Proton transport channels

ATP synthase activity can be restored by adding back the F] complex to the depleted membranes. The F[ complexes bind to membrane channels known as the F complex, which are also composed of multiple subunits. The polypeptides of the F0 component are very hydrophobic and form a proton transport channel through the membrane, which links the proton gradient to ATP synthesis. This channel appears to be lined with hydrophilic residues such as seryl, threonyl and carboxyl groups. The stalk that connects the F, to the F complex comprises one copy each of the polypeptide known as the oligomycin-sensitivity-conferring protein (OSCP) and another protein known as F6. 

Preparation method. The preparation method affects the formation of the electron transfer ehannels (eatalyst), gas diffusion channels (pores), proton transport channels (eleetrolyte), water transport ehannels (hydrophilic electrolyte), and most importantly, the three-phase interfaee in the CLs. 

Wang et al. studied the micromorphology of Nafion in different dilute solutions by TEM.3 Thin Nafion membranes were stained by mercury nitrate solutions. The size of the ionic domains and the morphology of the proton transport channels in the Nafion membrane varied with the solvent. 

Perfluorosulfonated membranes have a microscopic phase-separated structure with hydrophobic regions and hydrophilic domain. Hydrophobic regions provide the mechanical support and hydrophilic ionic domains provide proton transport channel. Many morphological models for PFSA have been developed based on SAXS and wide-angle x-ray scattering (WAXS) experiments of the membranes. However, because of the random chemical structure of the PFSA copolymer, morphological variation with water content and complexity of coorganized crystalline and ionic domains, limited characteristic detail proved by the SAXS and WAXS experiments, the structure of the PFSl has been still subject of debate. Here, a brief description of seven membrane structure models is provided. 

The low proton conductivity of aromatic PEMs, especially under low humidity conditions, results from a less pronounced phase-separated structure and lower connectivity of the proton transport channels. Therefore, the formation of a defined phase-separated structure and well-connected proton transport channels would be the key factors in improving the proton conductivity of the aromatic PEMs. 

Kumar et al. (2014) prepared SPEEK/sulfonaled graphene oxide composite membranes for PEMFCs. Sulfonated graphene oxide nanosheets were incorporated into the SPEEK structure to interconnect the proton transport channels. The MEA was hot-pressed at 125°C and a pressure of 5.88 MPa for 3 min. Both the anode and the cathode consist of 20% Pt/C with a Pt loading of 0.38 mg cm". The active electrode area was 1 cm. H2 and O2 were fed to each side of the cell at a flow rate of 100 and 70 mL min", respectively. The cell was conditioned at 0.3 V for 1 h before polarization studies. The OCV of both MEAs was greater than 0.95 V, indicating negligible gas crossover. SGO/SPEEK membrane showed a maximum power density of 378 mW cm" at 80°C and 30% RH, which is much higher than that of SPEEK (250 mW cm ). 

Protons reentering the cell via the proton-transport channel of ATP synthase (in the reverse... 

Conduction along water wires may as well be the dominant mechanism in the permeation of protons in channels an MD study of proton transport through a gramicidin channel can be found in, for example, [156]. 

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. 

Based on GebeTs calculations for Nafion (where lEC = 0.91 meq/g),i isolated spheres of ionic clusters in the dry state have diameters of 15 A and an intercluster spacing of 27 A. Because the spheres are isolated, proton transport through the membrane is severely impeded and thus low levels of conductivity are observed for a dry membrane. As water content increases, the isolated ionic clusters begin to swell until, at X, > 0.2, the percolation threshold is reached. This significant point represents the point at which connections or channels are now formed between the previously isolated ionic clusters and leads to a concomitant sharp increase in the observed level of proton conductivity. 

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. 

The catalytic cycle can be divided into three phases, through each of which the three active sites pass in sequence. First, ADP and Pj are bound (1), then the anhydride bond forms (2), and finally the product is released (3). Each time protons pass through the Fo channel protein into the matrix, all three active sites change from their current state to the next. It has been shown that the energy for proton transport is initially converted into a rotation of the y subunit, which in turn cyclically alters the conformation of the a and p subunits, which are stationary relative to the Fo part, and thereby drives ATP synthesis. 

The peptide subunit was easily incorporated into lipid bilayers of liposome, as confirmed by absorption and fluorescence spectroscopy. Formation of H-bonded transmembrane channel structure was confirmed by FT IR measurement, which suggests the formation of a tight H-bond network in phosphatidylcholine liposomes. Liposomes were first prepared to make the inside pH 6.5 and the outside pH 5.5. Then the addition of the peptide to such liposomal suspensions caused a rapid collapse of the pH gradient. The proton transport activity was comparable to that of antibiotics gramicidin A and amphotericin B. 

---