Redox orbital solvent effects

HNMR studies have shown that the Fe(II) form is particularly stable and the native conformation is essentially retained up to 97°. Cytochrome c III) has a more labile structure and the Fe coordination is more easily disrupted. It is known (see Sect. 4.2) that, upon exposure to higher pH, methionine-S is displaced from the Fe(III) by another (N) donor. Since the Fe donor-acceptor orbitals can mix with the porphyrin 7t system, the effective d-electron density is extended to the heme edge and thus to the molecular surface at which this is exposed. As evident from Fig. 1, the actual protein surface area taken up by the solvent-accessible heme is very small (amounting to less than 1 % of the total) so that if electron transfer to redox partners involves this entity (as it is widely believed) then the orientation of the protein during the encounter will be critical. The electron-transfer activity of the Fe centre may thus be termed anisotropic . 

Cyclic voltammetry (CV) can provide information about the thermodynamics of the redox process, kinetics of heterogeneous electron transfer reactions and coupled chemical reactions [32]. The reversible electron transfer steps inform us about the compound s ability to accept electrons however, experimental conditions, such as solvent and temperature also influence the voltammogram. The structure of the lowest unoccupied molecular orbital (LUMO) levels of the compound can be determined from the number of CV waves and reduction potentials ( 1/2)- Moreover, the CV can serve as a spectroscopy as demonstrated by Heinze [32], since the characteristic shapes of the waves and their unequivocal positions on the potential scale are effectively a fingerprint of the individual electrochemical properties of the redox system. 

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