【摘 要】
：
Photolyase enzyme absorbs blue light through an electron transfer reaction to repair the cyclobutane pyrimidine dimer(CPD)in DNA damaged by exposure to ultraviolet light,but an improved understanding
【机 构】
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College of Chemistry Beijing Normal University Beijing 100875 China
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Photolyase enzyme absorbs blue light through an electron transfer reaction to repair the cyclobutane pyrimidine dimer(CPD)in DNA damaged by exposure to ultraviolet light,but an improved understanding of its repair mechanism is required to provide a reasonable and unbiased interpretation for various steps in the photo-catalytic cycle.We used multi-configurational perturbation theory at the CASPT2//CASSCF//AMBER QM/MM level to propose an effective metallic-like conduction electron transfer mechanism through excited state proton motion controlled by intervening adenine and the assistance of a water wire.The formation of a double hydrogen bond locked minimum(DHBLM)through the change in the hydrogen bond pattern between CPD and its neighboring water molecule from single to double is determined to play the decisive role in lowering the energy level of the first carbon-carbon elongation,resulting in a downhill C5-C5 bond fission relaxation pathway.The released energy of single hydrogen bond recovery allows the second carbon-carbon(C6-C6)bond cleavage to proceed easily along a flat relaxation pathway through a hydrogen bond pattern change from 5 thymine-water-3 thymine in DHBLM to 3 thymine-water,which is followed by another downhill concerted asynchronous C6-C6 and C5-C5 bond splitting,producing the cleaved CPD.The productive/futile backward electron transfers are characterized by different downhill/moderate barrier relaxation pathways and are determined to occur from the 3/5 side,thereby ensuring a high repair quantum yield.The highly excited energy propagation in repaired/futile products to aqueous solvent is verified to contribute significantly to the nanosecond lifetime of FADH-in the DNA repair photo-cycle.
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