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UV Excited States of DNA: Photostability with a Twist

Speaker: Bern Kohler, Ohio Eminent Scholar, Professor, Department of Chemistry and Biochemistry, The Ohio State University Location: 402 N Blackford St. Indianapolis, IN 46202 LD 010

Host: Li

DNA absorbs UV radiation strongly, leading to the formation of excited electronic states that can decay to mutagenic photoproducts. In recent years, attention has focused on understanding why excited states in DNA strands are generally much longer-lived than excited states of monomeric nucleobases. Femtosecond transient absorption measurements with mid-infrared probing have revealed that UV excitation of single-stranded DNA efficiently forms charge transfer states in which an electron is transferred between stacked nucleobases. These states are identified by vibrational marker bands of nucleobase radical ions, which appear on a subpicosecond time scale and decay by charge recombination in tens to hundreds of picoseconds, depending on base sequence. In double-stranded DNA, intrastrand charge separation produces radical ion base pairs that undergo interstrand proton transfer in some sequences. Proton-coupled electron transfer (PCET) in the forward direction is complete in less than 200 fs, but charge recombination occurs on the picosecond time scale. Recent experiments suggest that reverse PCET occurs by a sequential mechanism in which rate-determining intrastrand electron transfer is followed by sub-picosecond proton transfer. In all DNA strands studied, the radicals and tautomeric base pairs formed transiently in high quantum yields by UV absorption decay efficiently and do not appear to pose a photochemical threat.