Electrodichroism of purple membrane: ionic strength dependence

The dichroism of purple membrane suspension was measured in dc and ac electric fields. From these measurements three parameters can be obtained: the permanent dipole moment, mu, the electrical polarizability, alpha, and the retinal angle, delta, (relative to the membrane normal). The functional dependence of the dichroism on the electric field is analyzed. There is a small decrease ( approximately 2 degrees ) in retinal angle going from dark adapted to the light adapted form. No measurable difference in mu, alpha, and delta was found under the photocycle. The dichroism was measured in two different salt solutions (KCl and CaCl(2)) in the range 0-10 mM. The retinal angle increases from 64 degrees to 68 degrees with increasing ionic strength going through a minimum. This is attributed to the changing (decreasing) inner electric field in the membrane. The polarizability, alpha, consists of two parts. One component is related to the polarization of the purple membrane and the second component to the ionic cloud. The second component decreases with ion concentration approximately as kappa(-3) (kappa is the Debye parameter) in agreement with a model calculation for the polarization of the ionic cloud. The origin of the slightly ionic strength dependent permanent dipole moment is not well understood.