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 t ryptophan fluorescence spectra in terms of static and tryptophan is th e key photophysical parameter for the interpretation dynamic dielectri c 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 tryptopha n containing peptide melittin, and 5-methoxytryptophan. The values ran ged from 5.9 to 6.2 +/- 0.4 Debye/f for NATA and melittin, where f rep resents the local field correction. The (1)L(b) \Delta<(mu)over right arrow>\ was much smaller. Application of Stark spectroscopy to these c hromophores required decomposition of the near-UV absorption into the (1)L(a) and (1)L(b) bands by measurement of the fluorescence excitatio n anisotropy spectrum and represents an extension of the method to sys tems where band overlap would normally preclude quantitative analysis of the Stark spectrum. The results obtained for 5-methoxytryptophan po int out limitations of this method of spectral decomposition. The rele vance of these results to the interpretation of steady-state and time- resolved spectroscopy of tryptophan is discussed.