A TWO-QUANTASOME THEORY OF CHLOROPHYLL-a FLUORESCENCE IN GREEN PLANT PHOTOSYNTHESIS

Abstract

The mean time for transfer of electronic excitation energy to a single-site, operative trap in a photosynthetic unit is calculated by direct solution of the weak-interaction equations. Both2- and 3-dimensional arrays of pigment molecules are considered. The PSU is assumed to consist of 2 subunits, psu1 and psu2, corresponding to the 2 pigment systems of green plant photosynthesis. If each subunit has its own trapping center, and the concentration of chlorophyll-a is higher in 1 psu than in the other, 2 trapping times, t1 and t2, occur. The 2-quantasome hypothesis, that each psu is embodied in a separate quantasome (chloroplast lamellar fragment), is shown to be a necessary adjunct of the assumption that both psu''s contain 3-dimensional pigment arrays. The trapping times for such arrays are t1 = 0.29 nsec, if psu1 contains 400 Chl-a molecules; and t2 = 1.2-2.2 nsec, if psu2 contains 100[long dash]50 Chl-a''s. The relation of these trapping times to the fluorescence lifetime of Chl-a in vivo is discussed. The theory explains the anomaly in the values of this lifetime as measured by steady-state fluorescence yield, phase fluorimetry, and direct flash (with second-moment analysis) experiments. Theoretical values of yield, phase, and flash lifetimes are 0.46, 1.0, and 1.5 nsec, respectively. The corresponding experimental values are 0.41, 0.8, and 1.7 nsec. Two- and 3-dimensional pigment arrays are compared in their abilities to correlate theory and experiment. It is shown that agreement of the theory with more than 1 of the 3 experiments simultaneously is only possible if psu1, at least, is 3-dimensional. The theory calculates 2 quantities, t1 and t2, and correctly predicts from them 3 independent experimental results. The 3rd correlation verifies the internal consistency of the theory. The theoretical value of the total Chl-a fluorescence yield is the most reliable quantity calculated, because it is independent of the (relatively unknown) apportionment of Chl-a''s between the 2 pigment systems. The present theory is completely consistent with interpretations of Chl-a-fluorescence yield changes based on the series formulation of photosynthesis. However, the theory predicts that, rather than being actually nonfluorescent, the Chl-a of psu1 has an intrinsic yield of 2%, about 1/5 that of psu2.