Mode-Specific Reorganization Energies and Ultrafast Solvation Dynamics of Tryptophan from Raman Line-Shape Analysis

Abstract

Tryptophan is widely used as an intrinsic fluorophore for studies of protein structure and dynamics. Its fluorescence is known to have two decay components with lifetimes of 0.5 and 3.1 ns. In this work we measure the ultrafast dynamics of Tryptophan at <100 fs through measurements and modeling of the Raman excitation profiles with time-dependent wave packet propagation theory. We use a Brownian oscillator model to simulate the water-tryptophan interaction. Upon photoexcitation to the higher singlet electronic state (Bb) the structure of tryptophan is distorted to an overall expansion of the pyrrole and benzene rings. The total reorganization energy for Trp in water is estimated to be 2169 cm−1 with a 1230 cm−1 contribution from the inertial response of water. The value of reorganization energy of water corresponding to the fast response is found to be higher than that obtained upon excitation to the La state by previous studies that used computational simulations. The long-time dynamics of Trp manifests as a conformational heterogeneity at shorter times and contributes to inhomogeneous broadening of the Raman profiles (315 cm−1).