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.