Purple fragments

Figure 5.27 Schematic representation of a model for the conformational change of hemagglutinin that at low pH brings the fusion peptide to the same end of the molecule as the receptor binding site. The fusion peptide (purple) is at the end of heUx A about 100 A away from the receptor binding site in the high pH form. In the low pH fragment this region of helix A has moved about 100 A towards the area where the receptor binding sites are expected to be in the intact hemagglutinin molecule. (Adapted from D. Stuart, Nature 371 19-20, 1994.)...

Figure 8.22 The lac repressor molecule is a V-shaped tetramer in which each arm is a dimer containing a DNA-hinding site. The helix-tum-helix motifs (red) of each dimer bind in two successive major grooves and the hinge helices (purple) bind adjacent to each other in the minor groove between the two major groove binding sites. The four subunits of the tetramer are held together by the four C-terminal helices (yellow) which form a four helix bundle. The bound DNA fragments are bent. (Adapted from M. Lewis et al., Science 271 1247-1254, 1996.)...

It is necessary to note that the initial conditions of the samples in solution were absolutely different. RC was extracted from the membranes by detergent (lauryldimethy-lamineoxide—LDAO) the solution contains the individual protein molecules surrounded by a detergent belt shielding the hydrophobic areas of the protein surface. In the case of BR the situation is different. BR is the main part of purple membranes (about 80%) and is already close packed in it. It is difficult to extract BR in the form of individual molecules, for they are very unstable (Okamura et al. 1974). Thus, the initial solution of BR was in reality the solution of sonicated membrane fragments. 

Fig. 8.19 SEM images of mesolamellar thin films produced by intercalation of nanosheets of (A) aminopropyl-functionalized silica or (B) AMP between stacked purple membrane fragments containing bacteriorhodopsin (scale bars= 10pm).

In the purple bacterium Halobactium halobium, light driven proton transfer through membranes is executed by bacteriorhodopsin [41a]. This protein consisting of seven helical fragments criss-crosses the membrane seven times,... 

Fig. 2 Molecular models of two conformers of the 14-mer duplex d(ATACATGGTACATA) d (TAT17G18TACCATGTAT) ruthenated at N7 of G18 with monofunctional fragment Ru(r 6-biphenyl)(en) 2+ (a) showing the intercalation of the arene between G18 and T17 (b) the arene is non-intercalated but stacked on a tilted T17. Colour code Ru purple-, en blue-, biphenyl green. Adapted from [92]...

This approach has been extended by Rupley et al. (1988) to study of the water-induced percolation in hydrated purple membrane fragments of Halobacterium halobium. The results and conclusions are qualitatively similar to those reported above for lysozyme. (1) The percolation is two-dimensional, judged by the value of the critical exponent (Fig. 15). (2) Certain regions of the surface provide preferred protonic conduction paths. (3) There is a correspondence between the onset of function—here, the photoresponse—and the establishment of long-range connectivity within the surface water clusters. 

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