Photoacoustic detection of flash-induced charge separation in photosynthetic systems. Spectral dependence of the quantum yield

Abstract A new high-sensitivity photoacoustic spectrometer is described. The volume changes occurring in a thin layer of photosynthetic suspension during the first microsecond following a monochromatic laser flash (3 ns duration) are detected by a piezoelectric ceramic. These volume changes result from two components of opposite signs: (1) thermal expansion due to the release of heat in the medium, and (2) contraction occurring in the neighbourhood of the charged photoproducts. The expansion signal measures the thermal losses in the light-conversion process. Under weak flash excitation, the difference between the amounts of heat released by an active and an inactive sample measures the energy stored by the photoreactions. As the contraction signal is also linearly related to the number of flash-induced charge separations, the overall volume change, when compared between the active and inactive states of the sample, gives a relative measure of the quantum yield. In the spectral range investigated (630–705 nm), the quantum yield is practically independent of wavelength in purified PS I and in PS II (BBY) particles. In spinach chloroplasts, a depression of the quantum yield in the absorption region of LHC II suggests that a significant fraction of the light quanta absorbed by LHC II are not transferred to open reaction centres. In intact cells of Chlamydomonas reinhardtii with closed PS II reaction centres, the quantum yield spectrum of the PS I centres was compared in state I and state II. Upon transition from state I to state II, a large proportion of the quanta absorbed by the PS II antenna was found to be transferred towards PS I reaction centres. When measured at two different temperatures, the overall volume change can be deconvoluted in its two components (expansion and contraction), and the absolute value of energy storage calculated. In purified PS I and in PS II (BBY) particles, this value was found close to 1 eV per absorbed quantum, which is approximately the energy required for the formation of the radical pairs P700 + (F A ,F B ) − and Z + Q − A .