Longer Bridges Research Articles

P–O Bond Destabilization Accelerates Phosphoenzyme Hydrolysis of Sarcoplasmic Reticulum Ca2+-ATPase

The phosphate group of the ADP-insensitive phosphoenzyme (E2-P) of sarcoplasmic reticulum Ca2+ -ATPase (SERCA1a) was studied with infrared spectroscopy to understand the high hydrolysis rate of E2-P. By monitoring an autocatalyzed isotope exchange reaction, three stretching vibrations of the transiently bound phosphate group were selectively observed against a background of 50,000 protein vibrations. They were found at 1194, 1137, and 1115 cm(-1). This information was evaluated using the bond valence model and empirical correlations. Compared with the model compound acetyl phosphate, structure and charge distribution of the E2-P aspartyl phosphate resemble somewhat the transition state in a dissociative phosphate transfer reaction; the aspartyl phosphate of E2-P has 0.02 A shorter terminal P-O bonds and a 0.09 A longer bridging P-O bond that is approximately 20% weaker, the angle between the terminal P-O bonds is wider, and -0.2 formal charges are shifted from the phosphate group to the aspartyl moiety. The weaker bridging P-O bond of E2-P accounts for a 10(11)-10(15)-fold hydrolysis rate enhancement, implying that P-O bond destabilization facilitates phosphoenzyme hydrolysis. P-O bond destabilization is caused by a shift of noncovalent interactions from the phosphate oxygens to the aspartyl oxygens. We suggest that the relative positioning of Mg2+ and Lys684 between phosphate and aspartyl oxygens controls the hydrolysis rate of the ATPase phosphoenzymes and related phosphoproteins.

Mono- and binuclear bipyridyl derivatives of the Mo(η 3-C 3H 5)(CO) 2 fragment: structural studies and fluxionality in solution

New mono- and binuclear complexes of the Mo(η 3-C 3H 5)(CO) 2 fragment, containing bipyridyl ligands (2,2′-bpy, 4,4′-Me 2-2,2′-bpy) as chelates, and mono- (4-CNpy, 4-Mepy, NCMe, Br) or bidentate nitrogen ligands (4,4′-bpy, bipyridylethylene, pyrazine) as terminal or bridging ligands, respectively, were prepared. The binuclear complex [{Mo(η 3-C 3H 5)(CO) 2(2,2′-bpy)} 2(μ-4,4′-bpy)][PF 6] 2 ( 2) was shown by X-ray diffraction to assemble in the crystal forming large channels with a rectangular section. A longer bridge, such as bipyridylethylene, led to a different structure ( 3). 4-CNpy behaved as monodentate ligand ( 4), coordinating through the pyridine nitrogen as a terminal ligand. NMR spectroscopy studies showed that the complexes exhibited a fluxional behavior in solution, the endo and exo forms of the more symmetrical equatorial isomers being usually present and interconverting in solution. The solid state structures of the complexes revealed a preference for the more symmetrical equatorial isomer, with the two chelate ligands in trans positions in the binuclear species. The rings tended to become parallel in the organized crystal.

Gelation in free-radical terpolymerization of poly(allyl methacrylate) crosslinked polymer nanosphere with allyl benzoate and vinyl benzoate

Microgel-like poly(allyl methacrylate) (PAMA nanosphere) was prepared by the emulsion polymerization of AMA as the reactive crosslinked polymer nanosphere with abundant pendant allyl groups. The terpolymerization of PAMA nanosphere with allyl benzoate (ABz) and vinyl benzoate (VBz) was conducted in bulk using benzoyl peroxide as initiator at 80 °C. The gelation depended on the feed ABz/VBz molar ratio since the molecular weights of resulting poly(ABz-co-VBz)s that act as the bridges increased with an increase in the mole fraction of VBz in the feed monomer. Beyond the gel point, the sol was rapidly incorporated into the gel and the increasing tendency of the gel fraction became steeper from 70 to 95 mol% of VBz as a reflection of increased chain length of bridge between PAMA nanospheres. Moreover, the swelling ratio became lower with increasing the mole percentage of VBz in the feed monomer. These results would support the preferential incorporation of PAMA nanosphere into the gel and the longer bridge can capture more nanospheres to give a much shrunken gel.

