Tautomerism and the Protonation/Deprotonation of Isocytosine in Liquid- and Solid-States Studied by NMR Spectroscopy and Theoretical Calculations

Isocytosine crystallizes as a 1:1 ratio of two tautomers in a manner similar to that of the guanine and cytosine pairs in DNA. The solid-state NMR chemical shifts of crystalline neutral isocytosine, the sodium salt of isocytosine, and those of isocytosine hydrochloride were measured and related to the NMR parameters in aqueous and organic solutions. The experimental chemical shifts were compared with those calculated by three different methods: (1) calculations on isolated molecules, (2) calculations on isocytosine clusters of various sizes, and (3) infinite crystal calculations, that is, the gauge-including projector-augmented wave (GIPAW) method. The data obtained with the GIPAW method we…