Hinge structures

In a note Bennett et al. (38, 39) report a red material of [IrCl(PFs)2]2 stoichiometry which they claim is polymeric in the solid state with metal-metal bonds, 12. This material may also have the hinged structure (112) of the isoelectronic [Rh(CO)2Cl]2,13. The former structure is plausible because of the greater propensity of third-row metals toward metal-metal bond formation. 

Change-can mixer (pony mixer) n. A type of planetary mixer comprising several paddle blades mounted on a vertical shaft rotating in one direction while the can or contained counter-rotates. The paddle shaft is usually mounted on a hinged structure so that it can be swung out of the can, permitting the can to b removed, emptied, and replaced easily. This type of mixer is employed for relatively small batches (12-480 L) of fluid dispersions and dry materials. 

Alisina and Trematobolus demonstrate more advanced hinge structures in obolel-lide valves. Paired denticles, composed entirely of secondary shell, flank a concave pseudodeltidium in these genera (Fig. 18.1(7)). Shallow, closely placed sockets are discernible on the anterolateral sides of a narrow notothyrial platform, which divides medially narrow dorsal propareas (Fig. 18.1(5)). Homologues of the internal oblique muscles of early oboleUides were attached dorsally behind the rotation axis so that they could have acted as diductors. The forces produced by the action of... 

Many experimental problems continue to be of vital importance in establishing the validity or otherwise of theoretical models. Neutron investigation of chain shape in different nematic comb melts would help establish the relation, if any, between variation in hinge structure and nematic main chain and side chain coupling and the resultant phase symmetry. With the advent of a wider range of syntheses, many more proposals and speculations for new types of behaviour and effects can be made. 

Fig. 5. Rigid-body analysis of citrate synthase, using two X-ray structures (after Hayward and Berendsen, Proteins 30 (1998) 144). The decomposition of the protein into two domains (dark gray and white) and two interconnecting regions (light gray) is shown, together with the hinge axis for the closing/opening motion between them.

The first technique is very intuitive. Out of the few proteins that could be crystallized in a number of different conformations, adenylate kinase is probably the best-studied example. By combining nine observed crystal structures and interpolating between them, a movie was constructed that visualized a hypothetical path of its hinge-bending transition (jVonrhein et al. 1995]). 

The effective moment of inertia / and the friction coefficient / could easily be estimated. The force constant k associated with the relative motion of the lobes was determined from an empirical energy function. To do so, the molecule was opened in a step-wise fashion by manipulating the hinge region and each resulting structure was energy minimized. Then, the interaction energy between the two domains was measured, and plotted versus 0. 

Example The distance between two ends of a large, flexible molecule can provide information about its structural properties or its interaction with solvent. Analysis of an angle can reveal a hinged motion in a macromolecule. 

Figure 8.21 Richardson-type diagram of the structure of one suhunit of the lac repressor. The polypeptide chain is arranged in four domains, an amino terminal DNA-hinding domain (red) with a helix-tum-helix motif, a hinge helix (purple), a large core domain which has two subdomains (green and hlue) and a C-terminal a helix. (Adapted from M. Lewis et al.. Science 271 1247-1254, 1996.)...

The polypeptide chain of the lac repressor subunit is arranged in four domains (Figure 8.21) an N-terminal DNA-hinding domain with a helix-turn-helix motif, a hinge helix which binds to the minor groove of DNA, a large core domain which binds the corepressor and has a structure very similar to the periplasmic arablnose-binding protein described in Chapter 4, and finally a C-terminal a helix which is involved in tetramerization. This a helix is absent in the PurR subunit structure otherwise their structures are very similar. 

Figure 8.23 The helix-turn-helix motifs of the subunits of both the PurR and the lac repressor subunits bind to the major groove of DNA with the N-terminus of the second helix, the recognition helix, pointing into the groove. The two hinge helices of each arm of the V-shaped tetramer bind adjacent to each other in the minor groove of DNA, which is wide and shallow due to distortion of the B-DNA structure. (Adapted from M.A. Schumacher et al.. Science 266 763-770, 1994.)...

Figure 15.17 The three-dimensional structure of an intact IgG. Hinge regions connecting the Fab arms with the Fc stem are relatively flexible, despite the presence of disulfide bonds in this region linking the heavy and light chains. Carbohydrate residues that bridge the two Ch2 domains are not shown. (Courtesy of A. McPherson and L. Harris, Nature 360 369-372, 1992, by copyright permission of Macmillan Magazines Limited.)...

IgG antibody molecules are composed of two light chains and two heavy chains joined together by disulfide bonds. Each light chain has one variable domain and one constant domain, while each heavy chain has one variable and three constant domains. All of the domains have a similar three-dimensional structure known as the immunoglobulin fold. The Fc stem of the molecule is formed by constant domains from each of the heavy chains, while two Fab arms are formed by constant and variable domains from both heavy and light chains. The hinge region between the stem and the arms is flexible and allows the arms to move relative to each other and to the stem. 

The molecular basis for quasi-equivalent packing was revealed by the very first structure determination to high resolution of a spherical virus, tomato bushy stunt virus. The structure of this T = 3 virus was determined to 2.9 A resolution in 1978 by Stephen Harrison and co-workers at Harvard University. The virus shell contains 180 chemically identical polypeptide chains, each of 386 amino acid residues. Each polypeptide chain folds into distinct modules an internal domain R that is disordered in the structure, a region (a) that connects R with the S domain that forms the viral shell, and, finally, a domain P that projects out from the surface. The S and P domains are joined by a hinge region (Figure 16.8). 

When they form the three subunits A, B, and C of the asymmetric unit, the identical polypeptides adopt different three-dimensional structures. The C subunit in particular is distinct from the A and B structures, its hinge region assuming a different conformation so that the S and P domains are... 

A comparison of the amino acid sequences of dystrophin, a-actinin, and spectrin. The potential hinge segments in die dystrophin structure are indicated. 

The investigator s choice of method (semi-empirical or ab initio) hinges on a number of factors, one of which is simple practicality concerning both time and expense. Semi-empirical methods usually give reasonable molecular structures and thermodynamic values at a fraction of the cost of ab initio calculations. Furthermore, molecular structures calculated by semi-empirical methods are the starting point for more complex ab initio calculations. 

---