Pentamers

Cruzan J D, Viant M R, Brown M G, Luoas D D, Liu Kand Saykaiiy R J 1998 Terahertz iaser vibration-rotation-tunneiing speotrum of the water pentamer-d(IO). Constraints on the bifuroation tunneiing dynamios Chem. Phys. Lett. 292 667-76... 

Liu K, Brown M G, Cruzan J D and Saykally R J 1996 Vibration-rotation tunnelling speotra of the water pentamer Science 271 62-4... 

Fig. 1. Model of a ligand gated ion channel (LGIC) where (a) is the structure of a generic LGIC subunit showing the two cysteine (Cys) residues common to all LGIC subunits, and (b) shows the arrangement of five such subunits as a pentamer having psuedo-cyclic symmetry delineating a gated, fluid-filled...

Chemicals responsible for odor in some PUR foams were synthesised by polymerisation of PO in CH2CI2 with Bp2(C2H )20 catalyst (114). The yield was 25% volatile material and 75% polymeric material. The 25% fraction consisted of dimethyldioxane isomers, dioxolane isomers, DPG, TPG, crown ethers, tetramers, pentamers, etc, and 2-ethy1-4,7-dimethyl-1,3,6-trioxacane (acetal of DPG and propionaldehyde). The latter compound is mainly responsible for the musty odor found in some PUR foams. This material is not formed under basic conditions but probably arises during the workup when acidic clays are used for catalyst removal. 

The ratio of cycHc to linear oligomers, as well as the chain length of the linear sdoxanes, is controlled by the conditions of hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents (60,61). Commercially, hydrolysis of dim ethyl dichi oro sil a n e is performed by either batch or a continuous process (62). In the typical industrial operation, the dimethyl dichi orosilane is mixed with 22% a2eotropic aqueous hydrochloric acid in a continuous reactor. The mixture of hydrolysate and 32% concentrated acid is separated in a decanter. After separation, the anhydrous hydrogen chloride is converted to methyl chloride, which is then reused in the direct process. The hydrolysate is washed for removal of residual acid, neutralized, dried, and filtered (63). The typical yield of cycHc oligomers is between 35 and 50%. The mixture of cycHc oligomers consists mainly of tetramer and pentamer. Only a small amount of cycHc trimer is formed. 

Lipase-catalyzed intermolecular condensation of diacids with diols results in a mixture of macrocycUc lactones and liuear oligomers. Interestingly, the reaction temperature has a strong effect on the product distribution. The condensation of a,(D-diacids with a,(D-dialcohols catalyzed by Candida glindracea or Pseudomonas sp. Upases leads to macrocycUc lactones at temperatures between 55 and 75°C (91), but at lower temperatures (<45°C) the formation of oligomeric esters predorninates. Optically active trimers and pentamers can be produced at room temperature by PPL or Chromobacterium viscosum Upase-catalyzed condensation of bis (2,2,2-trichloroethyl) (+)-3-meth5ladipate and 1,6-hexanediol (92). 

Grown Ethers. Ethylene oxide forms cycHc oligomers (crown ethers) in the presence of fluorinated Lewis acids such as boron tritiuoride, phosphoms pentafluoride, or antimony pentafluoride. Hydrogen fluoride is the preferred catalyst (47). The presence of BF , PF , or SbF salts of alkah, alkaline earth, or transition metals directs the oligomerization to the cycHc tetramer, 1,4,7,10-tetraoxacyclododecane [294-93-9] (12-crown-4), pentamer, 1,4,7,10,13-pentaoxacyclopentadecane [33100-27-6] (15-crown-6), andhexamer, 1,4,7,10,13,16-hexaoxacyclooctadecane [17455-13-9]... 

The asymmetric unit contains one copy each of the subunits VPl, VP2, VP3, and VP4. VP4 is buried inside the shell and does not reach the surface. The arrangement of VPl, VP2, and VP3 on the surface of the capsid is shown in Figure 16.12a. These three different polypeptide chains build up the virus shell in a way that is analogous to that of the three different conformations A, C, and B of the same polypeptide chain in tomato bushy stunt virus. The viral coat assembles from 12 compact aggregates, or pen tamers, which contain five of each of the coat proteins. The contours of the outward-facing surfaces of the subunits give to each pentamer the shape of a molecular mountain the VPl subunits, which correspond to the A subunits in T = 3 plant viruses, cluster at the peak of the mountain VP2 and VP3 alternate around the foot and VP4 provides the foundation. The amino termini of the five VP3 subunits of the pentamer intertwine around the fivefold axis in the interior of the virion to form a p stmcture that stabilizes the pentamer and in addition interacts with VP4. 

