AFM and FTIR Spectroscopy Investigation of the Inverted Hexagonal Phase of Cardiolipin

Abstract

Atomic force microscopy (AFM) and FTIR spectroscopy techniques have been exploited to investigate the inverted hexagonal phase (H(II)) of cardiolipin obtained by dehydration of a phospholipid water dispersion on a solid support. The characteristic cylinders of the H(II) phase have been imaged by AFM and the effects of different preparation conditions (temperature and the presence of chemicals) on the structural parameters and on the presence of local nanoscale defects have been studied. It has been found that the measured repeat spacing of the H(II) cylinders decreases upon increase of temperature and addition of pentachlorophenol (PCP), a chemical which is known to affect the structure and function of lipid bilayers. It has been shown that AFM can help in revealing some features of the mechanism of the inverted hexagonal phase formation, corroborating the results of a recent molecular dynamics study on the H(II) phase formation from multilamellar phospholipid structures.

Electrically induced lipid migration in non-lamellar phase

“…There is a high chance that the inner structure remains H II , since they were originally surface-bound H II structures before the sonication. A similar structure in air has been reported with Cardiolipin which is another molecule known for inverted hexagonal phase [29]. Nevertheless, further studies on the internal architecture are difficult, since the surface-bound microscopic lipid patches do not give enough signals in NMR or X ray diffraction, while cryoTEM is for samples in solution but not for surface-bound molecules.…”

Effects of Cardiolipin on Membrane Morphology: A Langmuir Monolayer Study

“…Previously the role of CL in modulating membrane properties and membrane integrity has been probed using biophysics model membranes, including monolayer, bilayer, and vesicle systems. Model membrane studies determined that the incorporation of CL strongly affected the thermodynamic properties of lipid monolayers (15,16) and bilayers (15,(17)(18)(19). Monolayer analyses indicate that a mixture of EPC (egg phosphatidylcholine) and CL in specific mixed ratios and at specific pressures is thermodynamically stable, even though the apparent area compressibility modulus and the lysis tension of vesicle structures decreases with increasing CL content.…”

“…Such an effect would likely decrease the energy required to create folds in the inner mitochondrial membrane. Due to its chemical structure, CL may form micellar, lamellar, and hexagonal states in an aqueous dispersion, depending on the pH and ionic strength of the environment (17)(18)(19)(20). The abundance of high curvature zones in mitochondrial cristae might also be attributed to the intrinsic curvature of the CL molecule (7,21).…”

Biomembrane models

“…However, the presence of cations, such as Ca 2+ , or lowering the pH, which protonates the lipid head groups, may promote the transition to the H II phase. 21,24,25 Hydration of the lipid may also affect the lipid phase preference. Lower hydration (as a result of decreasing the (a) Defi nition of the critical packing parameter ( v / α o l c where v is the hydrocarbon chain volume, α o is the optimal polar head group cross-sectional area and l c is the critical chain length).…”

“…(b) The geometrical shapes of lipid molecules determining their critical packing parameter values and the respective structures formed by the lipid molecules in water: a -micelles, bbilayer, c -inverted micelles (hexagonal phase H II ) amount of water added to the lipids or of using salting-out salts) promotes formation of the H II phase. 21,25 The most important physical factor that regulates lipid phases is temperature. At low temperatures, the C-C single bonds in lipid alkyl chains adopt the all-trans conformation in which lipid molecules are extended and are basically rodshaped.…”