Charge transfer complexes, biological examples

III. EXAMPLES OF CHARGE-TRANSFER COMPLEXES IN BIOLOGICAL SYSTEMS... 

In the realm of biology, a step in the chain of charge transfers in the photosystem II oxygen-evolving complex (PSII/OEC) may provide an example of pcet according to the following scheme (15). For a consecutive pathway, et followed by pt, the steps are... 

It is known that superoxide forms complexes with Ca ", thereby permitting formation of calcium peroxide. Since the majority of biological membranes translocate Ca or other alkali metal ions, the opportunity for interaction with superoxide within the membrane may be considerable (Ca " and calmodulin are required for superoxide production by leukocytes ). Under pathological circumstances calcium peroxide could be formed and lipid peroxidation initiated. The breakdown of membrane capacitance as a consequence of this may be deleterious in a number of ways. For example, the loss of a critical electric field may remove the driving force for energy-dependent processes associated with charge transfer. On the other hand, circumstances may be established which bring about unrestrained and unmodulated metabolic activity, as in cancer. 

It has been shown for some neutral carriers, for example, valinomycin and the actins, that most of the charge transfer across the membrane occurs by the process (aqueous ion plus adsorbed carrier) (adsorbed complex) as the diffusion of the complex from the aqueous phase to the membrane cannot be sufficient to supply current [47]. When carriers exist in biological membranes, then this type of reaction probably dominates, as it is unreasonable to expect biological carriers to be lost from the membrane. 

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