Non-adiabatic excited state dynamics of riboflavin after photoexcitation

Abstract

Flavins are chromophores in light-gated enzymes and therefore central in many photobiological processes. To unravel the optical excitation process as the initial, elementary step towards signal transduction, detailed ultrafast (femtosecond) experiments probing the photo-activation of flavins have been carried out recently [Weigel et al., J. Phys. Chem. B, 2011, 115, 3656-3680.]. The present paper contributes to a further understanding and interpretation of these experiments by studying the post-excitation vibrational dynamics of riboflavin (RF) and microsolvated riboflavin, RF·4H2O, using first principles non-adiabatic molecular dynamics. By analyzing the characteristic atom motions and calculating time-resolved stimulated emission spectra following ππ* excitation, it is found that after optical excitation C-N and C-C vibrations in the isoalloxazine rings of riboflavin set in. The Franck-Condon (vertically excited) state decays within about 10 fs, in agreement with experiment. Anharmonic coupling leads to Intramolecular Vibrational energy Redistribution (IVR) on the timescale of about 80-100 fs, first to (other) C-C stretching modes of the isoalloxazine rings, then by energy spread over the whole molecule, including low-frequency in-plane modes. The IVR is accompanied by a red-shift and broadening of the emission spectrum. When RF is microsolvated with four water molecules, an overall redshift of optical spectra by about 20 nm is observed but the relaxation dynamics is only slightly affected. For several trajectories, a tendency for hydrogen transfer from water to flavin-nitrogen (N5) was found.

Deep ultraviolet initiated excited state dynamics of riboflavin and flavin mononucleotide

“…Short‐time dynamics of RF in the S 1 state was studied by employing nonadiabatic molecular dynamics simulations . The decay time of the Franck–Condon state was found to be 10 fs, in excellent agreement with the experimental report .…”

“…We excited both the molecules within this 266‐nm absorption band using 10 different UV excitations at 257.6, 259.9, 263, 265.9, 268, 270.4, 272.9, 274.8, 277.4, and 280.1 nm. Theoretical calculations using various quantum chemical calculations have reported the MO's involved following excitation to the visible and UV excited states for different isoalloxazine derivatives. TDDFT calculations were done using B3LYP/TZVP/PCM method in DMSO on RF molecule to compute the vibrational spectra and excitation energies in the S 1 state .…”

“…Theoretical calculations using various quantum chemical calculations have reported the MO's involved following excitation to the visible and UV excited states for different isoalloxazine derivatives. TDDFT calculations were done using B3LYP/TZVP/PCM method in DMSO on RF molecule to compute the vibrational spectra and excitation energies in the S 1 state . Here, we have also applied this method in water (using PCM solvent model) on RF to determine the excitation energies and the nature of MO's involved upon photoexcitation to the excited state inside the 266‐nm absorption band and compared with other results.…”

“…Understanding the photo‐induced phenomenon in the flavin family is fundamentally important to elucidate the role of these molecules in the context of their biological functions. Several groups have provided significant insight to both experimental and theoretical research in the area of flavin excited state photophysics. Flavins have four electronic excitations at ~222, 266, 373, and 445 nm, which are of ππ* character (Figure b).…”

Trajectory‐Based Nonadiabatic Dynamics with Time‐Dependent Density Functional Theory

“…TSH/LR-TDDFT has been applied to a multitude of different photophysical/photochemical processes of organic molecules (see refs. [14,[159][160][161][162][163][164] for a nonexhaustive list). The nonradiative relaxation of the photoexcited oxirane molecule is taken here as a prototypical example of TSH/LR-TDDFT dynamics.…”