Redox switched site

This has been demonstrated in EQCM studies of PTh film redox switching in HC104 solutions of different concentration. Under conditions where thionine reduction is a 2e/3H+ process, electroneutrality alone predicts uptake of one anion (and three protons) per Th site a film mass increase of 102.5 g molTh"1, independent of solution composition. Experimentally, the mass change is less than 20 g mol 1 in 1 mol dm 3 HC104, decreases as the electrolyte is diluted, and even becomes negative at pH > 2 The variation of mass change with concentration is attributable to activity effects. Hydronium perchlorate is included within the film, to an extent dependent on polymer redox state and solution concentration. 

The spatially averaged shear modulus in the acoustic frequencies obtained simultaneously to polymer redox switching in cyclic voltammetry arises from the exchange of ions and solvent during oxidation-reduction of the ferrocene sites attached to the polymer backbone. 

After the desired coverage has been achieved (via deposition time), the PVF+-coated electrode can be rinsed with water and transferred to aqueous 0.1 mol dm 3 sodium perchlorate for characterization. Since both oxidation states of the film are insoluble in aqueous media, we are able to investigate the redox switching process of surface-confined material, as schematically illustrated in Scheme 13.4. The polymer coverage (I7mol cm-2, expressed in terms of immobilised ferrocene sites) is obtained coulo-metrically by integration of the current response to a slow voltammetric scan. We now discuss the behaviour of thin (<0.1 jtm) PVF films after... 

Flavin-dependent le -transfer in enzymes and chemical model systems can he differentiated from 2e -transfer activities, i.e., (de)hydrogenation and oxygen activation, by chemical structure and dynamics. For le -transfer, two types of contacts are discussed, namely outer sphere for interflavin and flavin-heme and inner sphere for flavinr-fenedoxin contacts. Flavin is the indispensable mediator between 2e - and le -transfer in all biological redox chains, and there is a minimal requirement of three cooperating redox-active sites for this activity. The switch between 2e - and le -transfer is caused by apoprotein-dependent prototropy between flavin positions N(l)/0(2a) and N(5) or by N(5)-metal contact. 

Amesano E, Band L, Bertini I et al (2003) A redox switch in CopC an intriguing copper trafficking protein that binds copper(I) and copper(II) at different sites. Proc Natl Acad Sci USA 100 3814-3819... 

Nuclearity of the metal active site is well defined and controlled in the enzyme pocket. In solution, complexes can be present as different forms in equilibrium depending on various parameters (pH increase can lead to oligomer formation, for instance) making the rationalization of the catalytic process at work difficult. In simple redox achve complexes, a poorly constrained environment can drashcally change upon redox switch. Mononuclear copper systems are known to dimerize upon O2 reaction in solution (unless the coordination sphere is saturated, which is useless for O2 activation), and oxygenated intermediates of porphyrin complexes are quickly deactivated by self-oxidation and formation of p-oxo dimers. 

The properties of the metal in natural systems are finely tuned by the specific and constrained environment. Water activation in hydrolytic enzymes is highly affected by the H-bond network in the receptor pocket, while redox potentials of iron or copper species in catalytic sites are exquisitely controlled by the geometrical constrains existing at the various redox states. In solution, the ligand binding mode can adapt to the redox switch. This results in a specific interlocking of the receptor and metal properties (redox or acid-base). 

The peak width in the voltammetric profile conveys useful information about the electrochemistry of redox switching in an electroactive polymer film. Theory developed in Section 2.3, assuming an ideal situation of zero interredox center interaction, and sites in the layer at equilibrium with the potential applied at support electrode surface (which means that the site oxidation is Nemstian), predicts that the peak width at half height S has the following value ... 

Taking this one step further, competitive inclusirm complexation of cyclodextrin with either polymer-bound, associating alkyl side chains or ferrocene is dependent on the different oxidation states of ferrocene. This leads to a switching between the gel state and molecularly dissolved polymer [254]. This very special case can be categorized as electrochemically induced complexation, although, interestingly, the polymer has no direct interaction with the redox active sites (Fig. 19). 

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