Calculating peptide concentration is more complicated than dividing the weighed powder by the solvent volume. Lyophilized peptide often contains non-peptide mass such as water, salts, absorbed solvents, and counterions. For accurate concentration, peptide content and sequence-specific absorbance may need to be considered.
A lyophilized peptide sample may contain 10–70% non-peptide material by weight depending on sequence, purification, and formulation. That means using the full measured powder weight can overestimate the true peptide concentration.
If the peptide content is 80%, then 1 mg of lyophilized material does not contain 1 mg of peptide. It contains about 0.8 mg of peptide.
If 1 mg of a peptide product has 80% net peptide content and you want a 1 mg/ml peptide solution, the solvent volume should be based on 0.8 mg of peptide rather than 1.0 mg of gross sample mass.
If the peptide contains tryptophan or tyrosine, concentration can often be estimated using the extinction coefficient of those chromophoric residues.
| Residue | Typical molar extinction coefficient at 280 nm |
|---|---|
| Tryptophan (W) | 5560 AU/mmole/ml |
| Tyrosine (Y) | 1200 AU/mmole/ml |
The extinction coefficients are generally additive, so the total coefficient depends on the number of W and Y residues in the sequence.
mg peptide/ml = (A280 × dilution factor × molecular weight) / total extinction coefficient
This approach assumes the chromophores are exposed. For short soluble peptides, that is often acceptable. If folding or aggregation is suspected, denaturing conditions may be needed to improve the reliability of the measurement.
If the peptide is hydrophilic, highly modified, aggregation-prone, or intended for quantitative studies, it is important to account for content and sequence-dependent absorbance rather than relying only on powder weight.
Analyze your sequence to better understand peptide behavior:
If your project requires more exact concentration determination or additional content analysis, please contact sales@lifetein.com.