Conformational behaviour of mannuronic acid based azasugars

A set of mannuronic acid based iminosugars, comprising the C-5-carboxylic acid, -methyl ester and -amide analogues of 1-deoxymannorjirimicin (DMJ), was synthesized and their pH dependent conformational behavior studied. Under acidic conditions the methyl ester and the carboxylic acid took up an "inverted" 1C4 chair conformation as opposed to the "normal" 4C1 chair at basic pH. This conformational change is explained by the stereoelectronic effects of the ring substituents and it parallels the behavior of the mannuronic acid ester oxocarbenium ion. Because of this solution phase behavior, the mannuronic acid ester azasugar was probed as an inhibitor for a Caulobacter GH47 mannosidase that hydrolyzes its substrates following a reaction itinerary that proceeds through a 3H4 transition state. No binding was observed for the mannuronic acid ester azasugar, but sub-atomic resolution data were obtained for the DMJ-CkGH47 complex, showing two conformations, 3S1 and 1C4, for the DMJ inhibitor.