Comparative kinetic behavior of the photoinduced EPR signal observed in whole cells and cell free preparations of Rhodospirillum rubrum

The time response of the photoinduced EPR signal observed in whole cell Rhodospirillum rubrum is compared with the signal observed in cell free preparations from the same bacterium. Although the spectral characteristics of these signals, g value, line width, and shape, appear identical, the formation and decay characteristics are quite distinct. The decay kinetics observed in whole cells are monotonic, with a half-time of approximately 30 msec, whereas those observed in subcellular particles are biphasic, with both millisecond and second decay times. The formation kinetics of the signal generated in whole cells are dependent upon the dark-light history and environment of the cells; the formation curve displays a plateau, two maxima, and a minimum before a steady state is reached. The formation kinetics of the signal obtained with cell free preparations are strictly monotonie and independent, of their dark-light history. A flash from a liquid dye laser produces in the whole cells a machine-limited (faster than 2 msec) EPR signal which decays with a half-time of approximately 11 msec. © 1970.