Energetics of the splitting of pyrimidine photodimers induced by electron transfer to rhodium(III) complexes. A quantum chemical study

Electron transfer (ET) to Rh(III) complexes intercalated in DNA is known to initiate the photorepair of cyclobutane-type pyrimidine photodimers Pyr do uble left right arrow Pyr. We analyzed the energetics of the elementary ste ps of the resulting splitting reaction Pyro Pyr + Rh(III) + hv --> Rh(III) + 2Pyr based on results of semiempirical quantum chemical calculations (AM1 and INDO/S). As a check, we also performed B3LYP hybrid density functional calculations on small- and medium-size model systems. The first excited st ates of the complexes [Rh(NH3)(4)(phi)](3+) and [Rh(phi)(2)(dmb)](3+) (phi = 9,10-phenanthrenequinone diimine, dmb = 4,4'-dimethyl-2,2'-bipyridine) ex hibit intraligand charge-transfer character, featuring an electron hole in the phenantrene moiety of the phi ligand. Thus, this complex, when intercal ating in the pi stack of DNA is ideally suited for reduction by ET from a p yrimidine photodimer in DNA. Environmental effects were found to play a cru cial role in preventing thermal ET to a Rh(III) complex, but they favor bad e ET (BET) from Rh(II) to a pyrimidine cation radical that results from dim er splitting. A driving force for the ET reaction in a polar environment ma y be gained by increasing the Ligand size of the Rh complex. Because of opp osite environmental effects on the thermodynamics of the ET and BET reactio ns, a certain balance has to be kept between various characteristics of the whole system (excitation energy and ligand size of the RR complex, polarit y of the environment) to dose the reaction cycle of the overall photorepair by restoring the Rh(III) state. (C) 2000 John Wiley & Sons, inc.