Infrared Study of the Structure and Composition of Rabbit Lens Membranes: a Comparative Analysis of the Lipids of the Nucleus, Cortex and Epithelium

Infrared spectra of the phospholipids extracted from the nuclear, cortical and epithelial regions of the rabbit lens membrane bilayers are examined for the first time. Major structural, conformational and compositional differences in the lens membrane are correlated at physiological temperature. Vibrational data distinguish two classes of the phospholipids present in the rabbit lens membranes. Sphingo-lipids with a sphingosine backbone are largely concentrated in the nucleus whereas phospholipids with a glycerol backbone (glycero-lipids) such as phosphatidylcholine and phosphatidylethanolamine are abundant in the cortical region of the lens. Nuclear lipids are more saturated by a factor of four over the lipids of the cortical region. Nuclear lipids also exhibit increased headgroup and interface hydration and stronger hydrogen bonding over cortical lipids which provide them additional structural stability needed for the clarity of the lens. The lipid composition of the epithelial membranes is structurally similar to that of the cortical region but the hydrocarbon chains are more saturated. Epithelial membranes are largely shielded from bilayer water indicating hydrophilic lipid-water interactions are not important for the stability of these membranes. These membranes exhibit much stronger lipid-lipid interactions, vital for many physical properties like membrane fluidity and permeability. The fiber cell membranes are stabilized by hydrogen bonding of the carbonyls of the interface region and by headgroup-specific lipid-water interactions, while epithelial membranes are stabilized primarily through hydrophobic lipid-lipid interactions.