Barrier, solvent-induced

Schroeder J, Tree J and Vdhringer P 1995 Photoisomerization of f/ a/rs-stilbene in compressed solvents Kramers turnover and solvent induced barrier shift Z. Phys. Chem. 188 287... 

The above 7AI analogs serve as one of the few experimental proofs for the solvent-induced barrier in proton transfer reaction. It is thus believed that through... 

When friction is present, there will be solvent-induced recrossings of the barrier and non-equilibrium solvation, and the frequency Ar by which products are formed depends both on the equilibrium curvature of the barrier and on the dynamics of the solvent as expressed by the time-dependent friction kernel. The following two limiting cases should be noticed. 

Irritant contact dermatitis is a public health problem. Skin exposure to irritants (e.g., water, detergents, and solvents) causes damage to the barrier and induces an inflammation, ultimately contact dermatitis.43 By combining microprobe analysis with other techniques it is possible to correlate physiological changes (e.g., barrier repair processes) with effects on the keratinocytes. In a series... 

Consider now the barrier crossing problem in the barrier controlled regime discussed in Section 14.4.3. The result, the rate expressions (14.73) and (14.74), as well as its non-Markovian generalization in which cor is replaced by k ofEq. (14.90), has the structure of a corrected TST rate. TST is exact, and the correction factor becomes 1, if all traj ectories that traverse the barrier top along the reaction coordinate (x ofEq. (14.39)) proceed to a well-defined product state without recrossing back. Crossing back is easily visualized as caused by collisions with solvent atoms, for example, by solvent-induced friction. 

The PMF for Li+ adsorption shows two solvent-induced local minima. In bulk solution, the hydration shell of Li+ forms a rather rigid octahedral complex. Li+ and the water molecules in its hydration shell move cooperatively. Consequently, the hydration complex is sensitive to the barriers formed by the two pronounced layers of water molecules around z = —6k. and z = —3 A. This leads to a local free energy minimum on the solution side of each of the two maxima in the oxygen density profiles. There, molecules from the water layers can be part of the hydration eomplex (see below). 

As with Mb/Hb, metHr is the thermodynamically stable form in aerobic solutions, and a metHr reductase has been reported. The rate of autoxidation of oxyHr decreases by approximately tenfold with a one-unit decrease in pH in the acidic region, consistent with reaction (6a). Certain exogenous anions, particularly azide, catalyze autoxidation of oxyHr by nucleophilic displacement of the peroxo ligand and/or by delivery of a proton to the coordinated peroxo. The slow rates of reactions (6a) and (6b) in the absence of these anions, however, indicate a substantial kinetic barrier to autoxidation. Presumably, autoxidation requires entry of solvent into the 02-binding pocket of oxyHr, i.e., transient, simultaneous occupancy of the pocket by both bound O2 and solvent. Consistent with this mechanism, the L98 variants of Hr in which either smaller or more polar side chains were substituted all led to significant increases in autoxidation rates.Thus, the main function of L98 and the other hydrophobic residues shown in Figure 17 may be as hydrophobic and steric barriers to solvent-induced autoxidation. 

The sole study involving barrier impairment induced by organic solvents used ethanol and another unidentified form of deUpidization and measured penetration of compoimds with assorted lipophUicities. Solvent damage pro-dueed enhancing effects, more dramatically for hydrophiUc molecules like caffeine than for hpo-philic molecules, such as methylparaben and butylparaben [117]. 

An equivalent contrast is the following. In the equilibrium solvation view, the reaction system is assumed to pass directly from the rgactants side to the products side of the barrier, with no recrossing. In the nonequilibrium solvation case, dielectic friction effects will lead to a solvent induced recrossing of the barrier. This leads to a reduction in k compared to k which is quantified by the transmission coefficient k ... 

---