Unitary rate constant

For a single, irreversible step in a chemical reaction, i.e., an elementary chemical process, the rate of the reaction is proportional to the concentrations of the reactants involved in the process. The constant of proportionality is called the rate constant, or the unitary rate constant to highlight the fact that it applies to an elementary process. A subtlety that may be introduced into rate expressions is to use chemical activities (see Chap. 10) and not simply concentrations, but activity coefficients in biological systems are generally taken to be near 1. 

Because k2 describes the number of molecules of substrate converted to product per second per molecule of enzyme, it is called the turnover number of the enzyme. Generally, in more complex enzyme mechanisms, the expression for Kmax is complicated by k2 being replaced by an expression that is a ratio of sums of products of unitary rate constants this grouped expression is then called kcal. 

Annotate all reaction arrows with the corresponding unitary rate constant. For forward reactions where a substrate is involved, place its letter of designation next to the rate constant in the scheme do the same for reverse reactions involving a product. 

NC-Cr(cyclam)-CN-Ru(bpy)2-NC-Cr(cyclam)-CN complex [72]. Given the unitary efficiency of population of the emitting state in both systems, this indicates that the radiative rate constant of Cr(III) phosphorescence increases from 10 s" to 20 s upon attachment of the chromophoric unit. This behavior seems to be quite general, as all the other Ru(II)-Cr(ni) complexes studied [69,87,88] exhibit radiative (and non radiative) rate constants for deactivation of the Cr(III) doublet which are larger than those of the free chromophore. 

With reference to a simple reaction with constant stoichiometric coefficients, and unless otherwise specified, the reaction rate R [moles time-1 volume-1] measures the specific velocity of destruction of those reactants (and of formation of those products) that appear with unitary stoichiometric coefficients. The reaction rates of each other component are proportional to R according to their stoichiometric coefficients. 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

For a quantitative description of tear process, the Jurkov s equation was used, within which the relaxation time, x, was replaced with the tear rate, /, x with - the value of unitary tear, and the constants y and a with X and X, respectively, where X - constant P - specific friction pressure and p - friction coefficient in the friction zone. In this case, the durability equation can be written as ... 

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