KINETIC-STUDIES OF INTERFACIAL PHOTOCURRENTS IN PLATINIZED CHLOROPLASTS

A photobioelectrochemical cell was constructed using platinized chloroplasts entrapped on a fiberglass filter pad as the photosensitive material. In this two-electrode device, a platinum gauze electrode made pressure contact with the chloroplasts, and a silver/silver chloride electrode made pressure contact with the electrolyte-impregnated filter paper pad. Upon illumination, an oriented photocurrent was observed that is consistent with the vectorial photochemical model of the reaction centers in photosynthetic membranes. The kinetics of interfacial photoelectron transfer in this cell were studied using the technique of repetitive flash illumination. By driving the photocurrent response into steady state, using multiple flash frequencies and normalizing the photoresponse to the rate of flashing, the frequency response of the photocurrent was determined. As expected, in the low-frequency, linear region, the normalized photocurrent response was constant. However, as the rate of flashing increased (above 100 Hz), the yield of photocurrent per frequency interval decreased. This decrease in yield was interpreted as the inability of the thermally activated electron transport chain of photosynthesis to keep pace with the higher rates of reaction center excitation. The reciprocal of the frequency at which the normalized photocurrent has fallen to one-half is called the turnover time. In these experiments this occurred at approximately 200 Hz, corresponding to a turnover time of 4 ms.