Symposium‐in‐Print: DNA Photodynamics Introduction

The bases in DNA absorb UV light strongly producing excited electronic states that can initiate damaging photochemical reactions. Unrepaired damage can cause diseases such as skin cancer. Interest in the threat to living organisms posed by solar UV light has grown in recent years. The incidence of skin cancer, the most common cancer in North America, has been climbing for decades. In addition, depletion of stratospheric ozone has led to regionally enhanced levels of UV-B sunlight, which is more injurious to DNA than the longer wavelength, but more abundant UV-A rays. Growing interest in UV photodamage has rekindled research into excited electronic states of DNA. Although the molecular structures of many of the photoproducts formed in DNA by UV light have been known for decades, the mechanisms behind their formation are still obscure. Excited states produced by UV photons are at the beginning of the complex pathways that lead to photodamage. Consequently, much of the difficulty in understanding the initial steps behind DNA photodamage results from a lack of knowledge of the deactivation pathways of excited electronic states in DNA. Investigation of excited electronic states of nucleic acids began in earnest in the 1960s. Many influential original research (1–7) and review articles (8–10) have appeared since then in Photochemistry and Photobiology. The six articles in this Symposium-in-Print on Excited States of DNA continue this tradition and provide a flavor of the rapid progress made in recent years. The authors of the papers in this symposium-inprint were speakers at a Pacifichem symposium on Photophysical Dynamics of Biological Molecules, which took place in December 2005 in Honolulu, Hawaii. This is a small and necessarily arbitrary subset of the considerably larger community of researchers who are actively investigating DNA excited states. Nevertheless, the papers collected in this issue define and address someof the newly emerging frontiers in the longstanding quest to understand excited electronic states in DNA from first principles. Photoproduct formation is just one of a number of nonradiative decay pathways for excited electronic states in DNA. Furthermore, molecular evolution has likely constrained it to be a minor pathway in order to make the genome as resilient as possible to photochemical damage. The quantum yields of photodamage are very low, making the detection and study of photoproducts difficult. In the first paper of this issue, Kundu and Loppnow (11) report a UV resonance Raman study of 8-oxo-deoxyguanosine, the most important oxidative lesion in DNA. They show that this guanosine-derived photoproduct has a distinctive and unique resonance-enhanced Raman spectrum, which could be the basis for a new and sensitive detection technique. Invariably, improved techniques for quantifying DNA photolesions have revised understanding of how DNA photoproducts are distributed throughout the genome. A striking aspect of DNA photophysics is the ultralow fluorescence quantum yields of single bases and base multimers. The remarkably low quantum yields for fluorescence are the result of highly efficient nonradiative decay, which is thought to be a major factor behind genomic photostability. Support for this idea was the experimental demonstration in 2000 by Pecourt et al. (12) that the singlet excited state lifetimes of monomeric bases are less than 1 ps in aqueous solution. Since then considerable work has been devoted to understanding the mechanism behind ultrafast nonradiative decay by the building blocks of DNA. A variety of ultrafast spectroscopic techniques have now been used to directly measure the dynamics of excited electronic states. These techniques provide exquisite insights into the time scale of excited-state deactivation, but cannot yet probe the underlying nuclear motions, which cause a photoexcited molecule to return nonradiatively to the electronic ground state. For this reason, efforts by theoreticians have been indispensable. Quantum chemical calculations like those reported in the second and third papers in this collection, by Blancafort (13) and Kistler and Matsika (14), respectively, provide a molecular-level view of nonradiative decay, which is arguably unsurpassed in molecules of this size and complexity. Both papers illustrate the remarkable capability to now map various pathways on multiple excited-state potential energy surfaces of these relatively large polyatomic molecules all at a relatively high level of theory. Theoretical studies like these also provide a straightforward way to compare theory with the many gas-phase experiments on isolated nucleobases and small nucleobase clusters. Blancafort (13) investigates several nonradiative decay pathways for electronically excited cytosine. A key concept is the existence of conical intersections connecting different electronic states. The energetically most favored pathway involves torsion about the C5,C6 double bond of cytosine. This motion resembles the widely discussed nonradiative decay pathway found in electronically excited ethylene. Kistler and *Corresponding author email: kohler@chemistry.ohio-state.edu (Bern Kohler) DOI: 10.1111/j.1751-1097.2007. 00131.x 2007 American Society for Photobiology 0031-8655/07 Photochemistry and Photobiology, 2007, 83: 592–594