Proton transport pathways

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

A number of differenf approaches have been used to try to overcome some of these disadvantages of existing membranes. One such approach is to try to prevent water loss from the proton transport pathways, thus maintaining proton conductivity above the boiling point of wafer. Typically, this is attempted by adding hydrophilic inorganic species into the membrane. Furthermore, these particles in themselves may also be capable of proton conduction. 

Tripp BC, Ferry JG. 2000. A structure-function study of a proton transport pathway in a novel gamma-class carbonic anhydrase from Methanosarcina thermophila. Biochemistry 39 9232 0. 

A pathway of protonatable residues connects the Qb binding site with the aqueous cytoplasmic compartment. The particular residues involved in the proton transport pathway have been identified in large part using site-directed mutants. Figure 15 shows the proposed pathway of H+ transfer in Rhodobacter sphaeroides " ... 

I-III as pictured in Fig. 18-5. One site of pumping is known to be in the cytochrome c oxidase complex. When reconstituted into phospholipid, the purified complex does pump protons in response to electron transport, H+/e ratios of 1 being observed. 437,i47,i9i As mentioned in Section B,3 a large amoimt of experimental effort has been devoted to identifying proton transport pathways in cytochrome c oxidase and also in the cytochrome bcj (complex II). Proton pumping appears to be coupled to chemical changes occurring between intermediates P and F of Fig. 18-11, between F and and possibly between O and r3 7138... 

B. Yue, L. Yan, S. Han, L. Xie, Proton transport pathways in acid-base complex consisting of phosphonic acid group and 1,2,3-triazolyl group, J. Phys. Chem. B 117 (2013) 7941-7949. 

J.W. Traer, G.R. Goward, Solid-state NMR studies of hydrogen bonding networks and proton transport pathways based on anion and cation dynamics, Magn. Reson. Chem. 45 (2007) S135-S143. 

L.S. Cahill, U.A. Rana, M. Forsyth, M.E. Smith, Investigation of proton dynamics and the proton transport pathway in choline dihydrogen phosphate using sohd-state NMR, Phys. Chem. Chem. Phys. 12 (2010) 5431-5438. 

In general, there are two methods to introduce the PA to the CL. The first one rehes on a highly PA-doped membrane. The PA is transferred between membrane and catalyst layer upon MEA preparation and/or cell assembly, respectively. The second method involves depositing (spraying, painting, etc.) PA directly onto the CL. The PA content in the membrane is one of the decisive factors for the selection of one of the two preparation methods. Overall, the CL needs to provide electron and proton transport pathways, gas accessibility, high catalytic activity towards HOR and ORR, a high electrochemically active surface area (ECSA) and has to withstand the harsh HT-PEMFC environment (acidity close to pH = 0, temperature up to 180 °C, and electrochemical potentials up to 1.5 V). 

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