Calculation of quantum parameters for nonadiabatic redox reactions. Application to photoreduction of flavin in DNA photolyase

A simple practical way to account for the effect of quantum modes on electr on transfer (ET) is to use the Jortner expression for ET rate. The expressi on includes two quantum parameters which describe the properties of high-fr equency modes in the system. In our recent paper (J. Chem. Phys. 2000, 112, 9015), we developed a method to calculate these parameters for redox cofac tors from ab initio data on their potential energy surfaces. In this paper, we extend our method to include the solvent, which is treated as a continu ous dielectric medium. As an example, two reactions describing photoreducti on of the flavin cofactor in DNA photolyase (an enzyme that repairs thymine dimers in DNA) are investigated. We calculated the quantum parameters for each cofactor (flavin radical, tryptophan, tryptophan cation radical, and t yrosine), as well as for water, which was used as a model medium. and then obtained the dependence of ET rates k(ET) On the driving force Delta G(0) i n a wide range of Delta G(0). As expected, the quantum modes tend to flatte n the log k(ET) vs Delta G(0) dependence in the inverted region, and the ca lculation provides a quantitative estimate of this effect. A similar effect has been predicted to originate from inelastic tunneling (Medvedev, E. S.; Stuchebrukhov, A. A, J. Chem. Phys. 1997, 107, 3821). This effect makes k( ET) relatively insensitive to Delta G(0) variations, compared with the clas sical Marcus theory, and might be of importance for biological ET systems t o ensure their stable performance.