Effect of lipid architecture on cubic phase susceptibility to crystallisation screens

Abstract

The proposed mechanism for in meso crystallisation depends, at least initially, on retention of the underlying cubic phase. However, a crystallisation trial requires screening across a wide range of crystallisation conditions, containing polymers, salts, buffers and at varying pH, all of which are known to drive structural changes in lipid phases. We have previously shown that the lipid monoolein (MO) is relatively robust to the components of the PACT crystallization screen. Here we extend our research to determine the susceptibility of the 3-D ordered cubic phase formed by four different lipids; monoolein, phytantriol, phytanoyl monoethanolamide and H-farnesoyl monoethanolamide, to two different crystallisation screens (the PACT and PEG-ion screens) in situ, within a 96-well crystallisation plate. Addition of screen is shown to result in rich and varied phase behaviour with the transformation to 1-D ordered lamellar, 2-D ordered hexagonal and disordered micellar phases in many wells. We have rationalized the structural changes for each lipid by a consideration of the osmotic stress exerted by the PEG components, and the position of various anions and cations present in the Hofmeister series. The nanostructure of the cubic phase is shown to be the most important parameter affecting the susceptibility of the cubic phase structure to the components of the screen. In particular, a reduction in lipid bilayer thickness and water channel diameter increases the susceptibility.