p<i>K</i>_a Determination of d‑Ribose by Raman Spectroscopy

Determination of the p<i>K</i>_a of OH groups present in d-ribose is crucial in order to elucidate the origin and mechanism of many catalytic processes that involve the ribose unit. However, there is hardly any reports about the experimental p<i>K</i>_a of the OH group due to the lack of an appropriate method. In this study we investigated the protonation state of OH groups in d-ribose by introducing C–D labeling and measuring the changes in the isolated C–D frequency in several isotopologues of the compound with pH. The large shift in the ν_C–D of d-ribose-C1-D in ionized condition compared to other deuterium-substituted d-riboses (e.g., d-ribose-C2-D, d-ribose-C3-D, etc.) confirmed that the C1–OH group preferred ionization, and the ionization p<i>K</i>_a was 11.8. Both the ionized and the unionized structures of d-ribose preferred the pyranose conformation, which was supported by ¹³C NMR experiments. Electronic redistribution via resonance and intramolecular hydrogen-bond formation were proposed to account for the stabilization of the ionized structure