Rhamnolipid B and its precursor A, both microbial and membranous constituent, reversibly changed morphologies of their molecular aggregates from vesicles under acidic conditions, to lamella, lipid particles, and finally to micelles under weakly acidic conditions within a narrow pH range of about 5–7.

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Mutagenesis studies were carried out to examine the effects of replacement of either the nucleophile Glu-236 or the acid/base Glu-128 residue of the F/10 xylanase by a His residue. To our surprise, the affinity for the p-nitrophenyl-L L-D-xylobioside substrate was increased by 10 3 -fold in the case of the mutant E128H enzyme compared with that of the wild-type F/10 xylanase. The catalytic activity of the mutant enzymes was low, despite the fact that the distance between the nucleophilic atom (an oxygen in the native xylanase and a nitrogen in the mutant) and the K K-carbon was barely changed. Thus, the alteration of the acid/base functionality (Glu-128 to His mutation) provided a significantly favorable interaction within the E128H enzyme/substrate complex in the ground state, accompanying a reduction in the stabilization effect in the transition state.z 1999 Federation of European Biochemical Societies.

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