Evidence for unbenignant nature of glucose as a replacement for water in purple membranes

The net angle (theta(alpha)) between the seven helical segments of the bacteriorhodopsin (bR) polypeptide and the normal to the membrane plane of the purple membrane (PM) is approximately 0 degrees when determined by oriented far-ultraviolet (UV) circular dichroism (OCD) and midinfrared linear dichroism (IRLD). However, theta(alpha) is approximately 11 degrees when determined by high-resolution electron cryo-microscopy and electron diffraction (EMD). The spectral studies are made with fresh hydrated PM films at ambient temperature, whereas diffraction studies are made with aged glucose-embedded PM at -120 to -268 degrees . The current study presents oriented far-UV OCD results of hydrated PM films embedded with glucose, which can best be interpreted as a change in the magnitude of theta(alpha) (Deltatheta(alpha)) from 0 to 23 degrees as a consequence of glucose embedment. Possible alternative explanations contrary to this conclusion are discussed and ruled out. Therefore, it is suggested that a theta(alpha) of approximately 11 degrees as determined by the EMD method may not be an intrinsic structural characteristic of the native PM but an induced one. The differences in the Deltatheta(alpha) value due to glucose embedment as determined by the two different approaches (23 vs. 11 degrees ) may be attributed to the drastic differences in the experimental conditions used, especially temperature. It is expected that at extremely low temperatures protein dynamics would be highly restricted and Deltatheta(alpha) relatively curtailed. It is concluded that glucose may not be as benign to biological structures as has been assumed in the past.