Reversible Photolysis of the Purple Complex in the Purple Membrane of Halobacterium halobium

Open Access
Publisher Website
Google Scholar
Abstract
A suspension of the purple membrane from Halobacterium halobium in concentrated salt solutions saturated with diethylether is bleached by light. During this process, the purple complex is converted in to a compound absorbing maximally at 412 nm (412‐nm complex). The purple complex reappears in the dark with a rate constant of 0.04 s−1 at 20°C. The initial rate of formation of the 412‐nm complex is proportional to the intensity of the bleaching light and a quantum yield of 0.79 is measured. The action spectrum of the photolytic reaction is identical to the absorption spectrum of the purple complex. The light reaction is temperature independent. The regeneration of the purple complex requires an activation energy of 11.4 kcal × mol−1. Illumination causes a decrease in the tryptophan fluorescence of the membrane. Regeneration of the purple complex restores the original fluorescence level. The half times of both processes correlate with the half times observed by absorption spectroscopy. The photochemical reaction is accompanied by release of protons during bleaching and uptake of the same amount of protons during regeneration of the purple complex. The half times of the measured pH changes again correspond to the one observed by absorption spectroscopy. The rapid release of protons upon illumination is temperature independent. For the dark uptake of protons an activation energy of 14 kcal × mol−1 has been determined. The significance of this isolated photochemical system is discussed with respect to a possible function of the purple membrane in vivo.

This publication has 9 references indexed in Scilit: