Base Composition Dependence of DNA Hypochromism

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Abstract
The low energy region hypochromicities of poly (dA) poly (dT) and poly (dG) poly (dC) were calculated to compare three coupling formalisms currently used to describe the electronic structure of polymers: first-order perturbation theory (FOPT), configuration interaction of singly excited monomer states (CI), and time dependent Hartree theory (TDH). Several scaled sets of monomer molecular orbitals, at the Hückel (HMO) and Pariser-Parr-Pople (PPP) levels, were used to describe the low energy transitions, and the Coulomb potential was represented with distributed monopoles. The HMO set fails to give the observed base composition dependence of hypochromism (HAT > HGC) for all three formalisms. The PPP sets give the correct order for all formalisms, and the difference between the experimental and calculated hypochromism converges to the same value in AT and GC helices as the power of the coupling theory increases (FOPT < CI < TDH), indicating a base independent contribution from solvent, backbone, and vacuum uv monomer states.