Hydrophobic microdomain

Morishima etal. [29 — 31] prepared amphiphilic copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) with various hydrophobic comonomers, and studied the tendency of their self-aggregation and the nature of the hydrophobic microdomains thus formed in aqueous solution. Chart 1 shows some of these amphiphilic copolymers. Here, the value of x indicates the mol% content of hydrophobic comonomer units. 

The formation of a microphase structure can be sensitively detected by using hydrophobic fluorescent probes. Hydrophobic microdomains tend to solubilize hydrophobic small molecules present together in aqueous solution. For example, diphenylhexatriene (DHT) is hydrophobically bound to the St aggregates in ASt-x in aqueous solution and, as a result, the fluorescence intensity is greatly enhanced. Figure 9 shows the fluorescence intensity of DHT in the presence of ASt-x relative to the intensity in its absence (I/I0) as a function of the ASt-x concentration [29],... 

A pronounced enhancement of the fluorescence intensity occurs when the St content in ASt-x is higher than 55 mol%, again, indicating that a hydrophobic microdomain is formed at the critical St content. 

Despite the fact that the Phen moieties are tightly incorporated in the compartment of the hydrophobic microdomain, the fluorescence from the Phen residues in poly(A/St/Phen) is very efficiently quenched by MV2+ in aqueous solution. The quenching efficiency is much higher than the APh-2 (8 with x = 2)... 

The time profiles of the absorbance due to MV+ at 600 nm are illustrated in Figures 18. Note that they depend on the MV2+ concentration. The curves for the poly(A/St/Phen)-MV2+ systems are biphasic and can be explained in terms of a simple mechanism illustrated in Scheme 2. Here, D A, A represents a compartmentalized Phen moiety (D) and MV2+ dications (A) bound to the hydrophobic microdomain. 

Drug Release from PHEMA-l-PIB Networks. Amphiphilic networks due to their distinct microphase separated hydrophobic-hydrophilic domain structure posses potential for biomedical applications. Similar microphase separated materials such as poly(HEMA- -styrene-6-HEMA), poly(HEMA-6-dimethylsiloxane- -HEMA), and poly(HEMA-6-butadiene- -HEMA) triblock copolymers have demonstrated better antithromogenic properties to any of the respective homopolymers (5-S). Amphiphilic networks are speculated to demonstrate better biocompatibility than either PIB or PHEMA because of their hydrophilic-hydrophobic microdomain structure. These unique structures may also be useful as swellable drug delivery matrices for both hydrophilic and lipophilic drugs due to their amphiphilic nature. Preliminary experiments with theophylline as a model for a water soluble drug were conducted to determine the release characteristics of the system. Experiments with lipophilic drugs are the subject of ongoing research. 

Benjelloun A, Brembilla A, Lochon P, Adibnejad M, Viriot ML, Carre MC (1996) Detection of hydrophobic microdomains in aqueous solutions of amphiphilic polymers using fluorescent molecular rotors. Polymer (Guildford) 37(5) 879-883... 

Contaminants bound to colloids also may lead to an increase in the apparent solubility of the compounds. Most colloidal phases are effective sorbents of low-solubility contaminants, due to their large surface area. For example. Fig. 8.21 depicts the solubilization of p-nitrophenol into hydrophobic microdomains, which defines the trace metal level in the groundwater of a coastal watershed (Sanudo-Wilhelmy et al. 2002). The authors emphasize that the (heavy) metals contained in the colloidal size fraction in some instances may reach more than 50% of what is considered dissolved metal this should be considered to properly understand the cycling of metals and carbon in the subsurface water. 

T. Okano, T. Aoyagi, K. Kataoka, K. Abe, Y. Sakurai, M. Shimada, I. Shinohara, Hydrophilic-hydrophobic microdomains surfaces having an ability to suppress platelet adhesion and their in vitro antithrombogenicity, J. Biomed. Mater. Res. 20 (1986) 919-927. 

It is important for the molecular design of a superior barrier materials to take into account the balance and distribution of hydrophilic and hydrophobic microdomains as well as the stiffness and flexibility of a macromolecule, in order to place the subject on the fundamental understanding of the interactions among solvents, solutes, and macromolecules. 

