Luzzati V, Vargas R, Mariani P, Gulik A, Delacroix H
Centre de Génétique Moléculaire, Université Pierre et Marie Curie, Gif-sur-Yvette, France.
J Mol Biol. 1993 Jan 20;229(2):540-51. doi: 10.1006/jmbi.1993.1053.
It has recently been shown that the structure of two of the six cubic phases so far identified in lipid-containing systems is micellar, one (Q223) of type I, the other (Q227) of type II. The micelles of both phases belong to two distinct classes, those of each class being centred at one of the special positions of the space group. From the chemical viewpoint, phase Q227 seems to require a heterogeneous mixture of water-miscible and water-immiscible lipids, whereas phase Q223 has been observed with chemically pure lipids. Also, the area/volume ratio measured at the polar/apolar interface takes the same value in the two types of micelles of phase Q223, different values in those of phase Q227, in keeping with the notion that the area/molecule ratio is closely related to the chemical activity of the lipid components. The topological properties of the micellar phases are profoundly different from those of the bicontinuous phases. The bicontinuous cubic phases (Q230, Q224, Q229) are often presented as paradigms of the infinite periodic minimal surfaces (IPMS). Some authors have generalized that notion and sought in the IPMS a unified theory underlying the entire field of lipid polymorphism. These analogies entertain some confusion between the mathematical concept of surface and the physical notion of interface. A few electron density maps are presented to document the distance that separates the polar/apolar interfaces from the IPMS. The maps also show that some of the geometric singularities (points, lines, surfaces) of the structures coincide with the locus of the CH3 ends of the chains and with the very centre of the water matrix, i.e. with the regions where the short-range disorder is highest. We introduce the expression chaotic zones to designate these regions. In all the lipid phases the chaotic zones are found to occupy special geometric positions, either related to the symmetry elements or to the IPMS. It thus appears that it is energetically more advantageous to adopt an orderly disposal of the short-range disorder than to minimize the area of the polar/apolar interfaces. Finally, regarding the possible biological significance of lipid polymorphism, the point is stressed that among the phases that are observed in equilibrium with excess water (these phases are also the most likely candidates for a biological role) those with a cubic symmetry deserve special attention.(ABSTRACT TRUNCATED AT 400 WORDS)
最近研究表明，在含脂体系中迄今所鉴定出的六个立方相中的两个相的结构为胶束状，其中一个（Q223）为I型，另一个（Q227）为II型。这两个相的胶束都属于两个不同的类别，每个类别的胶束都以空间群的一个特殊位置为中心。从化学角度来看，Q227相似乎需要水混溶和水不混溶脂质的非均匀混合物，而Q223相已在化学纯脂质中观察到。此外，在Q223相的两种类型胶束中，在极性/非极性界面处测得的面积/体积比具有相同的值，而在Q227相的胶束中该值不同，这与面积/分子比与脂质成分的化学活性密切相关的观点一致。胶束相的拓扑性质与双连续相的拓扑性质有很大不同。双连续立方相（Q230、Q224、Q229）常被视为无限周期最小曲面（IPMS）的范例。一些作者对这一概念进行了推广，并在IPMS中寻求脂质多态性整个领域的统一理论。这些类比在表面的数学概念和界面的物理概念之间造成了一些混淆。给出了一些电子密度图，以记录极性/非极性界面与IPMS之间的距离。这些图还表明，结构的一些几何奇点（点、线、面）与链的CH3末端位置以及水基质的正中心重合，即与短程无序最高的区域重合。我们引入“混沌区”这一表述来指代这些区域。在所有脂质相中，发现混沌区占据特殊的几何位置，要么与对称元素相关，要么与IPMS相关。因此，似乎采取短程无序的有序排列在能量上比最小化极性/非极性界面的面积更有利。最后，关于脂质多态性可能的生物学意义，强调了一点，即在与过量水处于平衡状态下观察到的相（这些相也是最有可能具有生物学作用的候选相）中，具有立方对称性的相应受到特别关注。（摘要截取自400字）
