Molar Extinction Coefficient of a Peptide

The molar extinction coefficient (ε, epsilon) is a constant that describes how strongly a molecule absorbs light at a given wavelength. For a specific peptide under fixed conditions, ε does not change — it is an intrinsic property of the chemical structure. What changes when different concentrations are placed in a spectrophotometer is the measured absorbance. Knowing ε makes it possible to calculate the actual concentration of a peptide in solution from an ultraviolet absorbance reading, without additional equipment.

The Beer-Lambert Law

The equation connecting absorbance, concentration, and molar extinction is the Beer-Lambert law:

A = ε × c × l

• A is the measured absorbance (dimensionless).
• ε is the molar extinction coefficient, in M⁻¹·cm⁻¹.
• c is the molar concentration of the solution (mol/L).
• l is the optical path length of the cuvette, normally 1 cm under standard conditions.

Rearranging for concentration gives c = A / (ε × l). With A read from the instrument and ε calculated from the peptide sequence, the concentration of the solution is determined. This is the reference method in analytical chemistry and in quality control for research-grade lots.

What Determines ε in a Peptide

At 280 nm — the wavelength most commonly used for peptides and proteins — absorbance is generated by aromatic amino acids and disulfide bonds. Other residues absorb very little at that wavelength and their contribution to ε is negligible.

The reference values established by Pace et al. (1995) under denaturing conditions are:

• Tryptophan (Trp, W): 5,500 M⁻¹·cm⁻¹ per residue.
• Tyrosine (Tyr, Y): 1,490 M⁻¹·cm⁻¹ per residue.
• Disulfide bond (Cys-Cys): 125 M⁻¹·cm⁻¹ per bond.

The total ε at 280 nm is calculated by summing the contribution of each residue:

ε₂₈₀ = (nTrp × 5,500) + (nTyr × 1,490) + (nSS × 125)

A peptide with two tryptophan residues and one tyrosine would have ε₂₈₀ = (2 × 5,500) + (1 × 1,490) = 12,490 M⁻¹·cm⁻¹. The ProtParam tool from ExPASy calculates ε directly from an amino acid sequence and is freely available. It is the standard starting point for any laboratory that needs to verify the concentration of a research lot.

Peptides Without Aromatic Residues

Several research peptides contain neither Trp nor Tyr. In those cases, ε₂₈₀ is effectively zero and measurement at 280 nm provides no information about concentration.

For those compounds, work is done at 205 nm or 214 nm, where the peptide bond itself absorbs. Absorbance at those wavelengths is higher, but the technique is more sensitive to interferents: the solvent, buffer, and salts also absorb in that region and can overlap with the peptide signal.

BPC-157, for example, contains neither tryptophan nor tyrosine in its sequence. Direct spectrophotometric quantification at 280 nm is uninformative for that compound. Laboratories working with it use alternative methods or measure at shorter wavelengths with the corrections those conditions require.

Why This Matters for a Certificate of Analysis

A certificate of analysis (COA) for a peptide lot typically reports purity as a percentage by HPLC and content in milligrams per vial. Those two figures are not the same as the concentration in solution once lyophilised powder is reconstituted.

When a laboratory takes an aliquot of the dissolved lot and reads it in the spectrophotometer at 280 nm, it can verify that the actual concentration matches what is declared — using the Beer-Lambert equation with the ε calculated for that sequence. If the measured absorbance does not match what is expected for the declared concentration, there is a discrepancy that should be investigated before the material is used in an experiment.

The Janoshik Analytical report for the MOTS-c lot that PeptoClinic publishes records a content of 11.31 mg against a 10 mg label. A result like that can only be verified with a well-defined quantitative analytical methodology. The quality and documentation section of the PeptoClinic site shows how lot purity and content are documented across the catalogue.

A Common Error: Conflating Absorbance with Concentration

The absorbance shown on a spectrophotometer readout is not the concentration. It is a dimensionless quantity that depends simultaneously on ε, c, and l. Measuring the same solution in a 0.5 cm cuvette and a 1 cm cuvette yields two different absorbance values; the concentration is identical in both cases.

The error becomes consequential when absorbance readings are compared across different peptides without adjusting for ε. A peptide with three tryptophan residues produces a much higher absorbance at the same molar concentration as one with none. Comparing raw readings under those conditions says nothing about concentration — it reflects the aromatic composition of each compound, nothing else.

Limitations of the 280 nm Method

Spectrophotometric measurement at 280 nm has conditions of application that should be understood before interpreting results.

Buffer interferents. Contaminating proteins, phenols, and some stabilising additives absorb at nearby wavelengths and can overlap with the peptide signal.

Folding state. The ε calculated using the Pace et al. formula corresponds to denaturing conditions (6 M guanidinium hydrochloride or 8 M urea). Under native conditions, the molecular environment can slightly shift the position and shape of the absorption peak of the chromophores. The difference is typically below 5%, but in high-precision applications it can matter.

Short peptides with low hydrophobicity. In some cases, aggregation in solution complicates the linear relationship between absorbance and concentration that Beer-Lambert assumes.

The standard reference for ε values at 280 nm is Pace et al. (1995) in *Protein Science* (PMID 8563639). The full catalogue of research peptides available from PeptoClinic covers compounds across several functional areas, each shipped with lot-specific documentation.

For Laboratory Research Only

The peptides PeptoClinic supplies are research material classified Research Use Only (RUO). They are intended exclusively for laboratory use: in vitro research and preclinical assays. They are not medicines, supplements, or foods. They are not approved or authorised for use in humans or animals by any regulatory authority, including the FDA or ANMAT.

PeptoClinic does not provide guidance on dosing, administration, or clinical protocols. Requests of that kind are declined. For information about available compounds or the documentation that accompanies each lot, contact [email protected].