Electron paramagnetic resonance studies on photosynthetic bacteria. I. Properties of photo-induced EPR-signals of Chromatium D.

Abstract (1) Photo-induced EPR-signals of the free radical type were investigated in a variety of preparations derived from the photosynthetic bacterium Chromatium D. Materials ranged from intact, actively photosynthesizing organisms to cell-free “simplified systems” such as chromatophore-type preparations to synthetic photochemical systems containing bacteriochlorophyll in solution. Closely similar signals were observed in terms of the general properties such as g -value, linewidth and line shape, microwave saturation behaviour, and lack of detectable hyperfine structure. (2) Improvements of the time response capabilities of the instrumentation made fast measurements possible — present time limit a few msec — and allowed detailed kinetic studies on all these materials. Characteristic time courses of both formation and decay of photo-induced EPR-signals were observed. They are seen to fall into two classes of patterns which are described in detail for intact photosynthesizing bacteria and for the classical chromatophore-type preparation. These patterns are termed the “cell pattern” and the “chromatophore-type pattern”, respectively. (3) The “cell pattern” is characterized by a relatively slow and complex time course of formation with a fast initial rise at the onset of irradiation and a much faster, comparatively simple time course of decay. It is only observed when intact bacteria are studied under physiological conditions and is strongly dependent on such metabolic parameters as age of the cultures, substrate levels, and preillumination history. Changes introduced by altering the metabolic situation are reversible. No evidence for the existence of lag periods in either formation or decay has been obtained. (4) The “chromatophore-type pattern” shows a fast monotonic rise with no complexities and a much slower and typically biphasic decay. It is a quite stable phenomenon, strictly repetitive, and relatively independent of the quality and intactness of the preparation used. This pattern is observed with the classical chromatophore-type preparations and also with a large number of further simplified systems. (5) The transition from the cell pattern to the chromatophore-type pattern is readily achieved by unspecific treatments and is found to be irreversible. It occurs at a level of organization very close to that of the intact organism and is accompanied by a gradual loss of the overall photosynthetic capacities of the materials. (6) A comparison of these results with other measurable parameters of bacterial photosynthesis shows that existence and demonstratability of these photo-induced EPR-signals parallel strictly the photochemical activities of the bacteriochlorophyll molecule, including its participation in bacterial photosynthesis. On the other hand, the existence of the two characteristic time course patterns parallels the overall photosynthetic capacities of the materials, but does not correlate with the known capacities of the materials to form ATP in the process of photophosphorylation. The results demonstrate clearly that the paramagnetic species detected by EPR-spectroscopy are closely linked to the overall processes of photosynthesis and are participating in the electron transfer system of these bacteria.