Practice question

Proteins absorb ultraviolet due to aromatic residues, yet tryptophan dominates intrinsic fluorescence because energy absorbed by phenylalanine and tyrosine transfers efficiently to tryptophan with higher quantum yield. Since tryptophan is encoded in polypeptide sequence, its emission provides intrinsic signal without external labeling, sensitive to nearby quenchers and solvent polarity. DAPI intercalates DNA, green fluorescent protein chromophore forms by autocatalytic maturation, and ANS binds hydrophobic patches, all extrinsic probes requiring addition. Intrinsic tryptophan fluorescence enables label-free studies of folding, binding and dynamics.