Hydrophobic domains, effect

It has been shown in Chapter 5, the fluorescence quenching of the DPA moiety by MV2 + is very efficient in an alkaline solution [60]. On the other hand, Delaire et al. [124] showed that the quenching in an acidic solution (pH 1.5-3.0) was less effective (kq = 2.5 x 109 M 1 s 1) i.e., it was slower than the diffusion-controlled limit. They interpreted this finding as due to the reduced accessibility of the quencher to the DPA group located in the hydrophobic domain of protonated PMA at acidic pH. An important observation is that, in a basic medium, laser excitation of the PMAvDPA-MV2 + system yielded no transient absorption. This implies that a rapid back ET occurs after very efficient fluorescence quenching. 

The inhibitory effects of PVA can also be found in degradation studies of polycaprolactones (PCLs). These polyesters can be readily split by lipase enzymes binding to hydrophobic domains of that linear substrate. PVA/PCL films in contrast are not biodegradable by PCL-degrading microorganisms. It can be assumed that the surface properties of PCL change upon interaction with PVA in a manner that enzymatic accessibility of the hydrolysable PCL backbone motifs is decreased. 

In general, the polymer ligand surrounding the metal complex constitutes an electrostatic or hydrophobic domain in aqueous solution. This polymer domain governs the chemical reactivity of the metal complex. This phenomenon is said to be an environmental effect of polymer on a chemical reaction. 

On the other hand, an attempt to accelerate the step of coordination of the substrate to the Cu catalyst was successful because it used the hydrophobic domain of the polymer ligand156 That was the oxidation catalyzed by polymer-Cu complexes in a dilute aqueous solution of phenol, which occurred slowly. The substrate was concentrated in the domain of the polymer catalyst and was effectively catalyzed by Cu in the domain. A relationship was found to exist between the equilibrium constant (Ka) for the adsorption of phenol on the polymer ligand and the catalytic activity (V) of the polymer-ligand-Cu complex for various polymer ligands K a = 0.21 1/mol and V = 1(T6 mol/1 min for QPVP, K a = 26 and V = 1(T4 for PVP, K a = 52 and V = 10-4 for the copolymer of styrene and 4-vinylpyridine (PSP) (styrene content 20%), and K a = 109 and V = 10-3 for PSP (styrene content 40%). The V value was proportional to the Ka value, and both Ka and V increased with the hydrophobicity of the polymer ligand. The oxidation rate catalyzed by the polymer-Cu complex in aqueous solutions depended on the adsorption capacity of the polymer domain. 

The environmental effects are caused by the micro-environments constituted by the domain of a polymer ligand. The electrostatic domain of a polymer-metal complex was demonstrated in the reaction of the polymer-Co(ni) complex with ionic species (Section IVA), and was shown to be utilized in the catalytic activity of the polymer-Cu complex (Section VIA). In another case, the hydrophobic domain was predominant, ie. in the reaction with hydrophobic substrates (Sections IVB and VIIC). The environmental effects of a polymer ligand also include dynamic effects, Which vary with the solution conditions (Section IIIC). 

Movileanu et al. [127] used reconstituted planar PC bilayers (black lipid bilayers) to study the effect exerted by quercetin (29) on their electrical properties. Quercetin inserted into model membranes, which resulted in an increase in their conductance and electrical capacitance. Clear pH dependence of quercetin (29) binding to membranes was observed. Capacitance changes were the most pronounced at low pH, which was attributed to the deeper insertion of quercetin (29) into the bilayer in acidic conditions. The authors postulated that quercetin inside the membrane interacted with both the hydrophobic domain and polar headgroups of PC. 

This instability can be avoided by adding a non-ionic surfactant to the surface of the latex, forming a hydrophilic layer (Triton x-405 of 30 units) on the surface of the latex [22]. In addition, this compound reduces the stacking effect by masking the hydrophobic domains (or properties) of the surface. Indeed, competition for adsorption between the ODN and the surfactant molecules can also lead to desorption. However, this effect was not observed in all reported studies, but it is in principle accessible by comparing the adsorption energies of ODN and the surfactant on the surface of the latex. 

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