The lactose permease from Escherichia coli (LacY) is a galactoside/H(+) symporter that catalyzes the coupled stoichiometric transport of a sugar and an H(+) across the cytoplasmic membrane. X-ray crystal structures of WT LacY and the conformationally restricted mutant Cys154→Gly exhibit an inward-facing conformation with a tightly sealed periplasmic side and a deep central cleft or cavity open to the cytoplasm. Although the crystal structures may give the impression that LacY is a rigid molecule, multiple converging lines of evidence demonstrate that galactoside binding to WT LacY induces reciprocal opening and closing of periplasmic and cytoplasmic cavities, respectively. By this means, the sugar- and H(+)-binding sites in the middle of the molecule are exposed alternatively to either side of the membrane. In contrast to the crystal structure, biochemical/biophysical studies with mutant Cys154→Gly show that the periplasmic side is paralyzed in an open-outward conformation. In this study, a rigid, funnel-shaped, maleimide-containing molecule was used to probe the periplasmic cavity of a pseudo-WT and the Cys154→Gly mutant by site-directed alkylation. The findings provide strong support for previous observations and indicate further that the external opening of the periplasmic cleft in the mutant is patent to the extent of at least 8.5 Å in the absence of sugar or about half that of the WT cavity with bound galactoside.

Experimental and theoretical evidence is reported for a rare type I dyotropic rearrangement involving a [1,2]-alkene shift, leading to the regio- and stereospecific ring contraction of bromocycloheptenes. This reaction occurs under mild conditions, with or without a Lewis acid catalyst. DFT calculations show that the reaction proceeds through a nonclassical carbocation-anion pair, which is crucial for the low activation barrier and enantiospecificity. The chiral cyclopropylcarbinyl cation may be a transition state or an intermediate, depending on the reaction conditions.