Branched insoluble polymers have more rigid backbones and more hydrophobic microdomains than branched soluble polymers such as PEI, which can be advantageous in designing some types of catalysts. In addition, such insoluble catalysts may be more useful in practical applications. 

Y. Morishima, M. Tsuji, M. Kamachi, and K. Hatada, Photochromic isomerization of azobenzene moieties compartmentalized in hydrophobic microdomains in a microphase structure of amphiphilic polyelectrolytes, Macromolecules 25, 4406-4410 (1992). 

The signal can be enhanced by so-called amplifiers, probably by an interaction of the relatively long-lived AMP D anion with hydrophobic microdomains on the enhancer molecules. The hydrophobic domains exclude water with its protons (which could quench chemiluminescence) from the direct environment of the anion emitter. Just the addition of 0.1% BSA may amplify the signal several-fold. This may show possible problems for quantification due to unknown effects of sample components with hydrophobic domains. Amplification with an independent system can be achieved by adding commercially available fluorescent micelles (Schaap et al., 1989), such as those prepared from CTAB (Section 3.1.2.5) and 5-A-tetradecanoyl-aminofluorescein, which sequester the more hydrophobic AMP D. Chemiluminescence transfers enough energy to the co-micellized fluorescer to provide for a = 100-fold increase in photon output. 

When positioned in hydrophobic microenvironments, anionic nucleophiles are partially desolvated. Whether this results in acceleration, however, depends on the type of the reaction involved. If the charge is more delocalized in the transition state than in the reactant, as in 13 and 14, desolvation of the nucleophile would selectively destabilize the reactant, leading to rate enhancement. On the other hand, if the charge is more localized in the transition state, the reaction would be retarded in hydrophobic microdomains. ... 

The simplest information obtained by qualitative solubilization is the existence (or the absence) of hydrophobic microdomains, using probes which are sensitive to the polarity of the environment. Such solubilization experiments are... 

Synthetic weak polyelectrolytes provide hydrophobic microdomains in the low pH range. Typical examples are poly(methacrylic acid) (PMA) (i-3), copolymers of maleic anhydride and n-alkylvinyl ethers (n = 4-9) 4, 5), and copolymers of maleic anhydride and styrene or methylstyrene (6). The compact polymer coils are formed only in acidic solution, but the compact conformation is converted into the extended rod or coil form at high pH because of repulsion of ionized carboxyl groups. At low pH, many hydrophobic molecules can be hosted by the polymer coils, a condition that disappears at high pH. However, a hydrophobic environment is required... 

In aqueous solutions of PA-I8K2 the ratio I3/I1 is almost constant at 1.09-1.04 over the entire pH range. These data contrast with those observed in aqueous solutions of PMA where the ratio I3/I1 sharply decreases from 0.88 to 0.55 at pH > 4. The data indicate that PMA acts as a host for pyrene only in the low pH range. However, PA-I8K2 provides hydrophobic microdomains and acts as a host over the entire pH range. Increasing pH increases the ionization of the polymers, and increased ionization leads to a repulsive... 

The fluorescence decay rate constants, k , of several fluorescent probes in various systems (Table I) provide further evidence for hydrophobic microdomains in PA-I8K2 aqueous solutions. All the decays are single exponential decays except for C PN in aqueous solutions of PMA at pH 8. All the values in PA-I8K2 aqueous solutions at pH 4, 8, and 10 are similar to those found in hexanol and much smaller than those in water and aqueous solutions of PMA at pH 8. In summary, PA-I8K2 readily solubilizes pyrene and some positively and negatively charged derivatives of pyrene, especially long-chain derivatives. 

These data indicate that the environment of CnPN in PMA and in PA-I8K2, both at pH 8, where both polyelectrolytes are almost fully ionized, is quite different. C PN experiences a water phase in PMA, and the interaction between CnPN and PMA is electrostatic in nature. In PA-I8K2 solution, CijPN was solubilized in polymer hydrophobic microdomains because of a hydrophobic interaction between the long hydrocarbon chains and because of electrostatic bonding. 

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