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| - Dynamic Conformations of Flavin Adenine Dinucleotide: Simulated Molecular Dynamics ofthe Flavin Cofactor Related to the Time-Resolved Fluorescence Characteristics
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| - Molecular dynamics (MD) simulations and polarized subnanosecond time-resolved flavin fluorescencespectroscopy have been used to study the conformational dynamics of the flavin adenine dinucleotide (FAD)cofactor in aqueous solution. FAD displays a highly heterogeneous fluorescence intensity decay, resulting inlifetime spectra with two major components: a dominant 7-ps contribution that is characteristic of ultrafastfluorescence quenching and a 2.7-ns contribution resulting from moderate quenching. MD simulations wereperformed in both the ground state and first excited state. The simulations showed transitions from “open”conformations to “closed” conformations in which the flavin and adenine ring systems stack coplanarly.Stacking generally occurred within the lifetime of the flavin excited state (4.7 ns in water), and yielded asimulated fluorescence lifetime on the order of the nanosecond lifetime that was observed experimentally.Hydrogen bonds in the ribityl−pyrophosphate−ribofuranosyl chain connecting both ring systems form highlystable cooperative networks and dominate the conformational transitions of the molecule. Fluorescencequenching in FAD is mainly determined by the coplanar stacking of the flavin and adenine ring systems,most likely through a mechanism of photoinduced electron transfer. Whereas in stacked conformationsfluorescence is quenched nearly instantaneously, open fluorescent conformations can stack during the lifetimeof the flavin excited state, resulting in immediate fluorescence quenching upon stacking.
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