pKa determination of D-ribose by Raman spectroscopy

Determination of the pKa 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 pKa 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 νC-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 pKa was 11.8. Both the ionized and the unionized structures of D-ribose preferred the pyranose conformation, which was supported by (13)C NMR experiments. Electronic redistribution via resonance and intramolecular hydrogen-bond formation were proposed to account for the stabilization of the ionized structure.