Charge and excitation migration in DNA chains

Abstract

A theoretical treatment of migration of electrons, holes, and singlet and triplet electronic excitations in ideal DNA helical chains is presented. A localized model is adopted, in which charge and excitation migration is considered to take place through discrete site to neighbor site transfer neglecting long‐range quantum mechanical coherence effects. Accordingly a probability function to describe the location of a charge or excitation is introduced. A stochastic equation is developed to describe the time evolution of this function and hence the average behavior of the migrating species. The formalism permits the introduction of base sequencing information through the use of probabilities for occurrence of each of the 16 distinct kinds of nearest‐neighbor pairs of bases in a single strand. Migration in a nonuniform but nonrandom DNA chain can then be treated to within a perturbation approximation. The fundamental base‐to‐base charge and excitation transfer parameters which enter the stochastic equation are calculated for each migrating species and possible pair of neighbor bases using SCF LCAO molecular orbital methods without configuration interaction. A noteworthy feature is the much larger magnitude of the triplet excitation exchange term in this work compared to previous work, leading to relatively large triplet transfer coefficients. Implications of the calculated transfer coefficients for excitation trapping are examined and found to be consistent with experiment. Numerical results for the time dependence of the probability function and for diffusion coefficients for migrating species in DNA's of biophysical interest are presented. The general aspect of these results is similar to that for a uniform DNA chain, but some important differences of detail are present. The mobility of electrons and holes moving along the DNA chain and the resultant electrical conductivity along the chain axis are estimated and found to be considerably larger than in organic crystals.