2 research outputs found
p<i>K</i><sub>a</sub> Determination of d‑Ribose by Raman Spectroscopy
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
C<sub>α</sub>–H Carries Information of a Hydrogen Bond Involving the Geminal Hydroxyl Group: A Case Study with a Hydrogen-Bonded Complex of 1,1,1,3,3,3-Hexafluoro-2-propanol and Tertiary Amines
Experimental measurement of the contribution
of H-bonding to intermolecular
and intramolecular interactions that provide specificity to biological
complex formation is an important aspect of macromolecular chemistry
and structural biology. However, there are very few viable methods
available to determine the energetic contribution of an individual
hydrogen bond to binding and catalysis in biological systems. Therefore,
the methods that use secondary deuterium isotope effects analyzed
by NMR or equilibrium or kinetic isotope effect measurements are attractive
ways to gain information on the H-bonding properties of an alcohol
system, particularly in a biological environment. Here, we explore
the anharmonic contribution to the C–H group when the O–H
group of 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) forms an intermolecular
H-bond with the amines by quantum mechanical calculations and by experimentally
measuring the H/D effect by NMR. Within the framework of density functional
theory, ab initio calculations were carried out for HFP in its two
different conformational states and their H-bonded complexes with
tertiary amines to determine the <sup>13</sup>C chemical shielding,
change in their vibrational equilibrium distances, and the deuterium
isotope effect on <sup>13</sup>C2 (secondary carbon) of HFP upon formation
of complexes with tertiary amines. When C2–OH was involved
in hydrogen bond formation (O–H as hydrogen donor), it weakened
the geminal C2–H bond; it was reflected in the NMR chemical
shift, coupling constant, and the equilibrium distances of the C–H
bond. The first derivative of nuclear shielding at C2 in HFP was −48.94
and −50.73 ppm Å<sup>–1</sup> for anti and gauche
conformations, respectively. In the complex, the values were −50.28
and −50.76 ppm Å<sup>–1</sup>, respectively. The
C–H stretching frequency was lower than the free monomer, indicating
enhanced anharmonicity in the C–H bond in the complex form.
In chloroform, HFP formed a complex with the amine; δC2 was
69.107 ppm for HFP–triethylamine and 68.766 ppm for HFP-<i>d</i><sub>2</sub>–triethylamine and the difference in
chemical shift, the ΔδC2 was 341 ppb. The enhanced anharmonicity
in the hydrogen-bonded complex resulted in a larger vibrational equilibrium
distance in C–H/D bonds. An analysis with the Morse potential
function indicated that the enhanced anharmonicity encountered in
the bond was the origin of a larger isotope effect and the equilibrium
distances. Change in vibrational equilibrium distance and the deuterium
isotope effect, as observed in the complex, could be used as parameters
in monitoring the strength of the H-bond in small model systems with
promising application in biomacromolecules