Evidence for RET between Trp105 and bound ANS in the L105W–ANS complex. (A) Key: dotted line, corrected excitation spectrum (emission at 500 nm) of ANS (1.5 μM) in ethanol; dashed line, corrected excitation spectrum (emission at 500 nm) of the W17Y (5.0 μM)–ANS (2.5μM) complex; solid line, corrected excitation spectrum (emission at 500 nm) of the L105W (4.4 μM)–ANS (2.5 μM) complex. (B) Key: dashed line, excitation spectrum (emission at 348 nm) of L105W (4.4 μM) without ANS; solid line, difference excitation spectrum at an emission of 500 nm [spectrum of L105W–ANS – 0.67(spectrum of the W17Y–ANS complex shown in (A))]. The arrow indicates the position of the 0–0 band of the 1Lb electronic absorption transition of Trp.