Sugar Placement

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Science  31 Aug 2012:
Vol. 337, Issue 6098, pp. 1020
DOI: 10.1126/science.337.6098.1020-b

Chemical bonds vibrate at frequencies that depend on the masses of the linked atoms. Because bond scission and formation are essentially extreme sorts of vibration, their rates also vary when the atom masses change, giving rise to kinetic isotope effects that offer insight into the order and extent of bond rearrangements underlying a reaction. The easiest, and thus most common, effects to study involve deuterium/hydrogen substitutions, given the factor of 2 mass difference. The 13/12 mass ratio of stable carbon isotopes induces a smaller rate distinction that is nonetheless discernible—even at the low natural abundance of 13C—using current nuclear magnetic resonance (NMR) technology. Huang et al. applied this technique to elucidate the precise mechanistic details of substitution reactions at the anomeric carbon of simple sugars, which bear on the selectivity attainable in the generating particular oligosaccharides for targeted biological studies. Specifically, they examined rate distinctions for 12C versus 13C centers in the displacement of trifluoromethanesulfonate by isopropanol to form the α and β anomers of mannopyranoside and glucopyranoside. For three of the four reactions, comparison of the NMR data to theoretical simulations supported a loosely associative mechanism, with the bond-cleaving and -forming events perhaps just shy of simultaneous. The α-mannopyranoside was unusual in appearing to form through initial bond scission before isopropanol binding.

Nat. Chem. 4, 663 (2012).

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