Ultraviolet absorbance is the quickest way to put a number on how much peptide is in a solution. It needs no standard curve and consumes nothing, but it depends entirely on one value: the extinction coefficient of the sequence at the chosen wavelength. Choose the wrong wavelength or the wrong coefficient and the concentration can be off by a large factor without any warning sign on the instrument.
This article covers how that coefficient is calculated from a sequence, when 280 nm works, when the far-UV range around 205 or 214 nm is needed, and what each method actually measures.
The Beer–Lambert relationship in one line
Absorbance A equals ε × c × l, where ε is the molar extinction coefficient (M−1 cm−1), c is molar concentration and l is the path length in centimeters. With a 1 cm cuvette, concentration is simply absorbance divided by ε. Converting to mass per volume requires the molar mass of the peptide as it exists in solution, which is where counter-ions and the difference between free base and salt start to matter.
The relationship holds only while the reading sits within the linear range of the photometer, the solution does not scatter light, and nothing else in the sample absorbs at that wavelength. Each of those conditions fails in a recognizable way.
Where the 280 nm coefficient comes from
At 280 nm, almost all absorbance from a peptide comes from three chromophores: the indole ring of tryptophan, the phenol ring of tyrosine, and disulfide-bonded cystine. The widely used values from Pace and colleagues (1995) are:
| Chromophore | ε at 280 nm (M−1 cm−1) |
|---|---|
| Tryptophan (each) | 5,500 |
| Tyrosine (each) | 1,490 |
| Cystine (each disulfide, not each Cys) | 125 |
The coefficient for a sequence is the sum: 5,500 × nTrp + 1,490 × nTyr + 125 × nCystine. An earlier set from Gill and von Hippel gives slightly different values, and the two can differ by a few percent for the same sequence, which is a useful reminder that the calculation is an estimate rather than a measured constant. Free cysteine thiols contribute almost nothing at this wavelength, so a reduced peptide and its oxidized disulfide form have slightly different coefficients.
A worked A280 calculation
Take a hypothetical 18-residue peptide containing one tryptophan, two tyrosines and no cysteine, with a molar mass of 2,000 g/mol.
- ε280 = 5,500 + (2 × 1,490) = 8,480 M−1 cm−1.
- A measured absorbance of 0.424 in a 1 cm cell gives c = 0.424 / 8,480 = 5.0 × 10−5 M, or 50 µM.
- Multiplying by the molar mass gives 0.100 g/L, which is 0.100 mg/mL of peptide.
If the same peptide had a single tyrosine and no tryptophan, ε would drop to 1,490 and the same absorbance would correspond to a concentration almost six times higher. Sequences with weak chromophores magnify every source of error in the reading, because a small stray absorbance is a large fraction of the total.
Peptides with no Trp or Tyr: A205 and A214
Many short sequences contain neither residue, and their absorbance at 280 nm is effectively zero apart from a weak phenylalanine band near 257 nm. For those, the peptide bond itself is the chromophore. Amide bonds absorb strongly in the far UV, so every peptide has measurable absorbance near 205 and 214 nm regardless of composition.
- A205. Scopes described an approximate absorbance of about 31 for a 1 mg/mL protein solution in a 1 cm cell. Anthis and Clore (2013) refined this into a sequence-specific calculation that adds contributions from the backbone and from aromatic and other absorbing side chains.
- A214. Kuipers and Gruppen (2007) published molar coefficients at 214 nm, with roughly 900 M−1 cm−1 per peptide bond plus much larger side-chain terms for Trp, Tyr, Phe and His. Proline in the chain is assigned its own value because its tertiary amide absorbs differently.
The far-UV methods are more sensitive, but they are far less selective. Many ordinary solution components absorb strongly below 220 nm, including trifluoroacetate, acetate, some buffer salts and dissolved organic solvents. Trifluoroacetate is often present in synthetic peptide salts, as explained in the TFA versus acetate counter-ion article, so a blank that matches the sample matrix exactly is essential. Standard glass and many plastic cuvettes absorb in this region too, which is why quartz cells are used for far-UV work.
Sources of error that inflate or deflate the result
- Light scattering. Aggregates or particulates scatter light and add apparent absorbance that rises toward shorter wavelengths. A reading at 320 or 340 nm, where the peptide itself should not absorb, is commonly used to detect and correct for it.
- Absorbing impurities. UV cannot tell the target from a truncated sequence or a protected by-product carrying the same aromatic residue. Absorbance measures all chromophores present, so a concentration from UV assumes the chromatographic purity is known.
- Environment of the chromophore. Tabulated values assume residues exposed to solvent. Tyrosine ionizes around pH 10, shifting its band toward 293 nm, and buried aromatic side chains in a structured peptide absorb differently.
- Coefficient choice. Using a 280 nm value for a sequence dominated by a single tyrosine, or applying a generic 1 mg/mL figure to a short peptide, can mislead by tens of percent.
How differing UV responses also affect purity calculations is covered in UV response factors and purity comparison.
What UV concentration does and does not tell you
A UV result estimates the amount of absorbing peptide in solution. It does not report the gross mass of powder, which also includes counter-ions and residual water, so it rarely matches the figure on a label. That distinction is the subject of purity versus net peptide content. When a concentration must be traceable rather than calculated, laboratories use amino acid analysis, which hydrolyzes the peptide and quantifies residues directly and can be used to validate an extinction coefficient for a particular sequence.
Because UV concentration assumes a known purity, it pairs with a chromatographic result. Every product we list carries its own published independent reverse-phase HPLC result, and vials are identified by crimp and cap color rather than by lot codes.
Frequently asked questions
Can A280 be used for a peptide with no tryptophan or tyrosine?
Not meaningfully. Without those residues the absorbance at 280 nm is near zero, so far-UV absorbance around 205 or 214 nm, or a different technique, is required.
Why count cystine rather than cysteine?
The 280 nm contribution comes from the disulfide bond linking two cysteines. Free thiols absorb very little at that wavelength.
Is A205 more accurate than A280?
It is more sensitive and works for any sequence, but it is more vulnerable to interference from counter-ions, buffers and solvents. Accuracy depends on matrix matching and a clean blank.
Why does a UV concentration differ from the weighed amount?
Weighed powder includes counter-ions, water and any impurities, while UV reports only the absorbing peptide in solution, calculated from an estimated coefficient.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.