All amino acids absorb at 200-220 nm due to the C=O group. Additionally, based on the solvent and concentration of the sample, hydrogen bonding may be present increasing the observed nm.

If you are using an aromatic amino acid (Tyr, Phe, Trp), it absorbs UV with two different peaks. The peak at ~270-280 nm is due to the conjugation system of the aromatic ring and the absorbance peak at lower wavelengths is attributed to carboxylic acid moiety of the amino acid.

Tryptophan

And many thanks! So, if I see decrease in the 220nm absorbance and broadening of the the absorbance ~260nm this will suggest change in the carboxylic side? I Oxidation?

Yoram

A single amino acid such as Trp also has an absorbance at 220 nm.