Helical rod

Figure 5.3 Electron micrographs of diastereomers of A-octyl-D-aldonamide (a) helical rods from D-Glu-8 (1, bar = 50 nm), (b) rolled-up sheets from D-Man-8 (2, bar = 300 nm), and (c) twisted ribbons from D-Gal-8 (3, bar = 300 nm). Reprinted with permission from Ref. 31. Copyright 1990 by the American Chemical Society.

Recently, a very interesting example of solvatochromism was reported by Fujiki and co-workers.206 Poly(methyl-3,3,3-trifluoropropylsilylene), 87, synthesized via Wurtz coupling, showed solvatochromism as a result of weak, non-covalent intramolecular Si- -F-G interactions which rendered the conformation of the polysilane uniquely controllable by solvent choice and molecular weight. UV, shown in Figure 18, photoluminescence, NMR, and viscosity studies on the polymer indicated a 73 helical rod-like conformation at room temperature in non-coordinating solvents (e.g., toluene and decane), since the intramolecular interaction resulted in constraining the chain in a rigid helix. 

This expression is made up so that p tends to unity at the limit of random-coil (fN = 0) and approaches the value of length/diameter at the limit of intact a-helix (fN = 1), with the assumption of 15 A for the diameter of the helical rod (2). It roughly accounts for the effect that the distribution of dipoles in the molecule becomes more spherically symmetric in going from helix to random-coil. Since p appears only in the correction terms for /g, its exact values are not needed. It has been shown (117) that the calculated values of depend little on the type of equation employed for p except in the region close to fN = 0. 

The gradual declines of x T/t]0 observed in Fig. 41 may be accounted for in part by departures of the actual polymer from the all-or-none model, and in part by the increase in the population of partially broken helical rods with the increase in helical fraction. At any rate, these experimental results on the system PCBL-ro-cresol are of great interest as the first affirmation of a system that exhibits a helix-coil transition close to the all-or-none type. 

Due possibly to the above mentioned heterogeneity, there is some variability with regard to the conclusions reached by various workers concerning the structure and configuration of bovine serum albumin. Brown (1977) proposed two possible models based on the primary sequence of the protein. He demonstrated that the molecule could possess a triple domain structure with three very similar domains residues 1-190, 191-382, and 383-582. Each domain could then consist of five helical rods of about equal length arranged either in a parallel or an antiparallel manner. His second model consisted of the following (1) a lone subdomain (1-101) (2) a pair of antiparallel subdomains, with their hydrophobic faces toward each other (113-287) (3) another pair of subdomains (314-484) and (4) a lone subdomain (512-582). These structures are supported by the observed helical content of bovine... 

Membranes contain many largely a-helical proteins. Cell surface receptors often appear to have one, two, or several membrane-spanning helices (see Chapter 8). The single peptide chain of the bacterial light-operated ion pump bacteriorhodopsin (Fig. 23-45) folds back upon itself to form seven helical rods just long enough to span the bacterial membrane in which it functions.189 Photosynthetic reaction centers contain an a helix bundle which is formed from two different protein subunits (Fig. 23-31).190 A recently discovered a,a barrel contains 12 helices. Six parallel helices form an inner barrel and 6 helices antiparallel to the first 6 form an outer layer (see Fig. 2-29).191-193... 

Each coat subunit in the Ff viruses is coiled into an a-helical rod of 7 nm length. These are arranged in the virus in a right-handed helical pattern with a pitch of 1.5 nm and with 4.4 subunits per turn (Fig. 7-7). The protein rods are inclined to the helix axis and extend inward. This arrangement permits a "knobs-in-holes" hydrophobic bonding between subunits. The helix of pitch 1.5 nm is the primary or one-start helix. However, in every regular helical structure we can also trace a two-start helix, a three-start helix, etc. In this instance the five-start helix is easiest to see. 

Trichohyalin contains extensive repeat sequence blocks, which are predicted to form an elongated, flexible single-stranded o-helical rod (Lee et al, 1993). These predictions are consistent with biochemical and electron microscopical analysis showing an extended shape with overall dimensions of 85 nm. Similar to profilaggrin, the N-terminus comprises an S-100 protein-like calcium-binding domain. 

Fig. 18 Schematic illustration of the preparation and photoresponsive behavior of the polypeptide XXIII, consisting oftwo amphiphilic helical rods linked by an azobenzene unit, a) Selective saponification of COOCH3 side...

The compound XXIII, consisting of two amphiphilic helical rods linked by an azobenzene moiety, was found to form micelles and ordered aggregates in aqueous solution in the dark, when the azo moiety is in the trans configuration. Photoisomerization of the azo linkage into the cis configuration, and the consequent bending in the structure of the molecules, induced disaggregation and disruption of the micelles.1118,1191... 

The poly(L-lysine) formed a-helical rods in the multilayers that are aligned along the scratching direction of the substrate. He applied attenuated total reflection-Fourier transform spectroscopy (ATR-FTIR) using polarized light for the determination of the orientation within the films. 

All three types of filament are polar helical rods (i.e., they have chemically distinct "heads" and tails ). In addition to the multiple contacts between subunits that serve to stabilize the filaments, additional multiple binding sites are available to bind auxiliary proteins. These auxiliary proteins can regulate the assembly and disassembly of the filaments, link filaments to one another and to other cellular components including membranes, and allow the filaments to participate in energy-dependent movement. 

The helices of the rhodopsin transmembrane domain are distinguished from those of bR by the large number of irregularities and kinks, due primarily to Pro and Gly residues (Figs. IB and 3). These kinks provide potential points of flexibility in the otherwise rigid helical rods. One minor kink, at Ser-127 in TM3, may be due to H bonding of the side chain —OH with the backbone carbonyl of Ile-123, and as such would not constitute a point of flexibility. Of particular interest is Pro-267 in TM6. This residue is completely conserved throughout the family of rhodopsin-like receptors and is likely to be a fundamental element of the activation switch (Baldwin et al., 1997 see Section V,C). 

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