Activation negative molar

High pressure can influence reactions characterized by negative molar and activation volumes in the following aspects (i) acceleration of the reaction, (ii) modification of regioselectivity and diastereoselectivity, and (iii) changes in chemical equilibria. The pressure dependence on the rate constant of the reaction is expressed as follows ... 

This form assumes that the effect of pressure on the molar volume of the solvent, which accelerates reactions of order > 1 by increasing the concentrations when they are expressed on the molar scale, has been allowed for. This effect is usually small, ignored but in the most precise work. Equation (7-41) shows that In k will vary linearly with pressure. We shall refer to this graph as the pressure profile. The value of A V is easily calculated from its slope. The values of A V may be nearly zero, positive, or negative. In the first case, the reaction rate shows little if any pressure dependence in the second and third, the applied hydrostatic pressure will cause k to decrease or increase, respectively. A positive value of the volume of activation means that the molar volume of the transition state is larger than the combined molar volume of the reactant(s), and vice versa. 

The value of is the difference in partial molal volume between the transition state and the initial state, but it can be approximated by the molar volume. Increasing pressure decreases the value of AV and if A V is negative the reaction rate is accelerated. This equation is not strictly obeyed above lOkbar. If the transition state of a reaction involves bond formation, concentration of charge, or ionization, a negative volume of activation often results. Cleavage of a bond, dispersal of charge, neutralization of the transition state and diffusion control lead to a positive volume of activation. Reactions for which rate enhancement is expected at high pressure include ... 

The negative sign of the molar volume (MgVol) suggests that the steric interactions are unfavorable to the activity. 

Comments There are several suggested controls for this assay, including use of yeast total RNA as a negative control (check for probe species specificity) and a no RNAse control to determine probe stability. In Fig. 6.3A, the positive control marker lane was produced by addition of R-luc-4 sites or F-luc mRNA only to the assay. Also, optimal times for RNAse digestion will vary from probe to probe. In addition, for maximum sensitivity a probe with high specific activity is preferable (yet still in molar excess to the mRNA). 

The correct ratio of lipid constituents is important to form stable liposomes. For instance, a reliable liposomal composition for encapsulating aqueous substances may contain molar ratios of lecithin cholesterol negatively charged phospholipid (e.g., phosphatidyl glycerol (PG)) of 0.9 1 0.1. A composition that is typical when an activated phosphatidylethanolamine (PE) derivative is included may contain molar ratios of phosphatidylcholine (PC) cholesterol PG derivatized PE of 8 10 1 1. Another typical composition using a maleimide derivative of PE without PG is PC male-imide-PE cholesterol of 85 15 50 (Friede et al., 1993). In general, to maintain membrane stability, the PE derivative should not exceed a concentration ratio of about l-10mol PE per lOOmol of total lipid. 

Converging lines of evidence have led to a general acceptance of the monomeric metaphosphate mechanism for the hydrolysis of phosphate monoester monoanions. The pH rate profile for aryl and alkyl phosphate monoester hydrolyses commonly exhibits a rate maximum near pH 4. where the concentration of the monoanion is at a maximum. The proposed mechanism is based on these principal points of evidence (a) a general observation of P-O bond cleavage (b) the entropies of activation for a series of monoester monoanions are all close to zero, which is consistent with a unimolecular rather than a bi-molecular solvolysis where entropies of activation are usually more negative by 20 eu7 (c) the molar product composition (methyl phosphate inorganic phosphate) arising from the solvolysis of the monoester monoanion in a mixed methanol-water solvent usually approximates the molar ratio of methanol ... 

Figure 5). (See also Refs. 49 and 78). The activation volume for conduction at 20°C. is negative and with large curvature away from the molar volume axis. Less negative values, but still with curvature away from the molar volume axis, are also observed at 50° and 60°C. However, at two intermediate temperatures—30° and 40°C.—the curvature is toward the molar volume axis. A change of this type is hard to reconcile with any other change than one in the structure of the solvent. 

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