Electronic absorption and emission spectra of nucleic acids and their components: Some questions of theory and experiment

Abstract

The results of calculations of the transition energies and polarization in nucleotide bases performed by the CNDO/S CI method are compared with experimental data for long-wavelength and vacuumabsorption bands. The calculations and the analysis of experimental data testify to the existence of n -T* transitions in the first absorption bands of the bases. The study of double-stranded polynucleotides and DNA hypochromism based on the theoretical electronic characteristics of the bases and perturbation theory is performed. The role of stacking and complementary interactions within the hypochromic effect is cleared up. Two mechanisms for the shift of the fluorescence band maximum are investigated: two-proton transfer along H bonds and excimer formation. The study of H-bond potential curves shows the disadvantage of two-proton phototautomerism in the nucleotide base pairs in contrast to model systems. The possibility of excimer state formation in stacked homo-and heterodimers of nucleotide bases is shown within the extended-Huckel treatment. The nature of excimer minimum for the excited-state potential curve is analyzed.To understand the absorption and luminescence spectra of DNA and also the nature of the related phenomena (hypochromism, energy migration, excimer states formation, etc.), a quantum-mechanical study of electronic structure of nucleotide bases is necessary. A series of important spectroscopic features points out that the knowledge of velectronic wave functions is not sufficient. In this connection the study of the excited singlet and triplet states of the bases with the help of the C N D O /~ method, which permits us to directly take into account the interaction of all valence electrons, has recently been undertaken [l, 21. To obtain detailed information on the electronic structure of the bases, the calculations of the ground and excited states of adenine (A), thymine (T), uracil (U), guanine (G), and The calculated energies of electronic transitions into singlet excited states A E and their oscillator strengths (f) are given in Tables I and 11. Experimental data for the absorption band maxima of the bases in the water solutions [5-81 and sublimated films of the bases [9,10] are also given. In can be seen that the *

Resonant two-photon ionization and laser induced fluorescence spectroscopy of jet-cooled adenine

“…The lowest electronic state of guanine is unambiguously characterized to be a * state, and the lowest n * state appears to be much higher in energy by about 0.5 eV to 0.9 eV. 13,14,16 Therefore, it is expected that the vibronic mixing between the n * and * states occurs in a much higher frequency region, perhaps well outside the range of the spectrum experimentally observed ͑Fig. 1 of Ref.…”

“…of the -* transition. 9,10 As for the relative locations of these states, Lipiński 13 and Hug and Tinoco 14 suggested that the n * state is located between the two lowest * states, whereas Stewart and Davidson, 10 Danilov et al, 16 and Takahata 17 proposed that the n * state is the lowest singlet excited state of adenine. Broo also concluded that the geometrically relaxed lowest excited state of adenine has an n * character, while a related compound, 2-aminopurine, has a strong * character that gives the molecule totally different photophysical properties.…”

“…15 Most of these studies agree, however, that the energy difference between the n * and * states is too small ͑and perhaps within the accuracy of the calculations͒ to allow unequivocal determination of the relative locations of the two states. 16 In contrast to the case of adenine, the electronic structure of guanine is quite well known. The lowest electronic state of guanine is unambiguously characterized to be a * state, and the lowest n * state appears to be much higher in energy by about 0.5 eV to 0.9 eV.…”

“…11 The locations and transition moments of the n * and * excited states were estimated by theoretical calculations. [12][13][14][15][16][17][18] Although most conventional studies in the condensed phase have found broad, structureless spectra, some recent investigations revealed better-resolved spectral features. For example, infrared spectra were obtained in a low-temperature inert gas matrix, 19 in a static gas cell, 20 and in a supersonic jet.…”

“…In contrast to the broad, structureless spectra of thymine and uracil even in a jet-cooled condition, 24 a well-resolved vibronic structure was observed in the electronic spectrum of adenine, as in the case of guanine. 25 It has been suggested by theory that the n * and * states lie close to each other in adenine, [13][14][15][16][17] while the identity of the lowest electronically excited state has been the subject of a long-standing debate. 13,17 Our findings strongly suggest that this state is of n * character, and that there exists extensive vibronic mixing between the n * and * states.…”