Practice question

Intrinsic protein fluorescence originates from aromatic side chains capable of absorbing UV and emitting without external dye. Tryptophan dominates because indole nucleus exhibits relatively high quantum yield near 0.2, excitation maximum near 280 nm, emission 308-355 nm highly sensitive to environment polarity. Phenylalanine quantum yield extremely low near 0.02, tyrosine emission often quenched via resonance energy transfer to tryptophan when both present. Extrinsic fluorophores like green fluorescent protein chromophore requires autocatalytic cyclization, DAPI and FAD are added ligands or redox cofactors. Hence tryptophan remains natural probe for folding, quenching and binding studies.