Prestressing Schemes for Incrementally Launched Bridges

Incremental launching is a competitive construction method for medium-span prestressed concrete bridges. Compared with other techniques for in situ casting, in short bridges it is an alternative to the use of falseworks and reduces the cost of labor with the same investment. In longer bridges it is an alternative to movable shuttering systems and reduces both investment and labor cost. Compared with segmental precasting, it may reduce both investment and the cost of prestressing, which may be partial instead of total, with the same advantages in terms of industrial production. Due to its competitiveness and overall quality, this construction method is widely used in Europe. During launch, the superstructure is moved over fixed bearings. The superstructure dead load produces temporary flexural and shear stresses within the cross section that are quite different from those produced by the service loads and, consequently, require special prestressing schemes, thoroughly illustrated in this paper.

Molecular Simulations of Tilted Chain Crystal - Amorphous Interfaces in Polymers

ABSTRACTSemi-crystalline polymeric materials exhibit metastable interfaces which must simultaneously feature molecular connectivity through chain bonding and disorder across the interface. To address this, off lattice molecular simulations have been constructed to capture the structure and properties of crystal-amorphous interfaces in polyethylene with freely rotating chains. Monte-Carlo simulations were performed on systems having {001 }, {201} and {502} interfaces (polymer chains tilted to the lamella normal by 0°, 34.4° and 41° respectively.)Results of the simulations reveal changes in interfacial structure with variation in the chain-tilt angle. The degree of tight folding decreases with increasing tilt, from predominantly [110] loops in the {001} interface to [200] and [310] loops on the other interfaces. The topology shifts towards an increase in loosely folded loops along with longer bridges and tails. The results also compare well with a Gaussian chain length distribution model. The {201} interface exhibits the lowest interfacial energy, in apparent agreement with experimental data.

Nose-Deck Interaction in Launched Prestressed Concrete Bridges

Incremental launching is a competitive construction method for medium-span prestressed concrete bridges. Compared with the other techniques for in situ casting, in short bridges it is an alternative to the use of falseworks and reduces the cost of labor with the same investment. In longer bridges it is an alternative to self-launching centering and reduces investment with the same cost of labor. Compared with segmental precasting, it may reduce both investment and the cost of prestressing. The design of a launched bridge is affected by temporary stresses arising from its movement over fixed bearings. These stresses tend to increase the cost of materials, reducing the competitiveness of the construction method. Of the various devices used to reduce temporary stresses, adopting a launching nose has virtually become standard, permitting an effective control at competitive costs. The techniques of optimizing the nose-deck interaction are illustrated here through several graphs, and the analysis of special cas...

Characterization of the bismuth(III) bromide–[2.2]-paracyclophane (2/1) adduct

The 2 : 1 charge-transfer molecular complex between bismuth bromide and [2.2]paracyclophane has been prepared and investigated by means of thermogravimetric, electronic and IR spectra and resistivity measurements. The X-ray structure was also determined: monoclinic, space group P21/c, a= 7.896(3), b= 14.198(6), c= 10.803(8)A, β= 101.27(5)°, Z= 2 and R= 0.0457. The structure is built up by polymeric linear chains of BiBr3 units interlayered by centrosymmetric hydrocarbon molecules. Two bromine atoms bridge adjacent metal atoms so that the co-ordination is very distorted octahedral with three short [2.595(3)–2.676(3)A] and two longer bridging [3.325(3), 3.402(2)A] Bi–Br bonds; the sixth site is occupied by a benzene ring of the [2.2]paracyclophane molecule. The complex shows high stability and characteristic electrical properties of a semiconductor at 25 °C.