Kinetics of starch hydrolysis and glucose mutarotation studied by NMR chemical exchange saturation transfer (CEST)

The –OH resonance on –C1 of the α and β-anomers of glucose that are normally invisible in 1H NMR spectra, was used to monitor the reaction kinetics of starch hydrolysis in aqueous solutions. The exchangeable protons on glucose were detected indirectly through their exchange with water protons, and hence the water resonance, by using selective radiofrequency (RF) saturation at the absorption frequency of the hydroxyl protons. The saturated population of –OH spins exchanged rapidly with the water proton population, leading to the partial suppression of the water signal. Mutarotation of the anomers of glucose and starch digestion kinetics measured this way were consistent with those readily obtained by other more direct, but less sensitive, NMR methods. The chemical exchange saturation transfer (CEST) technique is useful in favourable circumstances when signals from non-exchangeable protons are not resolved in a crowded region of the spectrum. Thus the 1H2O signal is so relatively large that its dynamic range is high and it gives greatly enhanced sensitivity for (indirectly) detecting much less abundant saccharide species.