Concentration quenching in chlorophyll

IN the primary process of plant photosynthesis it is generally accepted that efficient energy migration occurs between about 300 molecules of chlorophyll a , with subsequent light collection by a chemical trap. Solutions of chlorophyll in vitro , whether in fluid solvents 1 , monolayers 2,3 , multilayers 4,5 or, as shown recently in our laboratory, in rigid matrices of PMMA and in bilayer lipid vesicles, exhibit the phenomenon of concentration quenching of the excited state at concentrations much lower than those which are present in the chloroplast. At a chlorophyll concentration of 10 −1 M, which is comparable with that in the chloroplast, none of the in vitro systems has a fluorescent yield as high as is found in vivo , especially when the photochemical traps are closed. To try to understand the apparent absence of concentration quenching in vivo , we have re-examined its mechanism in vitro , and conclude that each chlorophyll molecule in the light-collecting system must be separated from other chlorophyll molecules, so as to prevent trap formation by orbital overlap and so that the minimum distance, averaged over all orientations in a random array, is 10 Å.