Hydrophobic/hydrophilic domains

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. 

ATP extraction has been used for the separation and large-scale purification of enzymes.45 47 Proteins and nucleic acids partition between the two phases differently due to differences in their surface charges and hydrophobic-hydrophilic domains. Proteins generally prefer the aqueous phase rich in... 

Due to the fact that, in SPEEK, the separation into hydrophilic and hydrophobic domains is less pronounced (Rikukawa and Sanui 2000), the hydrophobic/ hydrophilic domain difference is smaller, the backbone is less hydrophobic, and the sulfonic acid functional group is less acidic. Therefore, SPEEK is less polar, its conductivity is lower, and the flexibility of the polymer backbone is less. In order to improve the proton conductivity, the DS needs to be higher. However, this will also increase the methanol permeability and other problems of high swelling and loss of mechanical properties will arise. As a result, in order to overcome these problems. 

Results of methanol permeability at various DS, SiWA, and Si02 are plotted in Figure 13.15. It can be seen that no membrane possesses higher methanol permeability than the Naflon 112 membrane. The corresponding methanol permeability for Nafion 112 was about 15.07 x 10 cm%. The methanol permeabilities of composite membranes based on SPEEK are obviously lower than that of Naflon 112, which is possibly due to the microstructure differences between the SPEEK and the Naflon 112 membrane. Kreuer reported that the Naflon membrane has high hydrophobicity of the perfluorinated backbone and also high hydrophilicity of the sulfonic groups (Kreuer 2001). In the presence of water, this character is more pronounced and consequently increases the hydrophobic/hydrophilic domains of the Naflon membrane. 

Thyrotropin (TSH) regulates the production and secretion of thyroid hormones as well as thyroid epithelial cell growth via the TSH receptor. The TSH receptor belongs to the family of G protein-coupled receptors. It is composed of 764 amino acids. The receptor contains a long hydrophilic region orientated towards the exterior of the cell (ectodomain), 7 hydrophobic transmembrane domains and a short cytoplasmic region. 

Tropoelastin is the soluble precursor of elastin and consists of alternating hydrophobic and hydrophilic peptide domains. The most common amino acids in the hydrophobic domains are Gly, Val, Ala, and Pro, which are often present in repeats of tetra-, penta-, and hexapeptides, such as Gly-Gly-Val-Pro, Gly-Val-Gly-Val-Pro, Gly-Val-Pro-Gly-Val, and Gly-Val-Gly-Val-Ala-Pro, respectively [3, 4]. The hydrophilic domains are mainly composed of lysines interspersed by alanines. 

Tropoelastin is encoded by a single copy gene and the alternating hydrophobic and hydrophilic domains are generally encoded by different expressed regions, or exons (Fig. 1). Those exons are alternated by introns (intragenic regions), which are... 

Fig. 1 Primary structure of human tropoelastin isoform 3 (EBI accession no. P15502). The highlighted regions correspond to the signal peptide and hydrophobic and hydrophilic domains. Based on [2]...

Fig. 2 cDNA structure of human tropoelastin. The alternating hydrophobic and hydrophilic domains are generally encoded by different exons, shown in white and black, respectively. The arrows indicate the six exons that are subject to alternative splicing in a cassette-like fashion. Reproduced from [8] with permission from John Wiley and Sons, copyright 1998... 

Since some structural and dynamic features of w/o microemulsions are similar to those of cellular membranes, such as dominance of interfacial effects and coexistence of spatially separated hydrophilic and hydrophobic nanoscopic domains, the formation of nanoparticles of some inorganic salts in microemulsions could be a very simple and realistic way to model or to mimic some aspects of biomineralization processes [216,217]. 

Sequence data, in combination with functional studies, reveal several classes of E-IIs as shown in Fig. 3. These classes will most likely change and expand as future studies define additional domains with new functions. We introduce it here, only for the sake of our own convenience, in this chapter. The first class is represented by E. coli II . This protein is unique in that, as yet, it is the only representative of its class. It consists only of a single hydrophobic peptide with no hydrophilic domain attached at either end [7,9]. 

