TRYPTOPHAN UV RESONANCE RAMAN EXCITATION PROFILES

We report the first quantitative UV resonance Raman excitation profiles of tryptophan (Trp) in the spectral region between 207 and 250 nm. The Trp excitation profiles are narrow and show maxima at ca. 224 nm which are 4 nm red-shifted from the ca. 220-nm Bb absorption band maximum. The excitation profile band shapes indicate that Trp can be selectively enhanced in proteins. We see destructive interference between the ca. 195-nm Ba and the 220-nm Bb electronic transitions. This destructive interference bestows a spectral window at ca. 205 nm with little Trp enhancement, thus permitting selective ca. 205-nm-excitation studies of protein amide modes to examine protein secondary structure. We use the Raman intensity data to examine excited-state Trp geometry changes relative to the ground state. We detail the UV photophysics of Trp and describe the energy flux conditions appropriate for studying ground-state Trp species. Our Trp excitation profiles dramatically differ from those of Fodor et al. (J. Am. Chem. Soc. 1989, 111, 5509) because we avoid the high pulse energy flux conditions which both depopulate the Trp ground-state concentration and excite the Raman spectra of transient photochemical species. © 1990 American Chemical Society.