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Abstract
Issue Title: Trends and Critical Reviews/Mass spectrometry for the characterization of nanoparticles/Optical tweezers for medical diagnostics/Metrology for metalloproteins
In exploring the capability of nuclear magnetic resonance (NMR) spectroscopy for pomegranate juice analysis, the eight aromatic singlet resonances of [alpha]- and [beta]-punicalagin were clearly identified in the ^sup 1^H NMR spectra of juice samples. The four downfield resonances were found to be sensitive to small pH changes around pH 3.50 where the NMR spectra of the juice samples were recorded. To understand this unusual behavior, the ^sup 1^H and ^sup 13^C resonance assignments of the punicalagin anomers were determined in aqueous solution and pH titrations with UV and ^sup 1^H NMR detection carried out to characterize the acid-base properties of punicalagin over the pH range 2-8. Simultaneous fitting of all of the pH-sensitive ^sup 1^H NMR signals produced similar but significantly different pK ^sub a^ values for the first two deprotonation equilibria of the gallagic acid moiety of the punicalagin [alpha]- (pK ^sub a1^=4.57±0.02, pK ^sub a2^=5.63±0.03) and [beta]- (pK ^sub a1^=4.36±0.01, pK ^sub a2^=5.47±0.02) anomers. Equivalent pK ^sub a^ values, ([alpha]:6.64±0.01, [beta]:6.63±0.01) were measured for the third deprotonation step involving the ellagic acid group, in good agreement with a prior literature report. The punicalagin anomer equilibrium readjusts in parallel with the proton dissociation steps as the pH is raised such that [beta]-punicalagin becomes the most abundant anomer at neutral pH. The unusual upfield shifts observed for the glucose H3 and H5 resonances with increasing pH along with the shift in the [alpha]/[beta] anomer equilibrium are likely the consequence of a conformational rearrangement.
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