Extremely fast photoelectric signals from suspensions of broken chloroplasts and of isolated chromatophores

Abstract

This paper concerns transient photoelectric signals that are observed when a suspension of photosynthetic vesicles is illuminated with flashing light of nonsaturating energy using a pair of electrodes positioned at different depths along the path of the exciting light. With chloroplasts we observed two different types of signals. One rose extremely fast, the rise time (10–90%) was less than 200 ps under excitation with a single pulse from a mode-locked ruby laser, while the other rose more slowly (typically 10 μs). These signals displayed several different properties such as their polarity, kinetics, apparent source impedance, and sensitivity to structural integrity of the chloroplast lamellar system. Experimentally, signal ‘Fast’ could only be induced by very short light pulses (shorter than approx. 60 ns), whereas signal ‘Slow’ appeared only under longer excitation. The detection of signal Fast required special instrumentation, particularly fast preamplifiers with low input capacitance. Our results support the more recent idea that signal Slow did not reflect the primary transmembrane charge separation, as postulated in the earlier literature, but rather lateral movement of charge carriers along the intact lamellar system of chloroplasts. On the other hand, signal Fast may reflect the primary outwardly directed electron transfer across the thylakoid membrane. Its polarity, however, was opposite to that previously postulated to appear in response to this event. For comparison we also studied photoelectric effects in a suspension of structurally more homogeneous chromatophores from Rhodopseudomonas sphaeroides. These vesicles displayed only a signal of the same polarity and similar kinetics as signal Fast from chloroplasts. When the secondary quinone electron acceptor (Q) was chemically reduced, the decay time was shortened from approx. 30 ns to approx. 10 ns. The acceleration to approx. 10 ns is known for the rapid, supposedly transmembrane back-reaction of the primary charge separation. Therefore, we conclude that the electrodes monitor the primary charge separation. There is still the unresolved problem with the polarity of the electric signal which we attribute to an as yet unidentified property of the pick-up system. Because its signal-to-noise ratio under extremely high time resolution is superior to that obtained in flash spectroscopy, signal Fast represents a very good means to measure the spatial separation between the very primary electron carriers in the photosynthetic reaction center.