Structure and singly occupied molecular orbital analysis of anionic tautomers of guanine

Recently we reported the discovery of adiabatically bound anions of guanine which might be involved in the processes of DNA damage by low-energy electrons and in charge transfer through DNA. These anions correspond to some tautomers that have been ignored thus far. They were identified using a hybrid quantum mechanical-combinatorial approach in which an energy-based screening was performed on the library of 499 tautomers with their relative energies calculated with quantum chemistry methods. In the current study we analyze the adiabatically bound anions of guanine in two aspects: 1) the geometries and excess electron distributions are analyzed and compared with anions of the most stable neutrals to identify the sources of stability; 2) the chemical space of guanine tautomers is explored to verify if these new tautomers are contained in a particular subspace of the tautomeric space. The first task involves the development of novel approaches – the quantum chemical data like electron density, orbital and information on its bonding/antibonding character are coded into holograms and analyzed using chemoinformatics techniques. The second task is completed using substructure analysis and clustering techniques performed on molecules represented by 2D fingerprints. The major conclusion is that the high stability of adiabatically bound anions originates from the bonding character of the pi orbital occupied by the excess electron. This compensates for the antibonding character that usually causes significant buckling of the ring. Also the excess electron is more homogenously distributed over both rings than in the case of anions of the most stable neutral species. In terms of 2D substructure, the most stable anionic tautomers generally have additional hydrogen atoms at C8 and/or C2 and they don’t have hydrogen atoms attached to C4, C5 and C6. They also form an “island of stability” in the tautomeric space of guanine

Proton transfer, electron binding and electronegativity in ammonium-containing systems

Using modern electronic structure methods, the ammonia-hydrogen halide complexes and their anions are characterised to determine the number, type, and properties of their minima, and their electron binding energies. Methodological issues of determining the potential energy surfaces of reactive monomers are addressed in the course of this investigation. The energetic origins of the hydrogen-bonded minima are determined by evaluation of the one-body and two-body terms composing the total energy of the complexes, and a rationale for the drive to proton transfer is presented. It is concluded that the systems have qualitatively similar potential surfaces, and that the balance of the one-body and two-body forces determines the number and depth of minima, while the electron acts as a perturbing agent on the one- or two-body energy, depending upon the nature of the minimum encountered. The halogen-bonded structures of ammonia-hydrogen bromide, iodide, and astatide complexes are shown to be stable, and one may perhaps bind an electron. The concept of the ammonium radical as a pseudo-atom is presented and tested. It is found to show competing pseudo-atomic and molecular properties.Engineering and Physical Sciences Research Council (EPSRC