Subunits VP2 and VP3 from different pentamers alternate around the threefold symmetry axes like subunits B and C in the plant viruses (Figure 16.12b). Since VP2 and VP3 are quite different polypeptide chains, they cannot be related to each other by strict symmetry, or even by quasi-symmetry in the original sense of the word. To a first approximation, however, they are related by a quasi-sixfold symmetry axis, since the folded structures of the cores of the subunits are very similar. 

SV40 and polyomavirus shells are constructed from pentamers of the major coat protein with nonequivalent packing but largely equivalent interactions... 

X-ray studies at 22.5 A resolution of murine polyomavlrus by 1. Rayment and D.L.D. Caspar at Brandeis University confirmed the presence of these 72 capsomers at the expected positions, but even at low resolution the pentagonal shape of all 72 capsomers was evident (Figure 16.22). They concluded that each capsomer must be a pentameric assembly of the major viral subunit, known as viral protein 1 (VPl). Each of the 60 icosahedral asymmetric units contains 6 VPl subunits, not 7, and the complete shell contains 360 VPl subunits. The 12 VPl pentamers centered on icosahedral fivefold axes are identically related to their five neighbors, but the 60 pentamers centered on pseudosixfold positions "see" each of their 6 neighbors quite differently (Figure 16.23). How can such diversity of interaction be incorporated into the bonding properties of just one type of protein subunit, without compromising specificity and accuracy of assembly ... 

The links between VPl pentamers are quite tenuous. With one exception, (across the twofold axis between the y subunits in Figure 16.26), all... 

Figure 16.24 A six-coordinated pentamer of SV40 "extracted" from the model shown in Figure 16.23. The extended carboxy-termlnal arms are shown in the conformations they adopt in the assembled particle in the free pentamer they are disordered and flexible. (Courtesy of S. Harrison.)...

Figure 16.23 Overview of the structure of the SV40 virus particle, showing the packing of pentamers. The subunits of pentamers on fivefold positions are shown in white those of pentamers in six-coordinated positions are shown in colors. The six colors indicate six quite different environments for the subunit. (Courtesy of S. Harrison.)...

Figure 16.26 Schematic diagram illustrating the different conformations of the flexible arms that link the pentamers in SV40. The color scheme corresponds to Figure 16.23. The body of a pentamer is shown as a five-petaled flower the arms are cylinders (a helices) and lines.

The capsids of polyoma virus and the related SV40 have icosahedral symmetry, with 72 pentameric assemblies of the major capsid protein. The pentamers are linked to their neighbors by flexible arms, with a p strand that augments a p sheet in the invaded pentamer. These flexible arms allow the pentamers to be linked together with both fivefold and sixfold symmetry. 

Low molecular weight olefins ranging from to Q, are produced by the polymerization of propylene or butylenes over a supported phosphorie acid catalyst. The product of this polymerization is a series of highly branched olefins ranging from dimers to pentamers. Some fragmentation of the polymers formed takes place in the reactor, so appreciable quantities of olefins are obtained which are not integral multiples of the monomer units. 

The product is a mixmre of dimers, trimers, tetramers, and pentamers having an average RON (Research Octane Number) = 95. Table 3-14 shows the analysis of feed and products from dimerization of propylene. ... 

Fig. 2. ORTEP view of the cation in bis(trimethylphosphine)(diphenylsilanediyl)(pentame-thylcyclopentadienyl)ruthenium tetraphenylborate x acetonitrile 12 [37]...

Pentamidine isethionate (Pentam 300, the parenteral form NebuPent, die aerosol form) is used in die treatment (parenteral form) or prevention (aerosol form) of... 

The tetrameric macrocycle of 21 has Q-symmetry and its conformation is similar to that of 18 with one opposite pair of imidazole rings in the plane of the molecule, and the other pair perpendicular to it. In 22 all five imidazole rings are almost perpendicular to the molecular plane, so that a CH2CI2 molecule can be included in the cavity of the pentamer. Unfortunately, compounds 19-23 are unstable under influence of air and moisture [33]. 

USA Pentacarinate (Rhone-Poulenc Rorer) wfm Pentam (Fujisawa) wfm... 

The planar cyclic Pj anion isoelectronic with cyclopentadienyl anion has been prepared in the form of M Pj" salts (M = Li, Na) by Handler et al. [49], The penta-phosphole anion Pj" favors planar geometry [50] while the most stable structure of P/ is square-pyramidal [51], The negatively charged pentamers Sb and Bi are planar rings [52, 53],... 

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