Synchrotron x-ray study of the modulated lamellar phasePβ’in the lecithin-water system

Abstract

The results of a high-resolution, synchrotron x-ray powder diffraction study of lattice constants in the P& (" rippled" ) phase of lecithin-water multilamellar mixtures are given. The variation with water volume fraction P and hydrocarbon chain length N, of the modulation wave vector Q"suggests that membrane curvature and hydration interactions between membranes play a significant role in the modulation. The dependence of the membrane thickness and area per head group on N, indicates that the conformation of hydrocarbon chains is predominantly solidlike. The appearance of significant intensity in higher harmonics of the modulation wave vector rules out simple (e.g. , sinusoidal or triangular) membrane density modulations. The results are consistent with a Lifshitz phenomenological model for lamellar phases of interacting membranes proposed by Goldstein and Liebler [Phys. Rev. Lett. 61, 2213 {1988)]. The phase behavior predicted by the model includes a multicritical point called the Lifshitz point where the wavelength of the modulation diverges. The experimental results indicate that this multicritical point lies in the vicinity of N, =9 and P =0.18.Both the chain melting (or "main") transition T and the

Structure of the ripple phase in lecithin bilayers.

“…It has been shown from x-ray studies (9,10,12,16,18,19) that ripples in different bilayers are registered to form a two-dimensional oblique lattice as shown by the unit cell in Fig. 1.…”

“…The sixth parameter is a common scaling factor for all the I(h,k) peaks. For the simple 1G model, all the transbilayer profile parameters were taken from gel phase data (24) (18), an experimental relative electron density map p(r) is obtained using p(x,z) = E EF(h,k)cos(qhkx+qhkz). h20 k [13] RESULTS AND DISCUSSION The absolute form factors IF(h,k)I reported by Wack (18).…”

“…Many theoretical papers have attempted to explain the formation of the ripples (1)(2)(3)(4)(5)(6)(7)(8). Such understanding has been delayed because, despite many experimental studies (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(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).…”

“…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 structure within the ripple unit cell, including the shape and the amplitude of the ripples and the bilayer thickness.…”

Structural intermediates in phospholipid phase transitions

“…3 shows the structures, as derived from X-ray powder diffraction (Janiak, Small & Shipley, 1979;Wack & Webb, 1989), below and above this transition under equilibrium conditions. The main question addressed in the present studies concerns the structural pathways by which the symmetry changes from one dimensional (lamellar stack) to two dimensional (monoclinic, 'ripple phase').…”