The Hydrophilic Cluster

Glu-333 (or His-332). His-337 and Asp-189 (or His-190), which have been suggested as possible ligands fiom mutagenesis or chemical modification studies are also possibly coordinated to the Mn clusten 

Ca is an essential cofector in oxj en evolution. Depleting this cofactor suppresses OEC activity, which can be restored (up to 90%) by replenishing with Ca. Various cations compete vdth calcium for its binding site(s) in PSII. Sodium, potassium and cesium are weakly competitive with calcium, but they do not suppon oxygen evolution activity. Partial reactivation (up to 40%) results from addition of strontium to Ca-depleted PS II membranes and no other metal ions (excqjt VO, vanadyl ion) can restore activity.  

Most researchers addressing the stoichiometry of the Ca cofiictor in PS II now condude that functional water oxidase activity requires one Ca, whidi can be removed by low-pH/dttaie or 1.2 MNaClwash. -  

Biotechnological Applications of Photosynthetic Proteins Biochips, Biosensors and Biodevices 

Chloride plays an essential role in the ox) en-evolving process. Chloride depletion of PSII samples results in the inactivation of the OEC. Addition of certain anions restores the oxygen evolution activity. The eflFecriveness of the anions follows the order chloride bromide iodide - nitrate. The loss of the two polypeptides, 17 kDa and 23 kDa, induces an increased demand for Cl in order to retain optimal function of the water-oxidizing reaction, suggesting a role for Cl in maintaining the protein organization needed for 02-evolution.  

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Fig. 6.28 The hydrophilic clusters and surface areas increase with increasing humidity, (a) SPTES-70 membrane at 22°C and 35% RH (b) SPTES-70 membrane at 22°C and 65% RH...

Polyaniline (PANI) nanocomposite membranes are also prepared by a sol-gel process, embedding silica in the hydrophilic clusters (Nafion) followed by its deposition by redox polymerization [51]. PANI modified the membrane structure and reduced the methanol crossover, while silica Incorporation improved the conductivity and stability. Zeolite has been incorporated as potential filler for PEMs, either by blending or by infiltration in swelled membrane, to reduce the methanol permeability and enhance the thermal stabihty [52,53]. Although the fuel cell performance of these membranes was Inferior compared with pristine Nafion membrane, incorporation of semipermeable particles is an effective method to engineer the transport properties of composite membranes. Chen et al [54] reported nanocomposite membranes by in situ hydrothermal crystallization method, with similar proton conductivity, but low methanol permeability (40% less) in comparison with Nafion membrane. These membranes showed higher OCV (3%) and power density (21%) than Nafion. 

Good agreement between C(- and the dipole moment of the solvent (H20) molecules (i.e., by the hydrophilicity of metals) established by Trasatti25,31 was found and the reasons for this phenomenon were explained 428 The Valette and Hamelin data150 251 387-391 are in agreement with the data from quantum-chemical calculations of water adsorption at metal clusters 436-439 where for fee metals it was found that the electrode-H20 interaction increases as the interfacial density of atoms decreases. 

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

The hydrophilic Au SR (RSH = GSH, h-GSH, (PG)SH, (SA)SH) clusters were ionized by the ESI method [15,16,18,23,24] (Figures 2 and 3c). The details of the ESI source are depicted in Figure 3c, together with typical pressures of the chambers under operation. A 50%(v/v) water-methanol solution of the fractionated Au SR cluster with a typical concentration of 0.5mg/mL was electro-sprayed into the ambient atmosphere through the stainless steel needle of a syringe biased at ca. — 3kV. The solution was delivered by a syringe pump (SP310I, World Precision... 

Figure 7.22b shows that hydrophilic molecules, those with log Kj < 1, are much more permeable in octanol than in olive oil. The same may be said in comparison to 2% DOPC and dodecane. Octanol appears to enhance the permeability of hydrophilic molecules, compared to that of DOPC, dodecane, and olive oil. This is dramatically evident in Fig. 7.7, and is confirmed in Figs. 7.8c and 7.22b. The mechanism is not precisely known, but it is reasonable to suspect a shuttle service may be provided by the water clusters in octanol-based PAMPA (perhaps like an inverted micelle equivalent of endocytosis). Thus, it appears that charged molecules can be substantially permeable in the octanol PAMPA. However, do charged molecules permeate phospholipid bilayers to any appreciable extent We will return to this question later, and will cite evidence at least for a partial answer. 

By swelling with aqueous electrolyte, cations (and, to lesser extent, also anions) penetrate together with water into the hydrophilic regions and form spherical electrolyte clusters with micellar morphology. The inner surface of clusters and channels is composed of a double layer of the immobilized —SO3 groups and the equivalent number of counterions, M+. Anions in the interior of the clusters are shielded from the —SOJ groups by hydrated cations and water molecules. On the other hand, anions are thus... 

In general, incorporation of hydrophobic groups into PIPAAm chains decreases the LCST [29-31]. Hydrophobic groups alter the hydrophilic/ hydrophobic balance in PIPAAm, promoting a PIPAAm phase transition at the LCST, water clusters around the hydrophobic segments are excluded from the hydrophobicaUy aggregated inner core. The resulting isolated hydrophobic micellar core does not directly interfere with outer shell PIPAAm chain dynamics in aqueous media. The PIPAAm chains of the micellar outer shell therefore remain as mobile linear chains in this core-shell micellar structure. As a result, the thermoresponsive properties of PIPAAm in the outer PIPAAm chains of this structure are unaltered [23-27,32]. 

Fig. 24.2 summarizes the cleaning action of surfactants. The surfactant lines up at the interface and also forms micelles, or circular clusters of molecules. In both cases the hydrophobic end of the molecule gets away from water molecules and the hydrophilic end stays next to the water molecules (like dissolves like). When grease or dirt come along (primarily hydrophobic in nature) the surfactants surround it until it is dislodged from the substrate. The grease molecules are suspended in the emulsion by the surfactant until they can be washed away with freshwater. 

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