A Concerted Synchronous [2 + 2] Cycloreversion Repair Catalyzed by Two Electrons.

The current understanding of photoenzyme-catalyzed [2 + 2] cycloreversion repair of cyclobutane pyrimidine dimer (CPD) is that a photogenerated electron from the photolyase enzyme catalyzes the repair. This one-electron catalyzed repair is a sequential two-bond breaking cycloreversion of the cyclobutane center and involves a negative ion radical as an intermediate. Here, by resonantly capturing two exogenous low-energy electrons into the molecular field of a CPD, we show that the concerted synchronous two-bond breaking reaction, which is intermediate-free, and hence a safe repair, is feasible through two-electron catalysis.