Chapter 13 Transmembrane Polarity Profile of Lipid Membranes

Abstract

Abstract Biological membranes act as a barrier for permeation of solutes and solvents into the cell interior. This effect is largely due to the hydrophobicity of the membrane interior, which influences the transport of water, polar molecules and oxygen into the membranes. The penetration depth of water into the membranes, and the resultant transmembrane polarity profile, determine also interactions of small molecules, such as amino acids or proteins, with the lipid bilayer, and as well as the energetics of the insertion of proteins into membranes. Therefore, information on the hydrophobicity, which is inversely related to the polarity, of the biological membranes is crucial not only for understanding the processes in living cells, but also for development of an efficient system for the targeted drug delivery into the human cells or organs. In this chapter, recent studies on the transmembrane polarity profile of model and biological membranes by different physical methods are reviewed. In the first part, the physical methods (X-ray, neutron diffraction, capacitance, electron spin resonance (ESR) and fluorescence spectroscopies) in use for the study of the transmembrane polarity profile are described with the emphasis on the measurements of the permeation of water and polar molecules into the lipid bilayers. The second part reviews recent studies on the membrane polarity profile with respect to the lipid compositions, membrane domain structure, interactions with peptides and proteins. The emphasis is given on the results of different ESR and fluorescent methods that were applied recently to the measurements of polarity profile and the water penetration depth into membranes. As a general result it follows that the transmembrane polarity profile has a sigmoidal shape, which describes high polarity at the phospholipid head group region, and low polarity in the hydrophobic region of the membrane. The main factors that influence the transmembrane polarity profile are the fluidity of the membrane bilayer, dipole potential and hydration.