Movement, redox cycling

In mitochondria there are two types of mechanisms for coupling the electron transport to the movement of protons across the membrane. The first is based on anisotropic reduction and oxidation of a lipid-soluble quinone inside the membrane. The quinone, coenzyme Q, becomes protonated upon reduction and diffuses to an oxidation site on the other side of the membrane where removal of electrons leads to proton release. This is essentially a proton carrier system with the hydroquinone acting as the proton carrier in the lipid phase of the membrane. A further refinement of this system in mitochondria provides for a coenzyme Q redox cycle where the movement of one electron through the chain allows for two protons to cross the... 

In a more recent paper. Tipping and Woof (1983b) present evidence for a mechanism whereby humic substances accumulate by co-sedimentation with iron oxide particles. They also follow the movements of iron during its redox cycling in the hypolimnion and sediment. Their attempts to distinguish epilimnetic and hypolimnetic humic substances by spectroscopy and gel chromatography, however, were unsuccessful, and so provide little evidence for actual mechanisms responsible for the hypolimnetic accumulation af humic substances. 

The actuation force or movement generated during redox cycling is directly related to the concomitant changes in mechanical properties. Using a simple linear elastic model of the small-strain mechanical properties of PPy, it has been shown that the actuation strain (eo) at a constant applied stress (a) is accurately predicted from Equation 3.3... 

SCHEME 3 Potential movement of Cr(VI) on silica through two proposed mechanistic pathways (1) redox cycling, and (2) hydrolysis. 

Studied by the combined use of EQCM and ellipsometry measurements. In nitrated polystyrene films, increased solvation of the polymer with increasing redox cycling was observed/ The effect of electrolyte concentration and temperature on polymer swelling in TCNQ films was studied at the EQCM. In TCNQ films cation motion maintains electroneutrality within the polymer during redox cycling, and mass changes observed were found to be dominated by the concomitant movement of water of hydration. ... 

The catenate Cat-1+ was shown using redox stimulation to undergo reversible circumrotations. Starting from the four-coordinate Cu(I) state (Cu(I)N4) shown in the upper left comer of the square scheme, oxidation to Cn(II) (Cu(II)N4) caused the generation of the five-coordinate form (Cu(II)N5). This movement takes over 10 days. Thus, the oxidation could be accomplished separately from the measurement of the movement. As reported, the movement was monitored by UV-vis spectroscopy after chemical oxidation with Br2. In keeping with the theme of this chapter, the circumrotation was also examined using electrochemistry in a latter publication. Here, bulk electrolysis of a small solution sample, which usually takes 5-15 min to complete the oxidation, was used to initiate the movement. The system is then allowed to evolve, and an in situ linear sweep voltammogram (half a CV cycle) was recorded over 10 days. The peak current intensity for the four-coordinate species Cu(II)N4 at about -I-O.6V decreases in time, concomitant with an increase in the peak for the five-coordinate species at Cu(II)N5 at abont —0.1 V. 

Figure 19 Square scheme showing the cycle of redox-driven movements in catenane Cat-2 +.

TPB) [163, 164], The anode porosity (20 0 %) ensures good mass transport and improves the triple boundary by allowing 0 ion movement within the anode electrode [13, 160]. A small amount of ceria is added to the anode cermet to improve ohmic polarization loss at the interface between the anode and the electrolyte. This also improves the tolerance of the anodes to temperature cycling and redox changes within the anode gas [13,160]. 

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