Excited States of Nucleic Acids

Abstract

The literature on the electronic excited states of nucleic acids is reviewed covering the period approximately of mid 1973-mid 1975. Our interest focuses on the nucleic acids themselves as well as on their components and derivatives from two points of view which are closely related: (i) their spectroscopic properties, and (ii) their photochemical properties. The photobiological aspects of nucleic acids will not be considered here, however. Spectroscopy of nucleic acidsVery few papers have been concerned with the investigation of the excited singlet states of nucleic acid bases at high resolution. Sharp absorption and emission spectra are reported for crystalline purine at 4 K and an interpretation of the pattern of the excited singlet states is given using molecular orbital calculations and correlations with pyrimidine and aza substituted derivatives [63]. A study of the polarized singlecrystal reflection spectra of 6-azauracil shows that the 260 nm absorption band can be resolved into two bands [S]. Higher excited singlet states of nucleic acid bases have been investigated in thin films 1911 and the energy diagrams have been evaluated both in the neutral and ionized state [SS]. Theoretical calculations of the energies of the lower-out-of-plane transitions of the nucleic acid bases, their tautomers and ions have been made using the CNDO-CI method [35]. The CND0/2 method has also been used for these molecules [ 1011 and thiouracils [25].Studies of the fluorescence and phosphorescence emission of nucleic acids in water-alcohol glasses at low temperature, where the quantum yields are high, have received less emphasis than previously. In the case of the dinucleotide C3fP5C, the fluorescence polarimtion has been interpreted in terms of a charge resonance in the excimer state [lo]. Further work has been reported on the triplet emission of DNA [45] and yeast tRNAPhe [34]. In that case, the triplet emission of Ado can be observed at 1.2 K, but not at 77 K, partially due to energy transfer to the "Y" base. Energy transfer in Ado frozen aggregates to appropriate traps has been demonstrated [49] as well as triplet-triplet energy transfer from the bases to tryptophan in oligopeptide, poly rA (or DNA) complexes [33]. The large previously observed enhancement in the phosphorescence of DNA when Agf

Electron Configuration and Hydrogen-Bonding Pattern in Several Thymine and Uracil Analogues Studied by 1H–14N NQDR and DFT/QTAIM

“…In contrast to the natural bases, thio-nucleobases can be selectively photoactivated into the electronic triplet state with high affinity for cross-linking to other nucleic-acid bases and amino-acid residues . Thus thio-derivatives have been used as intrinsic photolabels to probe the structure and organization of RNA, , to identify the interactions between nucleic acids and proteins in nucleoprotein complexes, , and to follow the migration of electrons/holes in nucleic acids …”