photolyase 
 contains a flavin cofactor in a fully reduced form as 
 its functional state to repair ultraviolet - damaged dna upon blue light 
 absorption . 
 however , after purification , the cofactor exists in its 
 oxidized or neutral semiquinone state . 
 such oxidization eliminates 
 the repair function , but it can be reverted by photoreduction , a photoinduced 
 process with a series of electron - transfer ( et ) reactions . with femtosecond 
 absorption spectroscopy and site - directed mutagenesis , 
 we completely 
 recharacterized such photoreduction dynamics in the semiquinone state . 
 comparing with all previous studies , we identified a new intramolecular 
 et pathway , determined stretched et behaviors , refined all et time 
 scales , and finally evaluated the driving forces and reorganization 
 energies for eight elementary et reactions . 
 combined with the oxidized - state 
 photoreduction dynamics , we elucidated the different active - site properties 
 of the reduction ability and structural flexibility in the oxidized 
 and semiquinone states , leading to the dramatically different et dynamics 
 and photoreduction efficiency in the two states .