 1.497 mM.
3. Magnitude of the mistake: (1.761 − 1.497) ÷ 1.761 — about 15% high, and that bias carries into every dilution made afterwards and every concentration you report.
4. The remedy: either correct the number, or adjust the reconstitution volume. To obtain a genuine 2.5 mg/mL of peptide from 4.25 mg, add 1.70 mL of diluent (4.25 ÷ 2.5) rather than 2 mL.

For purely qualitative experiments the correction rarely changes the conclusion. But when you report an EC₅₀, an IC₅₀ or a binding constant, omitting it builds in a systematic error of 15–30%. Where the figure comes from is set out under third-party testing explained.

Reference table: µmol per milligram

If a label carries no molecular weight, you can derive it from the sequence by summing the average residue masses and adding 18.02 Da — the method described in how to read a sequence of amino acids.

Unit traps worth committing to memory

• A concentration of 1 mg/mL is 1 g/L. Getting this relationship inverted by 1,000 is the error that occurs most often.
• There are 1,000 mcg in a milligram. Vial labels use mg while working concentrations are frequently in mcg/mL — hence the unit converter.
• mM, µM and nM are each separated by a factor of 1,000. Freshly reconstituted vials generally sit in the mM range, while cell-culture experiments run in µM or nM.
• Daltons and grams per mole share the same numeric value: 1,419.55 Da corresponds to 1,419.55 g/mol.
• The volume you add is not the volume you get. Adding 2 mL of diluent to a lyophilised cake gives marginally more than 2 mL of solution; for a few milligrams of peptide the displacement can be ignored, but beyond about 50 mg in a vial it can no longer be.
• Metal complexes need care. GHK-Cu's quoted 403.93 Da includes the copper; working from the bare tripeptide sequence gives 340.4 Da instead and a 19% error.

The other half of the problem — choosing a diluent volume so that, in mg/mL and per-unit terms, your routine draws land on syringe graduations you can actually read — is covered in the reconstitution arithmetic explained and in the reconstitution calculator.