Chris Middleton

DNA Excited-State Dynamics: From Single Bases to the Double Helix

Ultraviolet light is strongly absorbed by DNA, producing excited electronic states that sometimes initiate damaging photochemical reactions. Fully mapping the reactive and nonreactive decay pathways available to excited electronic states in DNA is a decades-old quest. Progress toward this goal has accelerated rapidly in recent years, in large measure because of ultrafast laser experiments. Here we review recent discoveries and controversies concerning the nature and dynamics of excited states in DNA model systems in solution. Nonradiative decay by single, solvated nucleotides occurs primarily on the subpicosecond timescale. Surprisingly, excess electronic energy relaxes one or two orders of magnitude more slowly in DNA oligo- and polynucleotides. Highly efficient nonradiative decay pathways guarantee that most excited states do not lead to deleterious reactions but instead relax back to the electronic ground state. Understanding how the spatial organization of the bases controls the relaxation of excess electronic energy in the double helix and in alternative structures is currently one of the most exciting challenges in the field.

The inelastic incoherent neutron spectrum of crystalline oxamide: experiment and simulation of a solid

Abstract The inelastic neutron scattering spectrum of crystalline oxamide, CO(NH 2 )–CO(NH 2 ), is reported and compared with the results of Hartree–Fock and density functional theory calculations on a pentamer cluster model and the results of Car–Parrinello molecular dynamics calculations for the periodic crystal. All four hydrogen atoms of this centrosymmetric, planar sheet molecular crystal are involved in CO–H–N hydrogen bonds. It is shown that all three of these simulations provide reasonable descriptions of the inelastic neutron scattering spectra and thus of the hydrogen bond dynamics. There is no evidence for unusual structural or dynamic effects beyond those typically associated with H-bond formation. It is argued that a polymolecular approach is needed in order to provide an adequate treatment of such strongly hydrogen bonded systems.