The derivative 4-fluorotryptophan was confirmed to have negligible fluorescence at 25 degrees C and 285 nm (tryptophan/4-fluorotryptophan quantum-yield ratio greater than 100:1). However, photolysis experiments on tryptophan and 4-fluorotryptophan, in which loss of starting material was measured by reverse-phase h.p.l.c., demonstrated that 4-fluorotryptophan was significantly more photochemically active than the parent tryptophan, with the 4-fluorotryptophan photolysis quantum yield being 7 times larger than that of tryptophan at 25 degrees C and 285 nm. In addition, at 77 K and 275 nm 4-fluorotryptophan displayed strong fluorescence and phosphorescence, with emission quantum yields comparable with those of tryptophan at 77 K and 275 nm.

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