Hydration and Charge Transfer

In their natural state many proteins are bound into hydrophobic lipid matrices. The energetics of charge separation and mobility are governed largely by the dielectric constant of the surrounding medium. Calculations have shown that the internal structures have an effective 

Considerable differences in the resistivity between that of ordinary proteins and that of electron transfer proteins have been reportedand also between ferrocytochrome-c and ferricytochrome-c. This is attributed to the electronic state of the central metal atom. The anhydrous cytochromes exhibit resistivities many orders of magnitude higher thus anhydrous ferricytochrome has a room temperature of 4.1 x 10 Q cm and the ferrocytochrome Cj a value of 1.6 x 10 these data are even more remarkable because the ferri compound is said not to follow an Arrhenius type relationship in its temperature dependence while the ferro compound is reported to yield a negative value for its thermal activation energy. Cytochromes will be discussed in more detail in the next section. 

Another interesting example for severe hydration effects in relation to charge transfer is the purple membrane of Halobacterium halobium it acts as a photoelectret, producing at least semipermanent electric charges upon illumination this activity has been shown to be greatly affected by hydration. 

Water in proteins has been determined from hydration isotherms obtained gravimetrically and the more recent quartz crystal resonator techniques. The steady state conductivity of protein samples increases rapidly with water absorbed. The weight percentage, m, relates to the conductivity according to the Spivey equation, 

The polypeptide chain within the cytochrome molecule has virtually no conductivity at all, while the complete hemoprotein exhibits a conductivity of the order of 10 -10 (i2cm) suggesting a considerable contribution of the heme units to the overall electron transfer. Since the porphyrin subunit is about 13 A diameter, sufficient delocalization of n electrons should be expected to impart conductivity. It has been 

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