pH and pKDeterminations by High-Resolution Solid-State13C NMR: Acid−Base and Tautomeric Equilibria of Lyophilizedl-Histidine

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Abstract
Acid−base properties of lyophilized powders of l-histidine have been systematically investigated using parent solutions at pH varying from 1.8 to 10. For the first time, high-resolution solid-state 13C NMR was shown to allow separate observation of all three acid−base pairs in the successive deprotonations of the carboxylic end, the imidazolium cation, and the terminal ammonio group of histidine. 1H CRAMPS NMR spectra directly visualize the absence of the N3−H(π) tautomer in neutral and anionic species. Solid-state titration shifts are enlarged by ∼1−4 ppm with respect to those measured in solution, permitting unambiguous observation of conjugate acid−base pairs. Calculated pK's from solid-state acid-to-base ratios r are found equal to those classically measured in solution at 0 °C with a similar ionic strength of 0.1 mol dm-3. This proves that natural-abundance 13C solid-state determinations of r can be used to measure pK's in parent solutions without recourse to full titration curves and subsequent curve-fitting procedures. Such an approach also leads to noninvasive characterizations of the acidity of lyophilized powders, i.e., to the prediction of in situ pH of products obtained after rehydration and solubilization of powders. These results show the possibility of measuring the pK of nonvolatile acidic substrates dissolved in any sublimable solvent through lyophilization of the investigated solutions; this leads the way to pH and pK determinations when electrochemical or spectrophotometric measurements are impossible or ambiguous, e.g., for concentrated solutions, polyacids, or mixtures of acidic solutes, and possibly to the establishment of pK scales in nonaqueous solvents and in melts.

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