Purines, pyrimidines, and imidazoles. Part 62. Isotopic hydrogen exchange from the C-2 position in an lmidazole nucleoside related to intermediates in purine nucleotide de novo biosynthesis

The rate of hydrogen exchange at H-2 in ethyl 5-amino-l-(2,3-O-isopropylidene-β-D-ribofuranosyl)-imidazole-4-carboxylate has been measured in deuteriated phosphate and glycine buffers over a pH range at 65 °C. The bell-shaped rate curve produced is consistent with rate-determining hydroxide-ion attack on the protonated form of the nucleoside. The assignment of N-3 as the preferred site for protonation has been confirmed by calculations using the MNDO semi-empirical MO method.