Stark effect spectroscopy of tryptophan

The change in permanent dipole moment (IA1p I) for the transition from the 1La state to the ground state of tryptophan is the key photophysical parameter for the interpretation of tryptophan fluorescence spectra in terms of static and 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/ffor NATA and melittin, where f represents the local field correction.The 1Lb AlP was much smaller.Application of Stark spectroscopy to these chromophores required decomposition of the near-UV absorption into the 1La and 'Lb 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.where (Av) (AvA) -(AVF), and (AvA) and (AvF) are the shifts of the 0-0 absorption and fluorescence transition en- ergies, respectively, due to electrostatic interactions with the medium.These shifts are measured relative to the values obtained in a nonpolar solvent.The quantities Me and Mg are the excitedand ground-state permanent dipole moments, and A'i = Me-Mg.E0 is the static dielectric constant, n is the refractive index, a is the cavity radius of the solvent cavity (typically assumed to be spherical) required to accommodate the chromophore, and h is Planck's constant.This equation remains extremely useful but suffers from the same short- comings as all relations based on a continuum dielectric model of a solvent: all perturbations due to specific inter-