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

Abstract

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) + hν → 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 states of the complexes [Rh(NH 3 ) 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 dimer 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.

Local Excitation of the 5-Bromouracil Chromophore in DNA. Computational and UV Spectroscopic Studies

“…The stabilization of the CT states can be estimated with the Bell model. 36 Using a value of 4.8 for the dielectric constant (see, for instance, the modeling of electron transfer between rhodium(III) complexes and the pyrimidine photodimer in DNA) 37 and cavity radius 5 Å, the estimated stabilization of the CT state with respect to the locally excited states is 0.06 and 0.03 eV for A Br U and G Br U, respectively. Even if this shift is included in the calculation, the excitation energy of the CT states lies only at the tail of our measured difference spectra.…”

Synthesis, Characterization, and Photochemical Properties of Azobenzene-Conjugated Ru(II) and Rh(III) Bis(Terpyridine) Complexes

“…Molecular Orbital Calculation. Theoretical calculations have been carried out to determine the trans−cis reaction path of azobenzenes, ,16c, and to investigate the photochemistry and energy transfer of the complexes . In the present study, we calculated the molecular orbitals at the optimized structure of the azobenzene, Rh, and Ru complexes (trans form) at the semiempirical ZINDO/1 level.…”

Intermediate neglect of differential overlap for spectroscopy

“…The INDO/S scheme has been extended to other main‐group elements and TMs 3. − 6,22 Parameters for f elements were also developed 7,8. The number of atomic parameters considerably increases by passing from electronic configurations s k p m to s k p m d n and especially to s k p m d n f t .…”