Systematic Variation of Gel-Phase Texture in Phospholipid Membranes

Abstract

The tilted gel phase of lipid bilayers can display in-plane orientational texture due to long-range alignment of the molecular director. We explore systematic variations of texture defects in a series of binary phospholipid membranes. Using polarized two-photon fluorescence microscopy, the texture pattern of single domains is revealed. The appearance of a central vortex-type defect in each domain correlates with a particular range of hydrophobic mismatch values h > 1 nm at the domain border while domains with h < 1 nm correlate with uniformly aligned texture. The central vortex defect is characterized by a defect angle, indicating its bend or splay nature. Using image analysis, we measure the defect angle and find that it has primarily bend character for small mismatch values (h ≈ 1 nm) and primarily splay nature for larger values of h. For domains containing a vortex, the domain shape is decoupled from the texture while for uniformly textured domains there is a preferred texture orientation of ≃45° along the domain border. The results establish a foundation for understanding texture phenomena in compositionally complex membranes.

Nonlinear chiral rheology of phospholipid monolayers

“…Brewster angle 25 or polarized fluorescence microscopy 6,7,26 show the tilt orientation is uniform over large areas of the domains, with discontinuous jumps across “tilt-gradient” lines 6,26 . The tilt gradient lines originate from disclination pairs at the domain core 6,7 and terminate at the invaginations in the domains where the bright, dye rich lines abruptly end 6,7 . These regions of uniform tilt are relatively constant in width, suggesting a balance between the molecular correlations and the strain induced by the chiral twist.…”

“…One solution adopted by many materials 8–11 is to localize this required twist to defects 9–11 , or to expel the twist to the material boundaries 8 . In r-DPPC monolayers, this frustration between tilt and twist leads to “tilt gradient lines” across which tilt rapidly changes orientation 6,7 , as indicated in Fig. 1a.…”

“…1a. These discontinuous changes in tilt direction allow the chiral precession required globally, while maintaining a constant nearest-neighbor-directed tilt orientation locally 6,7 . Individual domains adopt a multi-lobed structure that winds in a counter-clockwise fashion (Fig.…”

“…Individual domains adopt a multi-lobed structure that winds in a counter-clockwise fashion (Fig. 1,2) 7,12 .…”

Shiga toxin induces membrane reorganization and formation of long range lipid order

“…18,21 The hydrophobic mismatch is correlated with the shape of gel domains and recently it was shown that differently shaped domains possess different texture of lipids. Different lipid combinations were chosen due to different hydrophobic mismatches, which exist between liquid and gel phases in the lipid bilayers.…”