A systematic solid-state 17O NMR study of a series of carboxylic compounds, maleic acid, chloromaleic acid, KH maleate, KH chloromaleate, K2 chloromaleate, and LiH phthalate.MeOH, is reported. Magic-angle spinning (MAS), triple-quantum (3Q) MAS, and double angle rotation (DOR) 17O NMR spectra were recorded at high magnetic fields (14.1 and 18.8 T). 17O MAS NMR for metal-free carboxylic acids and metal-containing carboxylic salts show featured spectra and demonstrate that this combined, where necessary, with DOR and 3QMAS, can yield site-specific information for samples containing multiple oxygen sites. In addition to 17O NMR spectroscopy, extensive quantum mechanical calculations were carried out to explore the influence of hydrogen bonding at these oxygen sites. B3LYP/6-311G++(d,p) calculations of 17O NMR parameters yielded good agreement with the experimental values. Linear correlations are observed between the calculated 17O NMR parameters and the hydrogen bond strengths, suggesting the possibility of estimating H-bonding information from 17O NMR data. The calculations also revealed intermolecular H-bond effects on the 17O NMR shielding tensors. It is found that the delta11 and delta22 components of the chemical shift tensor at O-H and C=O, respectively, are aligned nearly parallel with the strong H-bond and shift away from this direction as the H-bond interaction weakens.

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Using crystal structure data for the pyruvate decarboxylase from Saccharomyces uvarum (which is nearly identical with the enzyme from Saccharomyces cerevisiae), molecular modeling studies have been carried out to investigate the mode of action of the enzyme. Each step of the decarboxylation mechanism can be explained by assuming that the 4‘-amino group of thiamin diphosphate (TDP) acts as a general acid and, in its deprotonated form, as a general base. The carboxyl group of Glu 477 plays a key role in both pyruvate decarboxylation and acyloin formation. In the first case it interacts with the carboxylate group of pyruvate to stabilize the incipient dianion formed by attack of the thiazolium carbanion on pyruvate. In the second case, it interacts with the developing alkoxide anion arising from attack on acetaldehyde or benzaldehyde by the carbanion−enamine intermediate. These studies have permitted the assignment of configuration to all of the key intermediates in the catalytic process. Thus the carbanion−enamine intermediate 5 is found to have the E-configuration. The S-configuration is imposed on the 2-(2-hydroxypropionyl)ethyl)thiamin diphosphate intermediate 4 by the chiral conformation induced in the achiral cofactor through its interactions with the protein. The R-configuration is assigned to the 2-(1-hydroxyethyl)thiamin diphosphate intermediate 6 arising through protonation of the carbanion−enamine intermediate 5. The tight stereochemical control observed in acyloin formation from aromatic aldehydes and pyruvate is explained, as is the relaxed stereocontrol in acyloin formation from acetaldehyde and pyruvate.