Electrostatic stress mechanism

Whether it is the central position proposed for Asp70 in the electrostatic stress mechanism [12] or the role of Lys72 as proton-donor to the carbanion intermediate [24], it is obvious that these two residues are the most important ones in catalysis. Nevertheless, all Asp70 as well as Lys72 single-site mutants of M. thermoautotrophicum ODCase that were generated still retained some enzymatic activity (see below), a clear indication that it is not a sole residue carrying all of the catalytic load [21]. 

Since for liquids ry is of the order of 2 x 109 dyn/cm3, Sps must be of the order of 7,000 V/micron by this mechanism. Schultz and Wiech suggest that filaments of liquid may first be formed by Rayleigh instability from which are torn finer drops by local electrostatic stresses exceeding the tensile strength. 

This proposal of a ground state destabilization mechanism for ODCase (this type of mechanism was referred to earlier by Fersht as electrostatic stress 81 and by Jencks as the Circe effect ) sparked considerable controversy. In some circles it was seen as a prime example of the catalytic power of ground state destabilization,83 but several groups immediately questioned its validity on the basis of theoretical objections and apparent inconsistencies with biochemical experiments.23 26... 

Types of AppI ied Stress. Mechanical behavior of polymer-based materials depends on composition, structures, and interactions at molecular and super-molecular levels (5-7). The structures are much dependent on primary chemical (mostly covalent) bonding inside the chains and secondary bonding (dispersion van der Waals, induction, electrostatic, and hydrogen bonding, the last being the strongest in this category) forces in between chains (8). The composition often includes additives aimed at an improvement of a particular property. 

The two avenues above recalled, namely ab-initio computations on clusters and Molecular Dynamics on one hand and continuum model on the other, are somewhat bridged by those techniques where the solvent is included in the hamiltonian at the electrostatic level with a discrete representation [13,17], It is important to stress that quantum-mechanical computations imply a temperature of zero K, whereas Molecular Dynamics computations do include temperature. As it is well known, this inclusion is of paramount importance and allows also the consideration of entropic effects and thus free-energy, essential parameters in any reaction. 

CNT can markedly reinforce polystyrene rod and epoxy thin film by forming CNT/polystyrene (PS) and CNT/epoxy composites (Wong et al., 2003). Molecular mechanics simulations and elasticity calculations clearly showed that, in the absence of chemical bonding between CNT and the matrix, the non-covalent bond interactions including electrostatic and van der Waals forces result in CNT-polymer interfacial shear stress (at OK) of about 138 and 186MPa, respectively, for CNT/ epoxy and CNT/PS, which are about an order of magnitude higher than microfiber-reinforced composites, the reason should attribute to intimate contact between the two solid phases at the molecular scale. Local non-uniformity of CNTs and mismatch of the coefficients of thermal expansions between CNT and polymer matrix may also promote the stress transfer between CNTs and polymer matrix. 

PIEZOELECTRIC EFFECT. The interaction of mechanical and electrical stress-strain variables in a medium. Thus, compression of a crystal of quartz or Rochelle salt generates an electrostatic voltage across it, and conversely, application of an electric field may cause the ciystal to expand or contract in certain directions, Piezoelectricity is only possible in crystal classes which do not possess a center of symmetry. Unlike clcctrostricdon, the effect is lineal in the field strength. 

The model of a dipole in a spherical cavity can only provide qualitative insights into the behaviour of real molecules moreover, it cannot explain the effect of electrostatic interactions in the case of apolar molecules. More accurate predictions require a more detailed representation of the molecular charge distribution and of the cavity shape this is enabled by the theoretical and computational tools nowadays available. In the following, the application of these tools to anisotropic liquids will be presented. First, the theoretical background will be briefly recalled, stressing those issues which are peculiar to anisotropic fluids. Since most of the developments for liquid crystals have been worked out in the classical context, explicit reference to classical methods will be made however, translation into the quantum mechanical framework can easily be performed. Then, the main results obtained for nematics will be summarized, with some illustrative... 

The third mechanism by which proteins affect the stability of emulsions is rheological. This mechanism derives fundamentally from electrostatic and steric effects. The importance of viscosity has been described earlier. The viscosity of a caseinate solution is, inter alia, an indicator of the degree of bound water absorbed by the hydrophilic groups, as well as the water trapped inside the molecular aggregates (Korolczuk, 1982). The viscosity parameters (K, apparent viscosity at zero shear stress n, the power law factor and o-y, the yield stress) of sodium caseinate have been studied and found to be affected by concentration (Hermansson, 1975), precipitation and solution pH of caseinate (Hayes and Muller, 1961 Korolczuk, 1982), de-naturation (Hayes and Muller, 1961 Canton and Mulvihill, 1982), sodium chloride (Hermansson, 1975 Creamer, 1985), calcium chloride (Hayes and Muller, 1961) and temperature (Korolczuk, 1982). 

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