Excited States of Nucleotides and Singlet Energy Transfer in Polynucleotides

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Abstract
We have measured the absorption, fluorescence, and phosphorescence spectra of nucleotides at 77°K. The order of decreasing energies is A, U, T, G, C for the singlet, and U−, C, G, T−, A, T for the triplet. Besides the quantum yields of fluorescence and phosphorescence, we also determined the yield for intersystem crossing to the triplet, using the amplitude of the Δm=2 triplet ESR signal. We find internal conversion occurring from both the singlet and triplet states. From the excitation energies in the neutral and ionized molecules, the stability of the ionizable proton in the excited states can be compared to that in the ground state, and the results are applied to the question of proton transfer from T to A in DNA. From a discussion of the various cases of intermolecular energy transfer for singlets, and considering the broadband spectra of the nucleotides, it is shown that Förster's ``very weak case'' is applicable. We emphasize that two subcases must be distinguished, depending on whether transfer occurs faster than, or slower than, vibrational relaxation. The justification of the approximations in our treatment is given in the Appendix. Assuming dipole—dipole coupling, the rate of transfer is estimated in the nucleotides in the ``before relaxation'' subcase. In the ``after relaxation'' subcase the rate of transfer is calculated using the emission data obtained in this study. In a majority of the pairs of nucleotides, ``after relaxation transfer'' at the Watson—Crick distances is allowed. These considerations are discussed briefly in reference to transfer in polyadenylic acid and DNA.

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