EFFECT OF CATION RADICAL FORMATION ON REACTIVITY AND ACIDITY ENHANCEMENT OF CYTOSINE NUCLEOBASE: NATURAL BOND ORBITAL AND ATOM IN MOLECULE ANALYSIS

The radical cations of DNA constituents generated by the ionizing radiation initiate an alteration of the bases, which is one of the main types of cytotoxic DNA lesions. These cation radical spices are known for their role in producing nucleic acid strand break. In this study, the gas-phase intrinsic chemical properties of the gaseous radical cations of cytosine and its base pair with guanine were examined by employing density functional theory (B3LYP) with the 6-311++G(d,p) basis set. Structures, geometries, adiabatic ionization energies, adiabatic electron affinities, charge distributions, molecular orbital analysis and proton-transfer process of these molecules were investigated. The influence of cation radical formation on acidities of multiple sites in cytosine molecule was investigated. Results of calculations revealed that cytosine radicals formed by deprotonation of cytosine cation radicals can exothermically abstract hydrogen atoms from thiol groups, phenol, and alpha-positions of amino acid. Furthermore, comparison of acidity value of N-H sites of cytosine cation radical with the known proton affinities (PA) of organic and biological molecules implied that cytosine cation radical can exothermically transfer onto basic sites of amino acids and peptides.