Ladderane phospholipids form a densely packed membrane with normal hydrazine and anomalously low proton/hydroxide permeability.

Ladderane lipids are unique to anaerobic ammonium-oxidizing (anammox) bacteria and are enriched in the membrane of the anammoxosome, an organelle thought to compartmentalize the anammox process, which involves the toxic intermediate hydrazine (N2H4). Due to the slow growth rate of anammox bacteria and difficulty of isolating pure ladderane lipids, experimental evidence of the biological function of ladderanes is lacking. We have synthesized two natural and one unnatural ladderane phosphatidylcholine lipids and compared their thermotropic properties in self-assembled bilayers to distinguish between [3]- and [5]-ladderane function. We developed a hydrazine transmembrane diffusion assay using a water-soluble derivative of a hydrazine sensor and determined that ladderane membranes are as permeable to hydrazine as straight-chain lipid bilayers. However, pH equilibration across ladderane membranes occurs 5-10 times more slowly than across straight-chain lipid membranes. Langmuir monolayer analysis and the rates of fluorescence recovery after photobleaching suggest that dense ladderane packing may preclude formation of proton/hydroxide-conducting water wires. These data support the hypothesis that ladderanes prevent the breakdown of the proton motive force rather than blocking hydrazine transmembrane diffusion in anammox bacteria.

Chemical Synthesis and Self-Assembly of a Ladderane Phospholipid.

Ladderane lipids produced by anammox bacteria constitute some of the most structurally fascinating yet poorly studied molecules among biological membrane lipids. Slow growth of the producing organism and the inherent difficulty of purifying complex lipid mixtures have prohibited isolation of useful amounts of natural ladderane lipids. We have devised a highly selective total synthesis of ladderane lipid tails and a full phosphatidylcholine to enable biophysical studies on chemically homogeneous samples of these molecules. Additionally, we report the first proof of absolute configuration of a natural ladderane.

Enantioselective Dirhodium(II)-Catalyzed Cyclopropanations with Trimethylsilylethyl and Trichloroethyl Aryldiazoacetates.

Highly functionalized cyclopropanecarboxylates were readily prepared by rhodium-catalyzed cyclopropanation of alkenes with aryldiazoacetates and styryldiazoaceates, in which the ester functionality is either trimethylsilylethyl (TMSE) or trichlorethyl (TCE). By having labile protecting groups on the ester, chiral triarylcyclopropane carboxylate ligands were conveniently prepared. The asymmetric induction during cyclopropanation is dependent on the nature of the ester group and the chiral dirhodium tetracarboxylate catalyst. The prolinate catalyst Rh2(S-DOSP)4 was the optimum catalyst for asymmetric intermolecular cyclopropanation of TMSE diazoesters with styrene, while Rh2(R-BPCP)4 was the optimum catalyst for TCE diazoesters.

Rhodium(II)-catalyzed stereoselective synthesis of allylsilanes.

The rhodium-catalyzed decomposition of 2-(triisopropylsilyl)ethyl aryl- and vinyldiazoacetates results in the stereoselective formation of Z-allylsilanes. The transformation is considered to proceed by silyl-directed intramolecular C-H functionalization to form a ß-lactone intermediate followed by a silyl-activated extrusion of carbon dioxide.