Polyradicals of Polycyclic Aromatic Hydrocarbons as Finite Size Models of Graphene: Highly Open-Shell Nature, Symmetry Breaking, and Enhanced-Edge Electron Density

Properties of polyradicals (all CH bonds dissociated) of benzene and certain polycyclic aromatic hydrocarbons (PAHs) were studied. The occurrence of symmetry breaking is revealed in going from benzene and the PAHs to their polyradicals. Polyradicals would serve as finite size models of graphene with unpassivated edges in a more realistic way than the PAHs. Monoradicals (one CH bond dissociated) of benzene and all of the PAHs and higher radicals of benzene and one PAH (two to all CH bonds successively dissociated) were also investigated. Reliability of the methodology employed was ascertained by a comparison of our calculated single CH bond dissociation energy of benzene with the available previous experimental and theoretical results. Besides ground-state geometries, the aspects studied include single and successive CH bond dissociation energies, and electron density, molecular electrostatic potential (MEP), and spin density distributions. All of the monoradicals studied were found to have doublet spin multiplicity, while polyradicals with 4 to 16 rings and zigzag or mixed-type edges were found to have spin multiplicities varying from triplet to 11et. Bond lengths and bond angles of rings located at the edges are appreciably modified in going from PAHs to polyradicals. Electron density and spin density are found to be enhanced at the edges of monoradicals and polyradicals of PAHs, as found previously for PAHs. However, MEP maps of polyradicals have significantly different features from those of monoradicals and PAHs, which has a significant implication

Mechanism of methylation of 8-oxoguanine due to its reaction with methyldiazonium ion

<p>Anticancer drugs such as temozolomide (TMZ) and decarbazine (DTIC) and several detrimental methylating agents methylate DNA via the formation of methyldiazonium ion (). In the present contribution, density functional theory calculations have been carried out to investigate the mechanism of reactions of at the various nucleophilic sites of 8-oxoguanine (8-oxoG), which is formed extensively in oxidatively damaged DNA and can cause ageing, mutation and cancer. The reactant complexes, transition states and product complexes involved in different reactions have been fully optimised at the B3LYP/6-31G(d,p) level of theory in gas phase. Single-point energy calculations using each of the optimised geometries have been carried out at the B3LYP/aug-cc-pVDZ and M06-2X/aug-cc-pVDZ levels of theory in gas phase and at the polarizable continuum model (PCM)-B3LYP/aug-cc-pVDZ level of theory in aqueous media. Our calculations predict that the O8, O6 and N7 sites of 8-oxoG can easily get methylated by , with the O8 site being the most favourable site for methylation. This prediction is supported by the fact that the 6-methoxy-9-methyl-8-oxoguanine and several such secondary metabolites have been observed in the Red Sea marine tunicate Symplegma rubra Monniot, 1972, a natural source. It is found that methylation pattern is appreciably modified in going from guanine to 8-oxoG and guanine is more reactive towards than 8-oxoG. Biological implications of 8-oxoG methylation including effect of O6-methylation on 8-oxoG:adenine mispair have also been discussed. Thus, the present study suggests that intensive experimental studies be performed to ascertain whether 8-oxoG methylation occurs in biological media efficiently as it may be helpful in unravelling the mechanism of action of anticancer methylating agents which act via formation of such as TMZ and DTIC.</p