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 (B b ) 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 À1 with a 1230 cm À1 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 L a state by previous studies that used computational simulations. The longtime dynamics of Trp manifests as a conformational heterogeneity at shorter times and contributes to inhomogeneous broadening of the Raman profiles (315 cm À1 ).

Deep ultraviolet initiated excited state dynamics of riboflavin and flavin mononucleotide

“…It is attributed to the vibronic coupling of these UVRR modes with nearby high energy electronic state, which is not involved in resonance enhancement of UVRR bands. These types of shifts in experimental REP's with respect to the absorption maximum are previously reported in tryptophan and 6‐chloroguanine molecules upon deep UV excitations. We have shown that there are two weak electronic transitions (one higher and one lower in energy) close to the electronic state centered at around 266 nm (see Figure b).…”

“…The inertial solvent response time obtained in our simulation (26.5 fs) for RF and FMN is in the same order as earlier reports on 6‐chloroguanine (~15 fs) and tryptophan (~8 fs) . We have used same solvent correlation time (26.5 fs) for both the molecules, as we have shown in earlier reports that our model is not very sensitive to the timescale of dynamic solvent response …”

“…Time‐dependent stokes shift measurements on tryptophan and organic dyes have reported that a significant amount of stokes shift always happens in tens of femtoseconds, much before the fluorescence measurement . The inertial solvent response time obtained in our simulation (26.5 fs) for RF and FMN is in the same order as earlier reports on 6‐chloroguanine (~15 fs) and tryptophan (~8 fs) . We have used same solvent correlation time (26.5 fs) for both the molecules, as we have shown in earlier reports that our model is not very sensitive to the timescale of dynamic solvent response …”

“…The detailed process of UVRR cross section determination has been discussed elsewhere . The experimental absolute Raman cross section of each Raman band was estimated from the known Raman cross sections of the internal standard sodium sulfate using the formula given below: where N and S stand for sample and internal standard, respectively.…”

Reorganization energy and Stokes shift calculations from spectral data as new efficient approaches in distinguishing the end point of micellization/aggregation

“…26 But when the uorophore moves from the bulk aqueous phase to an organized assembly, its corresponding reorientation time slows down to 10 À9 second. So the difference in time scale affects the dynamics, and literature studies have indicated 27 that this could be related to the mode-specic reorganization of solvents. Hence there will be contributions from both the fast and inertial responses of solvent molecules which are manifested as summative to the change in the reorganization energy.…”