Mariani P, Rivas E, Luzzati V, Delacroix H
Istituto di Fisica Medica, Università di Ancona, Italy.
Biochemistry. 1990 Jul 24;29(29):6799-810. doi: 10.1021/bi00481a007.
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 mean value of (delta r)4 is dependent on chemical composition but not on physical structure, provided that the function delta r(r) satisfies the constraints mean value of (delta r) = 0 and mean value of (delta r)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 mean value of (delta r)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, mean value of (delta r)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.(ABSTRACT TRUNCATED AT 400 WORDS)
荧光假单胞菌脂质提取物的相图非同寻常，因为它显示出一个被六方相跨越的立方相。在较宽的浓度范围内对六方相进行了研究，并基于该结构包含已知半径的圆柱这一假设对反射进行了相位分析。通过参考空间群编号227（Fd3m）对消光符号为Fd--的立方相进行了分析。反射的相位是通过一种新颖的模式识别方法确定的，该方法基于这样一种观念，即电子密度对比度平均值(delta r)4的平均四次方取决于化学成分而非物理结构，前提是函数delta r(r)满足平均值(delta r)=0和平均值(delta r)2 = 1的约束条件。作为进一步的约束，采用了一种形状归一化方法，即对强度进行高斯变迹，这具有使自相关函数在原点处的曲率归一化的效果。我们分析了两个不同成分的立方相样品：对于每个样品，我们生成了所有与X射线散射数据兼容的相位组合，并寻找那些(delta r)4平均值与六方相最佳吻合的组合。利用相图的有利特性,我们仔细研究了各种参数的影响；我们得出结论，被比较的相的化学成分必须相同，X射线散射数据不应人为截断，并且变迹必须适度，以使曲率取值介于对应于两相原始数据的曲率之间。当采取所有这些预防措施时，发现(delta r)4的平均值非常稳定；鉴于这种新技术在解决更具普遍意义的从头算且分辨率极低的结构问题方面可能有用，这一结论很重要。立方相的结构由一个三维棒状网络组成，这些棒根据金刚石晶格以4对4的方式四面体连接，还有一族准球形不相交的胶束；棒和胶束的核心是极性的，间隙由烃链填充（II型结构）。所有尺寸（棒和胶束的直径、极性/非极性界面处每条链的面积）都与该系统的化学性质一致。（摘要截于400字）
