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Photophysics of Structurally Modified Flavin Derivatives in the Blue‐Light Photoreceptor YtvA: A Combined Experimental and Theoretical Study
Author(s) -
SilvaJunior Mario R.,
Mansurova Madina,
Gärtner Wolfgang,
Thiel Walter
Publication year - 2013
Publication title -
chembiochem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.05
H-Index - 126
eISSN - 1439-7633
pISSN - 1439-4227
DOI - 10.1002/cbic.201300217
Subject(s) - chemistry , flavin group , flavin mononucleotide , density functional theory , conical intersection , photochemistry , intersystem crossing , hydrogen bond , adduct , qm/mm , computational chemistry , molecular dynamics , molecule , excited state , organic chemistry , physics , singlet state , nuclear physics , enzyme
Abstract The light‐induced processes of two flavin mononucleotide derivatives (1‐ and 5‐deaza flavin mononucleotide, 1DFMN and 5DFMN), incorporated into the LOV domain of YtvA protein from Bacillus subtilis , were studied by a combination of experimental and computational methods. Quantum mechanics/molecular mechanics (QM/MM) calculations were carried out in which the QM part was treated by density functional theory (DFT) using the B3LYP functional for geometry optimizations and the DFT/MRCI method for spectroscopic properties, whereas the MM part was described by the CHARMM force field. 1DFMN is incorporated into the protein binding site, yielding a red‐shifted absorption band ( λ max =530 nm compared to YtvA wild‐type λ max =445 nm), but does not undergo any LOV‐typical photoreactions such as triplet and photoadduct formation. QM/MM computations confirmed the absence of a channel for triplet formation and located a radiation‐free channel (through an S 1 /S 0 conical intersection) along a hydrogen transfer path that might allow for fast deactivation. By contrast, 5DFMN‐YtvA‐LOV shows a blue‐shifted absorption ( λ max =410 nm) and undergoes similar photochemical processes to FMN in the wild‐type protein, both with regard to the photophysics and the formation of a photoadduct with a flavin‐cysteinyl covalent bond. The QM/MM calculations predict a mechanism that involves hydrogen transfer in the T 1 state, followed by intersystem crossing and adduct formation in the S 0 state for the forward reaction. Experimentally, in contrast to wild‐type YtvA, dark‐state recovery in 5DFMN‐YtvA‐LOV is not thermally driven but can only be accomplished after absorption of a second photon by the photoadduct, again via the triplet state. The QM/MM calculations suggest a photochemical mechanism for dark‐state recovery that is accessible only for the adduct with a C4aS bond but not for alternative adducts with a C5S bond.