Optical determination of the thickness of thin lipid films

Theoretical calculation of the dielectric constant of a bilayer membrane

The dielectric constant (epsilon) and refractive index (n) of a bilayer lipid membrane is determined from the known values of the polarizabilities of the carbon-carbon and carbon-hydrogen bonds. It is assumed that the hydrocarbon chains are hexagonally arranged in an all-trans conformation perpendicular to the plane of the membrane. The only variable in the calculation is the average separation between the chains and the theory relates epsilon to this separation. The calculation and results differ significantly from those presented in a 1968 publication by Ohki. It is shown that a thin membrane is not homogeneously polarized by the applied field. This effect is analysed and the dependence of epsilon on the membrane thickness is determined. The theoretical results are in good quantitative agreement with experimental measurements on bulk paraffins and on oriented multilayers of saturated fatty acids. The most important conclusion is that the dielectric constant for an applied field perpendicular to the membrane (which is the appropriate value for capacitance measurements) differs by only a few percent from the value for the macroscopic (bulk) liquid hydrocarbon. Thus the dielectric constant of a bilayer membrane can be approximated by the value for the appropriate bulk hydrocarbon.

Simultaneous measurements of optical and electrical properties of artificial membranes composed of mitochondrial lipids and their interaction with cytochrome c

A newly constructed cell, which allows simultaneous measurements of optical and electrical properties, was used to study bimolecular black membranes composed of beef heart mitochondrial lipids and their interaction with cytochrome c. The results show that these highly charged membranes are stable only in relatively limited ranges of boundary conditions. In 0.1 n KCl their maximum direct current (dc) resistance is 7 X 10(8) Ohm cm2 +/- 10%; the series capacity at 1 kHz is 0.43 muF/cm2 +/- 3%; the entire thickness, determined by optical reflectivity, is 5.8 +/- 0.2 nm. The interaction between oxidized cytochrome c and these lipid membranes is primarily of electrostatic nature, and dependent on the presence of highly charged phospholipids, such as diphosphatidyl glycerol (cardiolipin) and phosphatidyl ethanolamine. The attachment of cytochrome c maximally causes a 2.5-fold increase in reflectivity, without any noticeable change in the capacity. This leads to a subsequent instability of the membrane (i.e., rupture) preceded by a rapid increase of the dc conductivity. This behavior is far less pronounced with reduced cytochrome c.

Phase transitions and heterogeneity in lipid bilayers

The optical reflectivity of several well-characterized lipid bilayer systems has been correlated with calorimetric studies of the membrane components. There is a large increase in mean membrane thickness when a bilayer is cooled below the transition temperature of the membrane lipid. Similar studies on membranes generated from a mixture of two lipids possessing different degrees of unsaturation suggest that between the characteristic transition temperatures of the two lipids, the bilayer contains clusters of gel and liquid crystalline lipid which coexist within the plane of the membrane.