Polarographic Studies on Ubiquinone-10 and Rhodoquinone Bound with Chromatophores from Rhodospirillum rubrum

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Abstract
Redox components bound with chromatophores of Rhodospirillum rubrum, and pure samples of ubiquinone-10 and rhodoquinone were studied polarographically at 24°. In a mixture of ethanol and water (4: 1, v/v) at pH 7, ubiquinone-10 and rhodoquinone had half-wave potentials (E1/2) of +43 mV and −63 mV, respectively. For both quinones, values of the electron transfer number (n) were 2, and plots of E1/2versus pH formed straight lines with slopes of −30 mV/pH in the neutral pH range; thus, values of the proton transfer number (n-a) were estimated to be 1 for both quinones. When bound with chromatophores, ubiquinone-10 and rhodoquinone had E1/2 values of +50 mV (n=2) and −30 mV (n=2), respectively, at pH 7. Values of (n-a) were estimated to be 1 for ubiquinone-10 and 2 for rhodoquinone. A component (POC−170) thought to be one of the active center bacteriochlorophylls (Liac-890) was characterized; it has E1/2 value of −170 mV at pH 7 and its oxidation-reduction is possibly brought about by dehydrogenation-hydrogenation. Conceivably, the oxidation-reduction sites of ubiquinone-10, rhodoquinone and POC −170 partly, if not all, exist on the surface of chromatophore membrane or project outside the membrane, because of their accessibility to the polarographic electrode.

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