Phases and domains in sphingomyelin–cholesterol membranes: structure and properties using EPR spin-labeling methods

Abstract

EPR spin-labeling methods were used to investigate the order and fluidity of alkyl chains, the hydrophobicity of the membrane interior, and the order and motion of cholesterol molecules in coexisting phases and domains, or in a single phase of fluid-phase cholesterol/egg-sphingomyelin (Chol/ESM) membranes with a Chol/ESM mixing ratio from 0 to 3. A complete set of profiles for these properties was obtained for the liquid-disordered (ld) phase without cholesterol, for the liquid-ordered (lo) phase for the entire region of cholesterol solubility in this phase (from ~33 to 66 mol%), and for the lo-phase domain that coexists with the cholesterol bilayer domain (CBD). Alkyl chains in the lo phase are more ordered than in the ld pure ESM membrane. However, fluidity in the membrane center is greater. Also, the profile of hydrophobicity changed from a bell to a rectangular shape. These differences are enhanced when the cholesterol content in the lo phase is increased from 33 to 66 mol%, with clear brake-points between the C9 and C10 positions (approximately where the steroid-ring structure of cholesterol reaches into the membrane). The organization and motion of cholesterol molecules in the CBD are similar as in the lo-phase domain that coexists with the CBD.

Properties of membranes derived from the total lipids extracted from clear and cataractous lenses of 61–70-year-old human donors

“…For all these membranes the oxygen transport parameter from the membrane surface to the depth of C9 is as low as in gel-phase PC membranes; at locations deeper than the C9 it is as high as in fluid-phase membranes (Subczynski et al 1989, 1991). These profiles are typical for liquid-ordered-phase membranes saturated with Chol and are very different from the bell-shaped profile across model membranes without Chol (Mainali et al 2012b; Raguz et al 2008, 2009; Subczynski et al 2010; Widomska et al 2007). Similar to profiles of the order parameter (Fig.…”

Toward increased concentration sensitivity for continuous wave EPR investigations of spin-labeled biological macromolecules at high fields

“…High fields provide increased sensitivity to measure the structural parameters of membranes and membraneassociated proteins, such as fluidity, polarity, orientation and membrane order [26,27,30,31,74,75]. An improved sensitivity of sample holders at high fields will greatly facilitate the unraveling of these complex parameters.…”

Organization of lipids in fiber-cell plasma membranes of the eye lens

“…1. All profiles for membranes with a saturating amount of cholesterol differ drastically from profiles across membranes without cholesterol (see for example references [59, 60]). All these data have led to the conclusion that the extremely high (saturating) content of cholesterol in the fiber-cell membrane keeps the bulk physical properties of the lipid-bilayer portion of the membrane consistent and independent of changes in the phospholipid composition.…”

“…Human fiber-cell plasma membranes are built from highly saturated PLs [15, 32, 34], ~66% of which are sphingomyelins (SMs) and dihydrosphingomyelins (DHSMs) [32]. Membranes made of only saturated PLs [59], particularly SMs [60] exhibit very low hydrophobicity in the center of the membrane comparable to that of pentanol and octanol (with ε = 10–20). The hydrophobicity of these membranes reaches the maximal value, comparable at membrane centers to that of hexane (with ε = 2) only in the presence of a high amount of Chol (30 – 50 mol%) [59, 60].…”

“…Membranes made of only saturated PLs [59], particularly SMs [60] exhibit very low hydrophobicity in the center of the membrane comparable to that of pentanol and octanol (with ε = 10–20). The hydrophobicity of these membranes reaches the maximal value, comparable at membrane centers to that of hexane (with ε = 2) only in the presence of a high amount of Chol (30 – 50 mol%) [59, 60]. Thus, the high Chol content in fiber-cell plasma membranes ensure that the lipid bilayer portion of these membranes forms the high hydrophobic barrier for permeation of polar molecules.…”