Structure of the ripple phase in lecithin bilayers.

Abstract

The phases of the x-ray form factors are derived for the ripple (Pp.) thermodynamic phase in the lecithin bilayer system. By combining these phases with experimental intensity data, the electron density map of the ripple phase of dimyristoyl-phosphatidylcholine is constructed. The phases are derived by fitting the intensity data to two-dimensional electron density models, which are created by convolving an asymmetric triangular ripple profile with a transbilayer electron density profile. The robustness of the model method is indicated by the result that many different models of the transbilayer profile yield essentially the same phases, except for the weaker, purely ripple (0k) peaks. Even with this residual ambiguity, the ripple profile is well determined, resulting in 19 i for the ripple amplitude and 100 and 26°for the slopes of the major and the minor sides, respectively. Estimates for the bilayer head-head spacings show that the major side of the ripple is consistent with gel-like structure, and the minor side appears to be thinner with lower electron density.Lipids in water self-assemble into lamellar bilayers, which comprise the basic structural element of biomembranes. The supramolecular packing of the bilayers, in turn, forms lyotropic liquid crystals with rich phase behavior and diverse structures. Among the thermodynamic phases observed in lipid bilayer systems, the ripple (Pp,) phase in lecithin bilayers is especially fascinating to a broad range of researchers in condensed matter physics and physical chemistry as an example of periodically modulated phases. Many theoretical papers have attempted to explain the formation of the ripples (1-8). Such understanding has been delayed because, despite many experimental studies (9-19), the structure has not been as well characterized as have the structures for the lower temperature gel (Lp,) phase (20) and the higher temperature liquid crystalline (La) phase (21). When an accurate ripple structure is known, some indication of the energetics of the ripple formation may be obtained by comparing the relative size of the ripple wavelength and the ripple amplitude to the size of lipid molecules and the bilayer thickness (5). Also, correlation between the chirality of the constituent lipid molecules and the asymmetry of the ripples may lead to elucidation of the microscopic origin of the macroscopic properties of the ripple phase (8,19).Structural information about the ripple phase can be divided into two categories. The first category consists of the twodimensional lattice parameters, namely, the ripple wavelength Ar, the bilayer packing repeat distance d, and the oblique angle ,y for the unit cell that are illustrated in Fig. 1. These wellestablished parameters have been accurately obtained by indexing low-angle x-ray scattering peaks (9,10,12,16,18). For example, Wack and Webb (18) reported for dimyristoylphosphatidylcholine (DMPC) with 25% water by weight at 180C that Ar = 141.7 A, d = 57.94 A, and y = 98.40. The second category consists of the structu...

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