Synthesis and Biophysical Characterization of the Chlorosulfolipids of Ochramonas danica

Abstract

The freshwater algae Ochramonas danica produces a range of polychlorinated single-chain amphiphiles. Danicalipin A, a hexachlorosulfolipid makes up 90% of the polar lipid content of the flagellar membrane of O. danica. Its exotic structure presents both synthetic challenges and raises questions about its function within the membrane. To address these questions, we have combined total synthesis and membrane biophysics to investigate the effects of structural elements of chlorosulfolipids on their behavior in monolayers and bilayers. The discovery of a titanium-based catalytic, enantioselective dichlorination of allylic alcohols enabled the eight-step synthesis of (+)-Danicalipin A as a single stereoisomer in sufficient quantities for in vitro analysis. Nanoscale secondary ion mass spectrometry (NanoSIMS) confirmed that Danicalipin A is localized within plasma membrane of O. danica cells. Preliminary biophysical characterization of Danicalipin A has revealed that it alters the phase behavior and lateral organization in monolayers and bilayers of other lipids present in the membrane of O. danica. Danicalipin A incorporates into monolayers of phospho- and glycolipids at the air-water interface and increases the surface pressure at which the liquid-expanded to liquid-compact phase transition occurs, as well as increasing monolayer compressibility. Similarly, in giant unilamellar vesicles, Danicalipin A lowers the transition temperature of saturated phospho- and glycerolipids and causes phase separation. Natural and unnatural analogs to Danicalipin A are being synthesized to examine the effects of the stereochemistry, chlorination pattern, and sulfation on these biophysical properties. These results may reveal how the membrane of O. danica accommodates high concentrations of chlorosulfolipids, which are toxic to other organisms.