Tryptophan (W) is the Amino Acid That Absorbs Ultraviolet Light
Among the choices given, tryptophan (W) is the amino acid that can absorb ultraviolet light. 1
Biochemical Basis for UV Absorption
Tryptophan is one of three aromatic amino acids (along with tyrosine and phenylalanine) that absorb UV light, but it has the strongest absorption properties:
- Tryptophan has an absorbance maximum at 280 nm 1
- Tyrosine has an absorbance maximum at 275 nm 1
- Phenylalanine has an absorbance maximum at 258 nm 1
The UV absorption of tryptophan is primarily due to its indole ring structure, which contains conjugated double bonds that efficiently absorb ultraviolet radiation.
Photochemical Properties of Tryptophan
When exposed to UV light, particularly UVA radiation, tryptophan undergoes photochemical reactions:
- Tryptophan residues can be oxidized to form N-formylkynurenine and hydroxytryptophan 2
- The first step typically involves electron transfer from the amino acid to a photosensitizer triplet excited state 2
- This photochemical reactivity makes tryptophan particularly sensitive to UV exposure compared to other amino acids
Applications in Protein Analysis
The UV absorption properties of tryptophan are widely used in biochemical analysis:
- Protein concentration can be determined by measuring absorbance at 280 nm, which is primarily due to tryptophan residues 1
- The ratio of absorbance at different wavelengths [A280/A275 + A280/A258] can be used as a probe of protein folding state 1
- Changes in tryptophan fluorescence can indicate protein conformational changes
Biological Significance
Tryptophan networks in biological systems have important functional roles:
- Tryptophan residues are found in transmembrane proteins, cytoskeletal filaments, and photoreceptor complexes 3
- Under UV excitation, networks of tryptophan can exhibit cooperative effects including superradiance 3
- These properties may play roles in cellular signaling and control mechanisms 3
Comparative UV Sensitivity
When comparing the amino acid options in the question:
- W (tryptophan): Strong UV absorber with maximum at 280 nm 1
- S (serine): No significant UV absorption
- K (lysine): No significant UV absorption
- M (methionine): No significant UV absorption
- N (asparagine): No significant UV absorption
The answer is clearly tryptophan (W), as it is the only amino acid among the choices that strongly absorbs ultraviolet light.