p<i>K</i><sub>a</sub> Determination of d‑Ribose by Raman Spectroscopy
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Abstract
Determination
of the p<i>K</i><sub>a</sub> 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><sub>a</sub> 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<sub>C–D</sub> 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><sub>a</sub> was
11.8. Both the ionized and the unionized structures of d-ribose
preferred the pyranose conformation, which was supported by <sup>13</sup>C NMR experiments. Electronic redistribution via resonance and intramolecular
hydrogen-bond formation were proposed to account for the stabilization
of the ionized structure
