A receptor does not weigh anything. It counts. Two peptides prepared at the same mg/mL are not present at the same number of molecules per litre unless they happen to have the same molecular weight — and peptides in this catalog range from roughly 340 g/mol to over 9,000. Prepared at 1 mg/mL, the small one is present at nearly thirty times the molar concentration of the large one.
That is not a rounding difference. It is the difference between a saturating concentration and one that barely registers.
The conversion
Molar concentration is mass concentration divided by molecular weight:
- mol/L = (g/L) ÷ (g/mol)
- In working units: µM = (µg/mL) ÷ MW × 1000
Worked through: a peptide of MW 1,419 at 1 mg/mL is 1,000 µg/mL ÷ 1,419 × 1000 ≈ 705 µM. A peptide of MW 9,118 at the same 1 mg/mL is ≈ 110 µM. Same mass, same volume, six-fold different in the only term binding cares about.
| Approx. MW (g/mol) | 1 mg/mL equals | To reach 10 µM you need |
|---|---|---|
| 340 | ≈ 2,940 µM | ≈ 3.4 µg/mL |
| 1,000 | 1,000 µM | 10 µg/mL |
| 1,419 | ≈ 705 µM | ≈ 14 µg/mL |
| 3,368 | ≈ 297 µM | ≈ 34 µg/mL |
| 4,964 | ≈ 201 µM | ≈ 50 µg/mL |
| 9,118 | ≈ 110 µM | ≈ 91 µg/mL |
Which unit belongs where
When should a concentration be expressed in molarity?
Whenever the question is about binding, occupancy or potency. Affinity constants, IC50 and EC50 values, and receptor occupancy are all molar quantities, and published comparisons between compounds are molar for the same reason. Reporting an in-vitro result in µg/mL makes it incomparable with almost the entire literature it belongs to.
When is mg/mL the better unit?
When the question is about the material rather than the molecule — what is in the vial, what the certificate reports, how much stock remains, what solubility allows. Mass concentration is the language of handling, and converting it to molarity to describe a stock solution adds a step and an assumption without adding information.
Which molecular weight should be used for a blend?
None, as a single figure. A co-formulated blend contains two or more compounds with different molecular weights, and a molar concentration for "the blend" has no referent. Each component gets its own molar concentration from its own mass and its own MW — which is why a blend certificate has to report content per component. Why a blend is not three vials covers what that changes.
Does the salt form affect the conversion?
It affects the mass term, not the arithmetic. If the material is a trifluoroacetate salt at 80% net peptide content, a nominal 1 mg/mL solution holds about 0.8 mg/mL of peptide, and the molar concentration should be calculated from 0.8 rather than 1.0 — a 20% error otherwise, silently, in the direction of overestimating. Use the peptide's free-base molecular weight with the peptide mass, not the salt mass.
The mistake that survives peer review
The common failure is not a conversion error. It is comparing two compounds at equal mass and describing the result as a difference in potency.
If compound A (MW 1,000) and compound B (MW 3,000) are both tested at 10 µg/mL, A is present at 10 µM and B at 3.3 µM. If A produces a larger effect, that is the expected result of a three-fold concentration difference, not evidence that A is the more potent molecule. A fair comparison is run at matched molar concentrations, or across a range wide enough that the curves — not the single points — are what get compared. How to build that range is its own short discipline.
What about peptides quoted in international units?
IU is not a mass or molar unit at all. It is a measure of biological activity defined against a reference standard, and the conversion to mass is specific to the substance and only meaningful where such a standard exists. For research peptides without an IU standard, the unit does not apply, and seeing it used is a reason to ask what the number is actually referencing.
How do I find the molecular weight for a modified peptide?
Not by summing the residues from the name, which is the mistake that produces a quietly wrong molarity. Most engineered peptides carry modifications that change the mass: C-terminal amidation removes about one unit, N-terminal acetylation adds 42, and an acylation chain adds several hundred. The figure to use is the one on the certificate's identity panel, which reports the mass actually observed for the batch. Where a certificate is not to hand, the notation written around a sequence says which modifications are present, and why they were added — but the observed mass is the number to calculate from.
Why do published EC50 values for the same peptide disagree?
Often because the unit conversion was done differently, before any biology enters. One laboratory calculates molarity from the label mass, another from the peptide mass after net content, a third uses the salt's molecular weight instead of the free base. Each choice moves the reported concentration by ten to twenty percent, and those discrepancies compound with the genuine assay differences — cell line, receptor expression, whether albumin was present in the buffer. For acylated peptides that last variable alone can move a figure by orders of magnitude. Comparisons are meaningful within a paper, where every compound saw the same conversion and the same assay, and unreliable between papers.
A small discipline that prevents most of it
Record both. A stock written up as "1 mg/mL (705 µM, MW 1,419, net peptide content 82% per lot certificate)" cannot be misread, cannot be silently mis-converted downstream, and gives a reader everything needed to compare it to a published figure. It takes one line.
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