Peptide reconstitution math — concentration, volume, and storage

Reconstituting a lyophilized peptide is one of the most consequential moments in a research workflow — get the math wrong and every downstream measurement is off by a constant scaling factor that may not be discoverable until weeks later. This guide walks through the concentration math correctly, including the part that most online "peptide reconstitution calculators" handle incorrectly: net peptide content vs. lyophilized vial mass.

The basic math

Concentration is mass divided by volume. If you reconstitute a 10 mg vial of peptide in 2 mL of bacteriostatic water, the resulting solution is 5 mg/mL (or 5,000 µg/mL, or 5 µg/µL — all the same number expressed differently).

To express that same concentration in molar terms, divide mg/mL by the peptide's molecular weight in g/mol, then multiply by 1000 to convert to mM. For a peptide with MW 1,500 Da: 5 mg/mL ÷ 1,500 = 0.00333 mmol/mL = 3.33 mM = 3,333 µM.

The correction most calculators miss

The vial mass on a peptide product label is the lyophilized mass — total milligrams of powder in the vial. For a TFA-salt peptide, only 75–85% of that lyophilized mass is actual peptide (see our salt-form guide for the full breakdown). The rest is counter-ion, residual water, and other non-peptide material.

If a vial label says "10 mg peptide" and the COA reports a net peptide content of 80% by mass, then the vial contains 8 mg of actual peptide — not 10 mg. Reconstituting in 2 mL of solvent gives you 4 mg/mL of peptide, not 5 mg/mL. For research workflows that depend on accurate concentration (binding assays, in-vitro pharmacology, structural NMR), the 20% difference is material.

Reconstitution volume planning

To hit a target concentration, the volume to reconstitute is: (vial peptide mass) ÷ (target concentration). For a 10 mg vial at 80% peptide content (= 8 mg actual peptide) targeting 2 mg/mL: 8 mg ÷ 2 mg/mL = 4 mL bacteriostatic water.

Practical considerations: most research vials are 3-mL or 5-mL volumes, so you have limited room. If your target concentration would require more solvent than the vial can hold, you have two options — aliquot the lyophilized peptide into a larger vessel before reconstitution, or accept a higher initial concentration and dilute downstream.

Reconstitution solvent — what to use

Standard solvent for research peptide reconstitution is sterile bacteriostatic water with 0.9% benzyl alcohol. The benzyl alcohol acts as a mild preservative and the slightly acidic pH (~5.0) favors stability for most sequences. Sterile saline (0.9% NaCl) is acceptable for short-term work but does not preserve stability as well. Avoid: tap water, non-sterile USP water, and high-pH buffers above pH 7.0.

For peptides that are poorly soluble in aqueous solvent (rare in the standard research catalog but common for hydrophobic sequences), the cosolvent options are: 5–10% acetic acid (good for hydrophobic basic peptides), 5–10% DMSO (good for highly hydrophobic peptides; flag for cell-culture work because DMSO above ~1% in final assay concentration is mildly cytotoxic), or 5% ammonium bicarbonate (for peptides with acidic side chains that benefit from mild base).

Worked example

Suppose you ordered a 5 mg vial of a 15-residue peptide, MW 1,800 Da, TFA salt, COA reports 99% HPLC area and 78% peptide content by mass. You want to run an in-vitro assay at 10 µM working concentration in 100 µL well volume.

1. 1.Net peptide in vial: 5 mg × 0.78 = 3.9 mg actual peptide.
2. 2.Convert to molar: 3.9 mg ÷ 1,800 g/mol = 2.17 µmol (i.e. 2,170 nmol) of peptide in the vial.
3. 3.Choose a stock concentration. For ease of dilution, 1 mM stock is convenient: 1 mM × X mL = 2,170 nmol → X = 2.17 mL bacteriostatic water to reconstitute.
4. 4.To make 10 µM working solution: dilute 1 mM stock 1:100. For a 100 µL well, that means 1 µL of stock plus 99 µL of assay buffer.