STARK-EFFECT SPECTROSCOPY OF TRYPTOPHAN

The change in permanent dipole moment (\Delta<(mu)over right arrow>\) for the transition from the (1)L(a), state to the ground state of of tryptophan fluorescence spectra in terms of static and tryptophan is the key photophysical parameter for the interpretation dynamic dielectric properties of the surrounding medium. We report measurement of this parameter by means of electric field effect (Stark) spectroscopy for N-acetyl-L-tryptophanamide (NATA) in two solvents, the single tryptophan containing peptide melittin, and 5-methoxytryptophan. The values ranged from 5.9 to 6.2 +/- 0.4 Debye/f for NATA and melittin, where f represents the local field correction. The (1)L(b) \Delta<(mu)over right arrow>\ was much smaller. Application of Stark spectroscopy to these chromophores required decomposition of the near-UV absorption into the (1)L(a) and (1)L(b) bands by measurement of the fluorescence excitation anisotropy spectrum and represents an extension of the method to systems where band overlap would normally preclude quantitative analysis of the Stark spectrum. The results obtained for 5-methoxytryptophan point out limitations of this method of spectral decomposition. The relevance of these results to the interpretation of steady-state and time-resolved spectroscopy of tryptophan is discussed.