Three amino acids absorb UV light strongly enough to matter: tryptophan, tyrosine, and phenylalanine. All three contain aromatic ring structures, which is the key feature that allows them to interact with ultraviolet wavelengths. Among the 20 standard amino acids, these are the only ones with significant absorption in the near-UV range (250 to 300 nm), making them essential to how scientists measure and study proteins.
The Three Aromatic Amino Acids
Each aromatic amino acid absorbs UV light at a slightly different peak wavelength, and with different intensity. Tryptophan absorbs most strongly, with a peak near 280 nm and a molar extinction coefficient of 5,540 at that wavelength. Tyrosine peaks around 274 to 277 nm but absorbs roughly a quarter as strongly, with an extinction coefficient of 1,480. Phenylalanine absorbs at shorter wavelengths, peaking around 257 nm in its characteristic fine-structured band, and its absorption is the weakest of the three.
In practical terms, tryptophan dominates the UV signal of most proteins. A protein with even one or two tryptophan residues will have a measurable absorption peak near 280 nm. Tyrosine contributes meaningfully when present in larger numbers. Phenylalanine’s contribution at 280 nm is so small that it’s usually ignored in routine measurements, though it does absorb in the far-UV region around 204 to 210 nm.
Why Aromatic Rings Absorb UV
The reason these three amino acids absorb UV light, while the other 17 don’t, comes down to their ring structures. Aromatic rings contain electrons that are delocalized, meaning they aren’t locked in place between two specific atoms but instead spread across the entire ring system. These loosely held electrons can jump to higher energy states when they absorb a photon of UV light. The energy gap between the ground state and the excited state happens to match the energy carried by UV photons, which is why the absorption occurs specifically in the ultraviolet range.
Tryptophan has the largest aromatic system of the three, an indole ring with two fused rings. This extended system shifts its absorption to longer wavelengths and increases its intensity. Tyrosine has a phenol ring (a six-membered ring with a hydroxyl group attached), and phenylalanine has the simplest structure: a plain benzene ring. The larger and more electron-rich the ring system, the stronger and more red-shifted the absorption.
Other UV-Absorbing Groups in Proteins
Beyond the big three, two other parts of a protein can absorb UV light. Disulfide bonds, formed when two cysteine residues link together, absorb weakly near 250 nm. Their contribution is small but can be relevant in proteins with many disulfide bridges. When calculating a protein’s expected UV absorption, the number of disulfide bonds is sometimes included alongside tryptophan and tyrosine counts.
The peptide bond itself, the backbone linkage between every pair of amino acids, absorbs strongly near 190 to 210 nm. This falls in the far-UV range rather than the near-UV range. That absorption is the basis for circular dichroism spectroscopy, a technique used to study protein folding, but it sits at wavelengths too short to be useful for routine concentration measurements.
How pH Shifts the Absorption
Tyrosine’s absorption spectrum is notably sensitive to pH. At neutral pH, tyrosine’s phenol group is protonated and absorbs at 277 nm. When the pH rises high enough to remove that proton (above roughly pH 10), the peak shifts to 294 nm and becomes more intense. This 17 nm red shift happens because the negatively charged phenolate form has more electron density available in its ring, which lowers the energy needed for excitation.
This pH sensitivity is actually useful. Researchers can track the ionization state of tyrosine residues inside a protein by watching for this spectral shift, which reveals information about the local chemical environment around each tyrosine. Nearby charged groups from other amino acids can raise or lower the pH at which this shift occurs, so the exact behavior varies depending on where the tyrosine sits in the protein’s structure.
The Local Environment Matters
The absorption peaks listed above are averages measured for free amino acids in water. Inside a folded protein, the actual wavelengths can shift. When an aromatic residue is buried in the hydrophobic interior of a protein, away from water, its absorption typically shifts to slightly longer wavelengths compared to the same residue sitting on the protein surface exposed to water. A polar environment like water tends to stabilize different electronic states than a nonpolar one, altering which wavelengths are absorbed and by how much.
These solvent-dependent shifts are generally small, on the order of a few nanometers, but they’re large enough to be detected with careful spectroscopy. This is one reason why UV absorption spectra can reveal whether a protein is properly folded or has become denatured. When a protein unfolds, buried tryptophan and tyrosine residues suddenly become exposed to water, and their absorption characteristics change accordingly.
Practical Use: Measuring Protein Concentration
The most common real-world application of amino acid UV absorption is measuring how much protein is in a solution. Scientists routinely measure absorbance at 280 nm (often called A280) and apply Beer-Lambert’s law to calculate concentration. The formula is straightforward: absorbance equals the extinction coefficient multiplied by the concentration and the path length of light through the sample.
To use this method, you need to know the protein’s extinction coefficient, which can be calculated from its amino acid sequence. You count the number of tryptophan residues, multiply by 5,540; count the tyrosine residues, multiply by 1,480; add a small correction for any disulfide bonds; and sum the results. This gives a predicted extinction coefficient that’s accurate enough for most laboratory work. The approach assumes the protein is either fully denatured (so every residue behaves like a free amino acid) or that the native-state coefficient has been experimentally calibrated.
This method is fast, nondestructive, and requires no reagents, which is why it remains one of the most widely used assays in biochemistry. Its main limitation is that proteins with no tryptophan or tyrosine residues have negligible absorbance at 280 nm, and proteins with very few aromatic residues give unreliable readings at low concentrations.