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. Sphingolipids 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. Data also exhibit two phase transitions around 16 and 35 degrees C corresponding to the melting of the hydrocarbon chains from two pools of the phospholipids i.e. glycero- and sphingophospholipids, respectively. The first transition is caused by disruption of the carbonyl hydrogen bonding and disordering of the acyl chains of the glycero-lipids, while second transition is driven mainly from the hydrogen bonding effects of the carbonyls of the sphingolipids followed by acyl/sphingosine chain disordering. Upon transition to the liquid-crystalline phase, a sizable amount of the fiber cell membranes are disordered (approximately 33%) due to increased conformational motion of the acyl chains resulting from a loss of extended CH2 trans segments. These chains of the fully hydrated lipids pack in an hexagonal or triclinic unit cell in rabbit lens membranes.