Ultrafast excited-state dynamics in nucleic acids.
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The scope of this review is the nature and dynamics of the singlet excited electronic states created in nucleic acids and their constituents by UV light. Interest in the UV photochemistry of nucleic acids has long been the motivation for photophysical studies of the excited states, because these states are at the beginning of the complex chain of events that culminates in photodamage. UV-induced damage to DNA has profound biological consequences, including photocarcinogenesis, a growing human health problem.1-3 Sunlight, which is essential for life on earth, contains significant amounts of harmful UV (λ < 400 nm) radiation. These solar UV photons constitute one of the most ubiquitous and potent environmental carcinogens. This extraterrestrial threat is impressive for its long history; photodamage is as old as life itself. The genomic information encoded by these biopolymers has been under photochemical attack for billions of years. It is not surprising then that the excited states of the nucleic acid bases (see Chart 1), the most important UV chromophores of nucleic acids, are highly stable to photochemical decay, perhaps as a result of selection pressure during a long period of molecular evolution. This photostability is due to remarkably rapid decay pathways for electronic energy, which are only now coming into focus through femtosecond laser spectroscopy. The recently completed map of the human genome and the ever-expanding crystallographic database of nucleic acid structures are two examples that illustrate the richly detailed information currently available about the static properties of nucleic acids. In contrast, much less is known about the dynamics of these macromolecules. This is particularly true of the dynamics of the excited states that play a critical role in DNA photodamage. Efforts to study nucleic acids by time-resolved spectroscopy have been stymied by the apparent lack of suitable fluorophores. In contrast, dynamical spectroscopy of proteins has flourished thanks to intrinsically fluorescent amino acids such as tryptophan, tyrosine, and phenylalanine.4 The primary UVabsorbing constituents of nucleic acids, the nucleic acid bases, have vanishingly small fluorescence quantum yields under physiological conditions of temperature and pH.5 In fact, the bases were frequently described as “nonfluorescent” in the early literature. * To whom correspondence should be addressed. E-mail: kohler@ chemistry.ohio-state.edu. Phone: (614) 688-3944. Fax: (614) 2921685. 1977 Chem. Rev. 2004, 104, 1977−2019