ACOUSTIC DETECTION OF TRIPLET-STATES FORMED BY RADICAL PAIR RECOMBINATION IN QUINONE-DEPLETED PHOTOSYNTHETIC REACTION CENTERS, BY MAGNETIC-FIELD MODULATION (MAGNETOPHOTOACOUSTIC EFFECT) - A FEASIBILITY STUDY

An acoustic method is outlined to detect triplet states formed by radi cal pair recombination in photosynthetic reaction centers. It is based on magnetic field effect on the probability of triplet state formatio n by recombination. Using a periodically modulated magnetic field in t he presence of constant exciting light, a periodic modulation of the t riplet state concentration is set in the sample, which is detected thr ough the corresponding modulated heat emission, transduced to acoustic vibration of the gas phase around the sample. This effect is similar to the photoacoustic effect, except that here the light is not modulat ed. The feasibility of detecting such an effect was proven experimenta lly, by obtaining a signal from quinone-depleted reaction centers of R hodobacter sphaeroides. The signal had twice the frequency of the magn etic field modulation; it was proportional to the light intensity and significantly stronger at the lower temperatures (in the investigated range 113-278 K). No signal was obtained from quinone-containing react ion centers, which do not produce triplets. A theoretical outline of t he effect and the experimental set-up are described. The magnitude of the effect was calibrated against ordinary photoacoustic measurements, allowing numerical evaluation of certain parameters of the triplet st ate (e.g. triplet energy or yield) with the aid of auxiliary informati on from the literature.