Activation reactant-solvent interactions

Polarity of solvents — If applied to solvents, this rather ill-defined term covers their overall -> solvation capability (solvation power) with respect to solutes (i.e., in chemical equilibria reactants and products in reaction rates reactants and activated complex in light absorptions ions or molecules in the ground and excited state), which in turn depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules, excluding interactions leading to definite chemical alterations of the ions or molecules of the solute. Occasionally, the term solvent polarity is restricted to nonspecific solute/solvent interactions only (i.e., to van der Waals forces). 

The concept of cohesive pressure (or internal pressure) is useful only for reactions between neutral, nonpolar molecules in nonpolar solvents, because in these cases other properties of the solvents, such as the solvation capability or solvent polarity, are neglected. For reactions between dipolar molecules or ions, the solvents interact with reactants and activated complex by unspecific and specific solvation so strongly that the contribution of the cohesive pressure terms of Eq. (5-81) to In /r is a minor one. It should be mentioned that cohesive pressure or internal pressure are not measures of solvent polarity. Solvent polarity refiects the ability of a solvent to interact with a solute, whereas cohesive pressure, as a structural parameter, represents the energy required to create a hole in a particular solvent to accommodate a solute molecule. Polarity and cohesive pressure are therefore complementary terms, and rates of reaction will depend... 

The breakdown of the simple linear relationship between g(k/ko) and (fir — 1)/ (2fir + 1), required by Eq. (5-87), is obviously due to the failure of the approximations involved in deriving this equation, neglection of non-electrostatic and specific solute-solvent interactions, and, in the case of binary solvent mixtures, due also to the selective solvation of the reactants and activated complex by one component of the mixture cf. 

In conclusion, it can be stated, according to a general rule proposed by Palit in 1947 [270], that solvents which impede the active centre of a reactant through hydrogen bonding or by some other means, will suppress the reactivity of that reactant. Conversely, solvents capable of promoting a favourable electron shift, necessary to the reaction, by specific solute/solvent interactions, will enhance the reaction rate. 

According to IUPAC the definition solvents polarity is the overall solvation capability (or solvation power) for (1) educts and products, which influences chemical equilibrium, (2) reactants and activated complexes ( transition states ), which determines reaction rates, and (3) ions or molecules in their ground and first excited state, which is responsible for light absorptions in the various wavelength regions. This overall solvation capability depends on the action of all, non-specific and specific, intermolecular solute-solvent interactions, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute [53],... 

An enzyme, in general, functions by first binding the reactant to a site on its surface called the active site. It is here that the catalytic chemistry occurs. The boxmd reactant then interacts and reacts with the side chains of the amino acids that make up the enzyme, and it is this interaction that brings about the chemical transformation. When the reaction is complete, the bound product diffuses away from the active site. Enzyme reactions take place in water, the biological solvent, at ambient temperatures. They often occur at rates a million or more times faster than those of uncatalyzed reactions. Hundreds of thousands of reactions can occur at the site of a single enzyme in one second. Many enzymes require the assistance of a molecule called a coenzyme if the catalytic reaction is to occur. Other enzymes require metal cations, such as Zn, at their active sites. 

Optimization of reaction complex topology implies the positioning of charged enzyme groups such that there is an optimum interaction with (activated) reactants. Significant protonation (in protolytic enz5unes) or electron transfer (in redox catalysis) can already occiu in the adsorbed state. This is due to the unique electrostatic properties of the enzyme, which is shielded from the solvent by its hydrophobic peptide framework. 

An alternative approach is to combine QM and MM methods such that the reacting system (or the active site in an enzyme) is treated explicitly by a quantum mechanical method, while the surrounding environmental solvent molecules (or amino acids), which constitute the most time-consuming part in the evaluation of the potential energy surface, are approximated by a standard MM force field. " Such a method takes advantage of the accuracy and generality of the QM treatment for chemical reactions - and of the computational efficiency of the MM calculation.Because the reactant electronic structure and solute-solvent interactions are determined quantum mechanically, the procedure is appropriate for studying chemical reactions, and there is no need to parameterize potential functions for every new reaction. Furthermore, the solvent polarization effects on the solute are naturally included in the... 

The role of the solvent in organic reactions is of the utmost unportance. Its effect can Just be limited to a physical effect in making possible the solubilization of the reactants, with no direct interaction with the active center. More interesting are the cases in which the solvent interacts through specific forces, such as hydrogen bonds, thus altering the mechanism, the rate, and eventually the selectivity of the reaction in stabilizing certain reaction intermediates. 

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