Optical activity of biological membranes: scattering effects and protein conformation

THE FOLLOWING QUESTION HAS RECENTLY ARISEN IN THE LITERATURE CONCERNING THE INTERPRETATION OF THE OPTICAL ACTIVITY OF BIOLOGICAL MEMBRANES: do the characteristic spectral distortions observed for diverse membrane systems reflect some common and unique aspect of membrane architecture or are they the result of scattering effects owing to the particulate nature of membranous systems? We have confirmed the latter interpretation on the basis of the following experimental observations: (a) red blood cell membranes give a normal circular dichroism spectrum when scattering is reduced and (b) nonaggregated, nonmembranous helical proteins give distorted membranelike spectra when scattering is introduced. An improved estimate of secondary structure on the basis of undistorted spectra results in about 50 per cent alpha-helix for red blood cell membrane protein. In addition we conclude that the distortions in optical activity spectra offer no evidence in support of various proposed membrane models.