3-PHOTON EXCITATION OF A TRYPTOPHAN DERIVATIVE USING A FS-TI-SAPPHIRELASER

We found that N-acetyl-L-tryptophanamide (NATA) could be excited at 84 0 nm using a mode-locked Ti : sapphire laser. The emission spectra of NATA were the same for one-photon (1 hv) excitation at 280 nm as for e xcitation at 840 nm. The emission intensity of NATA was found to depen d on the cube of the laser power at 840 nm, consistent with simultaneo us absorption of three 840-nm photons. The intensity-decay times were the same for 280- and 840-nm excitation, suggesting the same excited s tate is reached for each mode of excitation. However, the anisotropies were different for 280- and 840-nm excitation. At 280 nm the time-zer o anisotropy was 0.130, whereas at 840 nm the time-zero anisotropy was negative (-0.06), indicating different orientations of the transition moments for one- and three-photon excitation. These results suggest t hat the intrinsic fluorescence of proteins may be excited using the fu ndamental output of a Ti : sapphire laser. (C) 1996 John Wiley & Sons, Inc.