An experimental and theoretical vibrational study of the interaction of cytosine and uracil with artificial seawaters: A prebiotic chemistry experiment

Abstract

Solids of uracil and cytosine lyophilized from different solutions were characterized using FT-IR and Raman spectroscopies and the frequencies in the spectra were compared to theoretical calculations. The effect of pH and of cations in artificial seawaters on the frequencies of FT-IR and Raman spectra were studied. The solid cytosine and uracil obtained from alkaline solutions indicated the formation of a mesomeric anion as a result of the deprotonation of N1 and N3, respectively. The spectroscopic analysis of cytosine obtained from an acid solution showed bands characteristic of cytosine hydrochloride. For uracil, no changes were observed because of its low pKa1. Cytosine interacts with the cations (Na+, Mg2+, Ca2+ and Sr2+) in the different seawaters. For these samples a shift of a band attributed to ν(C2N1) stretching was observed. The theoretical calculations indicate an interaction of the metallic cations through C2O, forming an “A” structure complex. For uracil, an interaction with the cations present in the artificial seawaters was also observed. The spectroscopic analysis showed new bands attributed to the C4O group which indicates that the interaction occurs at a carbonyl resulting in a “B” structure complex. The reactivity of cytosine and uracil were studied using 2 parameters the nucleophilicity and energy of the molecular orbital HOMO. The Mulliken charges at N1 for the two tautomers of both cytosine and uracil were compared. It was concluded that the enol-imino form has lower energy and, consequently, higher nucleophilicity. Thus, these complexes could have played an important role on nucleoside formation. The HOMO has a contribution to N1 in the enol-imino and keto-amino tautomers for both nucleic acid bases.