STUDY OF ELECTRONIC STRUCTURE OF CYTOSINE

Abstract

Experimental studies of excitation of thymine, guanine, cytosine, uracil, which are the basis of the biological structure of DNA and RNA, contribute to the study of the mechanisms of their destruction under the influence of low-energy β-radiation and ultraviolet radiation. Crystal chemical analysis of the structure of the cytosine molecule showed that it exists in two crystalline forms that crystallize in orthorhombic syngony, but in different space groups (form Cyt (I) – Pccn (56) and Cyt (II) – P212121 (19)). The crystal structure of cytosine (form Cyt (I)) is formed by repeating fragments consisting of two parallel chains, each of which forms three cytosine molecules. Parallel chains are connected by hydrogen bonds formed between the N atoms of the amino group of one molecule and the N heterocyclic ring of another N–H¼N molecule (1,897 Å). The individual fragments are perpendicular to each other (along the x-axis and the y-axis) and are connected by hydrogen bonds of the N amino group and the O carbonyl group of the N–H¼O (2.158 Å). Linear chains of cytosine molecules (form Cyt (II)) are located along the y-axis and are connected by single cytosine molecules of adjacent chains by hydrogen bonds N–H¼O (2,127 Å). Chains are formed from cytosine molecules that are connected by hydrogen bonds N–H¼O (2.153 Å) and N–H¼N (1.898 Å). Ab initio quantum chemical calculations of the electronic structure using the Quantum Espresso (QE) software package based on density functional theory (DFT) showed that Cyt (II) is characterized by a direct conductivity type, band gap Eg=3.52 eV, crystal cell energy Ecrys= –571.81 Ry, energy per atom E/at= –142.95 Ry, Fermi energy EFermi=2.67 eV. The optical band gap (Eg) of cytosine Cyt (II) is formed by electron transitions N 2p(O 2p) → C 2p(N 2p).