1H NMR: A Novel Approach To Determining the Thermodynamic Properties of Acetaldehyde Condensation Reactions with Glycerol, (+)-Catechin, and Glutathione in Model Wine

As wine oxidizes, ethanol is converted to acetaldehyde, but its accumulation is not predictable, due to poorly characterized reactions with alcohols, SO2, thiols, flavanols, and others. Measurement of these components has been thwarted by equilibria into the other forms during sample preparation. NMR spectra can be taken on intact samples and is thus ideal for this situation. Equilibria of acetaldehyde with glycerol, (+)-catechin, and glutathione were studied separately in model wine solutions at pH 3–4 by 1H NMR and 2D (1H–1H) COSY spectra. Glycerol acetals had equilibrium constants between 1.14 ± 0.056 and 2.53 ± 0.043 M–1, whereas ethylidene-bridged (+)-catechin dimers and glutathione thiohemiacetals had more favorable equilibria: from (3.92 ± 0.13) × 103 to (6.13 ± 0.32) × 103 M–2 and from 10.18 ± 0.22 to 11.17 ± 0.47 M–1, respectively. These data can be used to create accurate measures of acetaldehyde in its various forms and, consequently, offer insight into wine oxidation.