Channel hydrate

Proton conductivities of 0.1 S cm at high excess water contents in current PEMs stem from the concerted effect of a high concentration of free protons, high liquid-like proton mobility, and a well-connected cluster network of hydrated pathways. i i i i Correspondingly, the detrimental effects of membrane dehydration are multifold. It triggers morphological transitions that have been studied recently in experiment and theory.2 .i29.i ,i62 water contents below the percolation threshold, the well-hydrated pathways cease to span the complete sample, and poorly hydrated channels control the overall transports ll Moreover, the structure of water and the molecular mechanisms of proton transport change at low water contents. 

As mentioned in the introduction, Mb also forms an Fe =0 heme on reaction with peroxides (7). Of particular interest is the mechanism of proton delivery to the oxene ligand on reduction because the distal heme pocket of Mb lacks proton donors (Figure 2b) and is isolated from the bulk solvent (43), unlike CCP where the hydrated substrate channel connects the distal heme cavity to the solvent (44). Thus, it is expected that solvent-assisted PT to the heme of Mb at physiological pH is severely restricted. A spectacular example of solvent-assisted PT to a buried redox center has been highlighted in the X-ray structure of the bacterial reaction centers from Ehodobacter sphaeroides (45). A narrow hydrated channel extends from the cytoplasmic side of the reaction center to quinone Qb, which is buried -23 A in the L-subunit. 

Preliminary MD simulation of sepiolite interfacial water structure, as seen in Figure 4.16, clearly shows that sepiolite exhibits interesting interfacial properties due to its unique crystal structure, which is composed of a discontinuous magnesium-oxygen/hydroxyl octahedral sheet with alternating 2 1 magnesium-silicate modules (ribbons) and hydrated channels. Each ribbon exhibits a... 

Palygorskite and sepioHte minerals are 2 1 layered phyUosiHcates that differ from the above mentioned clays because the octahedral sheets have significant intracrystalline void space caused by discontinuous octahedral layers. The basal tetrahedral unit is connected to an adjacent inverted basal tetrahedral creating a void space or channel. Charge deficits are balanced by hydrated cations in the intracrystalline space. 

D. Hydrated monovalent cation approaching the carbonyl oxygens of a transmembrane channel. The carbonyl oxygens at the mouth replace water in the first coordination shell. As the ion moves through the channel, it retains one bound water molecule preceding and following it and the walls of the channel provide for lateral coordination. (Parts A through D reproduced with permission from Ref. 6>. 

E. Hydrated divalent cation approaching a channel with a slightly larger diameter than in D, but the energy of interaction with the divalent cation is sufficient to deform the channel drawing the walls in to make lateral coordination with the divalent cation. Since the channel is too small for a monovalent cation to pass through with its first hydration shell and since the monovalent cation channel interaction is insufficient to make the channel small enough for lateral coordination of the monovalent cation, the channel is selective for divalent cations. (Part E reproduced with permission from Ref. 68 )... 

The protein that stores iron in the body is called ferritin. A ferritin molecule consists of a protein coat and an iron-containing core. The outer coat is made up of 24 pol3q5eptide chains, each with about 175 amino acids. As Figure 20-27 shows, the pol q5eptides pack together to form a sphere. The sphere is hollow, and channels through the protein coat allow movement of iron in and out of the molecule. The core of the protein contains hydrated iron(HI) oxide, FC2 O3 H2 O. The protein retains its shape whether or not iron is stored on the inside. When filled to capacity, one ferritin molecule holds as many as 4500 iron atoms, but the core is only partially filled under normal conditions. In this way, the protein has the capacity to provide iron as needed for hemoglobin s mthesis or to store iron if an excess is absorbed by the body. 

Scheme 4 Representation of equilibria between Ti04 framework species and ri complexes inside TS-1 channels upon dosage of anhydrous H2O2 (left) and between rj and rj complexes upon hydration (right). Adapted from [50] with permission. Copyright (2004) by Wiley-VCH...

Summarizing, the in situ UV-Vis, XANES, and EXAFS studies of Bonino et al. [49] and of Prestipino et al. [50] on hydrated and anhydrous peroxo/hy-droperoxo complexes on crystalhne microporous and amorphous meso-porous titanosilicates have shown, for the first time, the equilibriiun between r] side-on and end-on complexes. The amount of water is the key factor in the equilibrium displacement. In this regard please note that, owing to the hydrophobic character of TS-1, substrates such as olefins are the dominant species in the channels. This fact assures a relatively local low concentration of water, which in turn guarantees a sufficient presence of the active end-on... 

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