The hydrated Li+–adenine–thymine complex by IRMPD spectroscopy in the N–H/O–H stretching region

Abstract: The interaction of lithium ions with the nucleic acid bases which make up the A:T base pair, adenine and thymine, as well as the hydration of the complex by one water molecule has been studied in the gas phase. An IRMPD spectrum is reported for [A–T–Li+]–H2O (A=adenine, T=thymine) over the N–H stretching region, 3200–3800cm−1. Calculations were performed using the B3LYP density functional and 6-31+G(d,p) basis set as well as MP2/6-311++G(2d,p) theory to model the thermochemistry and infrared spectra of potential structures. Theory and experimental results were used to determine possible structures of each complex. These structures, along with their corresponding thermochemical and spectroscopic data, are reported here. The lithium cation was found to bond most favorably to the O4 oxygen of thymine, and the water molecule was found to bind to the lithium cation. The adenine moiety of the complex is that of the A7 tautomer, leading to an A:T base pair which is not the canonical form. Based on the analysis, a number of low energy, intramolecular hydrogen-bonded structures are suggested as being present in the gas phase and thus responsible for generating the experimental infrared multi-photon dissociation (IRMPD) spectrum. [ABSTRACT FROM AUTHOR]