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Abstract
Monolayers of sphingomyelin (SM), ceramide (Cer) and cholesterol (Ch) and binary mixtures SM–Ch, SM–Cer and Cer–Ch were investigated at the air–water interface. SM, Cer and Ch were used in the experiment. The surface tension values of pure and mixed monolayers were used to calculate π-A isotherms. Surface tension measurements were carried out at 22 °C using a Teflon trough and a Nima 9000 tensiometer. Interactions between sphingolipid and Ch as well as sphingolipid and another sphingolipid result in significant deviations from the additivity rule. An equilibrium theory to describe the behavior of monolayer components at the air–water interface was developed in order to obtain the stability constants and Gibbs free energy values of SM–Ch, SM–Cer and Cer–Ch complexes. We considered the equilibrium between the individual components and the complex and established that sphingolipid and Ch as well as sphingolipid and another sphingolipid formed highly stable 1:1 complexes.
Highlights
Cholesterol (Ch), ceramide (Cer) and sphingomyelin (SM) are important constituents of cellular plasma membranes
An equilibrium theory to describe the behavior of monolayer components at the air–water interface was developed in order to obtain the stability constants and Gibbs free energy values of SM–Ch, SM–Cer and Cer– Ch complexes
We present evidence for the formation of 1:1 SM–Ch, SM– Cer and Cer–Ch complexes at the air–water interface
Summary
Cholesterol (Ch), ceramide (Cer) and sphingomyelin (SM) are important constituents of cellular plasma membranes. These molecules are chemically as well as functionally different, and still they appear to colocalize in the same membrane compartment and even to be attracted to each other. Cers are relatively minor polar components of cell membranes, varying between 1 and 10 % in proportion to glycerophospholipids (Hannun 1996). Researchers for many years have been trying to define the physical properties of mixtures of Ch and sphingolipids in monolayer membranes. This quasi-2D system is of intrinsic interest to physical chemists. Studies on the monolayer lipid membranes help us to understand certain properties of the membranes of animal cells (Slotte 1999; Bernholz 2004; Epand and Epand 2004; Halling and Slotte 2004; Guan et al 2009)
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