PubMed · 13172197
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R E MATTHEWS, J D SMITH. 1954-03-20. Interconversions between 8-azapurines.. https://pubmed.ncbi.nlm.nih.gov/13172197/
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The CASPT2//CASSCF method with the 6-31G basis set and an active space up to (16,12) was used to calculate the excitation energies for six tautomers of guanine. Our calculations provide further support on the recent reassignment of the near-UV resonant two-photon ionization (R2PI) spectrum, in which two rare tautomers of the 7H-oxo-imino form were proposed to replace the previously assigned 7H/9H-oxo-amino tautomers. The adiabatic excitation energies of the 7H-oxo-imino tautomers are calculated to be 0.3-0.5 eV higher than those of the 7H/9H-oxo-amino tautomers. Our calculations also indicate that the missing most stable tautomers (7H/9H-oxo-amino tautomers) in the R2PI experiment is possibly due to the existence of an ultrafast nonradiative deactivation process in the excited-state of these two tautomers.
Infrared laser spectroscopy is used to study the four lowest energy tautomers of guanine, isolated in helium nanodroplets. The large number of vibrational bands observed in the infrared spectrum are assigned by comparing the corresponding experimental vibrational transition moment angles with those obtained from ab initio theory. The result is the conclusive assignment of the spectrum to the N9H-Keto, N7H-Keto, N9Ha-Enol(trans), and N9Hb-Enol(cis) tautomers. The dipole moments of these tautomers are also experimentally determined and compared with ab initio theory.
Nucleic acid quadruplexes, based on the guanine quartet, can arise from one or several strands, depending on the sequence. Those consisting of a single strand are usually folded in one of two principal topologies: antiparallel, in which all or half of the guanine stretches are antiparallel to each other, or parallel, in which all guanine stretches are parallel to each other. In the latter, all guanine nucleosides possess the anti conformation about the glycosidic bond, while in the former, half possess the anti conformation, and half possess the syn conformation. While antiparallel is the more common fold, examples of biologically important, parallel quadruplexes are becoming increasingly common. Thus, it is of interest to understand the forces that determine the quadruplex fold. Here, we examine the influence of individual nucleoside conformation on the overall folding topology by selective substitution of rG for dG. We can reverse the antiparallel fold of the thrombin binding aptamer (TBA) by this approach. Additionally, this substitution converts a unimolecular quadruplex into a bimolecular one. Similar reverse substitutions in the all-RNA analogue of TBA result in a parallel to antiparallel change in topology and alter the strand configuration from bimolecular to unimolecular. On the basis of the specific substitutions made, we conclude that the strong preference of guanine ribonucleosides for the anti conformation is the driving force for the change in topology. These results demonstrate how conformational properties of guanine nucleosides govern not only the quadruplex folding topology but also impact quadruplex molecularity and provide a means to control these properties.