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◇Pyr. We analyzed the energetics of the elementary steps of the resulting splitting reaction Pyr◇Pyr + Rh(III) + hv → Rh(III) + 2Pyr based on results of semiempirical quantum chemical calculations (AMI and INDO/S). As a check, we also performed B3LYP hybrid density functional calculations on small- and medium-size model systems. The first excited states 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) exhibit intraligand charge-transfer character, featuring an electron hole in the phenantrene moiety of the phi ligand. Thus, this complex, when intercalating in the π stack of DNA, is ideally suited for reduction by ET from a pyrimidine photodimer in DNA. Environmental effects were found to play a crucial role in preventing thermal ET to a Rh(III) complex, but they favor back ET (BET) from Rh(II) to a pyrimidine cation radical that results from dinner splitting. A driving force for the ET reaction in a polar environment may be gained by increasing the ligand size of the Rh complex. Because of opposite environmental effects on the thermodynamics of the ET and BET reactions, a certain balance has to be kept between various characteristics of the whole system (excitation energy and ligand size of the Rh complex, polarity of the environment) to close the reaction cycle of the overall photorepair by restoring the Rh(III) state.