EVIDENCE FOR UNBENIGNANT NATURE OF GLUCOSE AS A REPLACEMENT FOR WATERIN PURPLE MEMBRANE

The net angle (theta(alpha)) between the seven helical segments of the bacteriorhodopsin (bR) polypeptide and the normal to the membrane pla ne of the purple membrane (PM) is approximately 0-degrees when determi ned by oriented far-ultraviolet (UV) circular dichroism (OCD) and midi nfrared linear dichroism (IRLD). However, theta(alpha) is approximatel y 11-degrees when determined by high-resolution electron cryo-microsco py and electron diffraction (EMD). The spectral studies are made with fresh hydrated PM films at ambient temperature, whereas diffraction st udies are made with aged glucose-embedded PM at -1 20 to -268-degrees- . The current study presents oriented far-UV OCD results of hydrated P M films embedded with glucose, which can best be interpreted as a chan ge in the magnitude of theta(alpha) (DELTA(theta)alpha) from 0 to 23-d egrees as a consequence of glucose embedment. Possible alternative exp lanations contrary to this conclusion are discussed and ruled out. The refore, it is suggested that a theta(alpha) of approximately 11-degree s as determined by the EMD method may not be an intrinsic structural c haracteristic 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, especiall y temperature. It is expected that at extremely low temperatures prote in dynamics would be highly restricted and DELTAtheta(alpha) relativel y curtailed. ft is concluded that glucose may not be as benign to biol ogical structures as has been assumed in the past.