POLYMORPHISM OF A LIPID EXTRACT FROM PSEUDOMONAS-FLUORESCENS - STRUCTURE-ANALYSIS OF A HEXAGONAL PHASE AND OF A NOVEL CUBIC PHASE OF EXTINCTION SYMBOL FD--

The phase diagram of the Pseudomonas fluorescens lipid extract is unusual, in the sense that it displays a cubic phase straddled by a hexagonal phase. The hexagonal phase was studied over an extended concentration range, and the reflections were phased on the assumption that the structure contains circular cylinders of known radius. The cubic phase, whose extinction symbol is Fd--, was analyzed by reference to space group No. 227 (Fd3m). The phases of the reflections were determined by using a novel pattern recognition approach, based upon the notion that the average fourth power of the electron density contrast 〈(Δr)4〉 is dependent on chemical composition but not on physical structure, provided that the function Δr(r) satisfies the constraints 〈(Δr)〉 = 0 and 〈(Δr)2〉 = 1. As a further constraint, a shape normalization is used, in the form of a Gaussian apodization of the intensities, which has the effect of normalizing the curvature of the autocorrelation function at the origin. We analyzed two cubic samples of different composition: for each of them we generated all the phase combinations compatible with the X-ray scattering data and we searched for those whose 〈(Δr)4〉 best agrees with the hexagonal phase. Taking advantage of the favorable properties of the phase diagram, we carefully explored the effects of various parameters; we concluded that the chemical composition of the phases being compared must be identical, that the X-ray scattering data should not be truncated artificially, and that the apodization must be mild so that the curvature takes a value intermediate between those corresponding to the raw data of the two phases. When all these precautions were taken, 〈(Δr)4〉 was found to be remarkably invariant; this conclusion is important in view of the possible usefulness of the novel technique in tackling ab initio-and at very low resolution-structural problems of more general interest. The structure of the cubic phase consists of a 3D network of rods joined tetrahedrally 4 by 4 according to a diamond lattice and of a family of quasi-spherical disjointed micelles; the core of the rods and of the micelles is polar, and the interstices are filled by the hydrocarbon chains (structure of type II). All the dimensions (diameter of rods and micelles, area per chain at the polar/apolar interface) are consistent with the chemical properties of the system. The structure-which is bicontinuous like that of most of the cubic phases-may be visualized as a 3D generalization of the lipid monolayer. The structure, moreover, does not belong to the class of the infinite periodic surfaces without intersections. © 1990, American Chemical Society. All rights reserved.