| Abstract
| - Ab initio molecular orbital theory at the G2(MP2,SVP)level has been used to study several conformations,defining part of the pseudorotational itinerary, of equatorial2-oxanol. Half-chair (3H4) andboat (B1,4) transitionstates lie 23.7 and 14.3 kJ mol-1 above thechair conformation (1C4),respectively, while the twist-boat conformers(3S1 and5S1) lie 6.9 and 5.9 kJmol-1 above the chair, respectively.Protonation of the glycosidic oxygen of thechair conformer yields an oxonium ion in the chair conformation.All other conformations collapse to give anoxocarbonium ion−water complex upon protonation. The axialanomer in the chair conformation lies 12.0 kJmol-1lower than the equatorial anomer. Protonation of the axial anomerin the chair conformation does not yield a chair,but collapses to the oxocarbonium ion. A clear role is shown forring distortion in enzymes which perform acid-catalyzed hydrolysis of equatorial glycosides. In addition toavoiding high-energy oxonium ion intermediates, distortionof the ring also reduces the glycosidic bond-stretch energy whichdelays the transition state and reduces the reactionbarrier. Enzymes which hydrolyze the axial anomer do not requirering distortion to achieve a concerted pathwayto the oxocarbonium ion. These results are discussed in relationto three enzymes, lysozyme, neuraminidase, andβ-amylase.
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