Excited States of Nucleic Acids at 300K and Electronic Energy Transfer
Investigation of the Excited States of Nucleic Acids appears to be a major step in the understanding of the photochemical changes induced in DNA by ultraviolet radiation. The details of the mechanisms initiated by the absorption of a photon by a base and ending with the formation of a photoproduct on the same or on another base cannot be understood without a knowledge of their excited states. This, together with the amount of information which can be obtained on their ground states as well, certainly accounts for the fact that many luminescence studies have been carried out on nucleic acids for the last ten years. However the main feature of these molecules is that the systems are quenched to a high degree under physiological conditions. The fluorescence quantum yields are so weak that until recently nucleic acid bases were simply considered not to fluoresce at room temperature. In this respect, they differ from many aromatic compounds for which internal conversion from the first excited singlet state is unimportant. Therefore, most of the work has been performed either at extreme pH values where the nucleic bases exhibit measurable fluorescence emission or at 77K in glasses where the quantum yields are of the order of 10−1 or 10−2, thus permitting normal recording of the luminescence spectra. Under such conditions a good understanding of the lowest excited singlet and triplet states has been thus achieved (for a review, see for example Guéron et al (1).
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