Molecular Mechanism of Flavin Photoprotection by Archaeal Dodecin: Photoinduced Electron Transfer and Mg2+-Promoted Proton Transfer.

Photoinduced biochemical reactions are ubiquitously governed by derivatives of flavin, which is a key player in a manifold of cellular redox reactions. The photoreactivity of flavins is also one of their greatest disadvantages as the molecules are sensitive to photodegradation. To prevent this unfavorable reaction, UV-light-exposed archaea bacteria, such as Halobacterium salinarum, manage the task of protecting flavin derivatives by dodecin, a protein which stores flavins and efficiently photoprotects them. In this study, we shed light on the photoprotection mechanism, i.e., the excited state quenching mechanism by dodecin using computational methodology. Molecular dynamics (MD) simulations unraveled the hydrogen bond network in the flavin binding pocket as a starting point for proton transfer upon preceding electron transfer. Using high-level ab initio quantum chemical methods, different proton transfer channels have been investigated and an energetically feasible Mg2+-promoted channel has been identified fully explaining previous experimental observations. This is the first extensive theoretical study of archaeal dodecin, furthering the understanding of its photocycle and manipulation.

Explaining level inversion of the La and Lb States of indole and indole derivatives in polar solvents.

Quantum chemical methods are used to study the solvent effects on the spectra of indole and a series of methyl-substituted indoles. We focus on the low-lying L(a) and L(b) states and study their interplay. We find that the solvent mainly affects emission from the L(a) state, by stabilizing its energy in its excited-state geometry. The stabilization of the L(a) state increases with increasing solvent polarity, which accounts for the large fluorescence shift observed in indoles and leads to an inversion in the nature of the lowest emitting state, from L(b) in vacuum to L(a) in water. To the best of our knowledge, this is the first theoretical evidence for level inversion done for a series of indoles. The underlying mechanism of level inversion is analyzed in detail. The usual interpretation of level inversion in terms of their static dipole moment is criticized and an improved predictive measurement is suggested.

Ballistic charge transport through bio-molecules in a dissipative environment.

The question whether dissipative bio-molecular systems can support efficient coherent (phase-conserving) charge transport is raised again following recent experiments on electron-energy transfer in bio-molecules. In this work we formulate conditions under which the current due to coherent ballistic resonant charge transport through DNA molecular junctions can be measured in spite of coupling to the dissipative environment.