The second class, represented by E. coli 11°, consists of a hydrophobic domain of approximately 360 residues followed by hydrophilic domain of approximately 100 residues [10], Other representatives of this class are E. coli 11° [11], S. aureus 11 [12], E. coli [13,14] B. subtilis 11 [15] and 11 encoded by the plasmid pUR400 [16]. 

The third class, represented by E. coli consists again of a single hydrophobic domain of approximately 360 residues but with two covalently attached hydrophilic domains, equal, together, in size to the hydrophobic domain [17], The A domain is proposed to function as a covalently attached E-III. Other representatives of this class include B. subtilis II° [18,19], 5. mutans 11 [20], E. coli II [21,22] and [23,24],... 

The 4TM receptors are pentameric complexes composed of subunits of 420 to 550 amino acids. The subunits exhibit sequence identities from 25 to 75%, with a similar distribution of hydrophobic and hydrophilic domains (Table 3.1). The hydrophilic 210 to 230 amino-acid N-terminal domain is followed by three closely spaced hydrophobic and putative transmembrane domains, then a variable-length intracellular loop, and finally a fourth putative transmembrane region shortly before the C-terminus (Figure 3.1). Of the four candidate transmembrane regions, evidence suggests that TM2 forms an a-helix, while the other hydrophobic regions more likely are folded as (3-sheets. 

The activity coefficients in the membrane phase are set to one. This assumption is justified at low concentrations of ions in the membrane, especially when considering their location at the interface of the hydrophobic and hydrophilic domains [134], but might be inappropriate at concentrations near saturation. 

Figure 3.2. Structure of some G-protein-linked receptors of the rhodopsin superfamily (a) the structural organisation of the predicted hydrophilic and hydrophobic domains of the receptor (b) how the hydrophilic regions form extracellular and intracellular loops, being anchored by the seven hydrophobic transmembrane domains.

It is well known that Nafion ionomer contains both hydrophobic and hydrophilic domains. The former domain can facilitate gas transport through permeation, and the latter can facilitate proton transfer in the CL. In this new design, the catalyst loading can be further reduced to 0.04 mg/cm in an MEA [10,11]. However, an extra hydrophobic support layer is required. This thin, microporous GDL facilitates gas transport to the CL and prevents catalyst ink bleed into the GDL during applications. It contains both carbon and PTFE and functions as an electron conductor, a heat exchanger, a water removal wick, and a CL support. 

Two other important factors that control the conductivity of PEMs are polymer microstructure and morphology. Within this section, Nafion will serve as the prime example to describe how the formation of hydrophobic and hydrophilic domains relates to proton transport. The microstructures of a few PEMs will then be described to highlight the importance of this area upon proton conductivity. 

In Nafion, the hydrophobic perfluorinated segments of the polymer are incompatible with the hydrophilic sulfonic acid groups and thus phase separation occurs. When exposed to water, the hydrophilic domains swell to provide channels for proton transport, whereas the hydrophobic domains provide mechanical integrity and, at least in the case of lower lEC samples. 

The structure of this interface determines fhe sfabilify of PEMs, the state of water, the strength of interactions in the polymer/water/ion system, the vibration modes of side chains, and the mobilities of wafer molecules and protons. The charged polymer side chains contribute elastic ("entropic") and electrostatic terms to the free energy. This complicated inferfacial region thereby largely contributes to differences in performance of membranes wifh different chemical architectures. Indeed, the picture of a "polyelectro-lyfe brush" could be more insighttul than the picture of a well-separated hydrophobic or hydrophilic domain structure in order to rationalize such differences. ... 

Snapshots of the final microstructure in hydrated Nafion membrane at different water contents. Hydrophilic domains (water, hydronium, and side chains) are shown in gray, while hydrophobic domains are shown in black. 

Okano, T., Katayama, M., and Shinohara, 1. The influence of hydrophobic and hydrophilic domains on water wettability of 2-hydrooxyethyl methacrylate/styrene copolymers, J. Appl. Polmer Sci, 1978, 22, 361-ill. 

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