Stoichiometric determination of chlorophyll a-water aggregates and photosynthesis. symbiotic roles of the magnesium atom and the ring v cyclopentanone group in the structural and photochemical properties of chlorophyll a monohydrate and dihydrate

X-ray photoelectron spectroscopy has been employed to probe the water content of aggregates of pheophytin a and chlorophyll a. The O ls spectra of these compounds show a distinct shoulder at 533.7 eV when water is present, which is used to calculate the number of bound waters of hydration of the aggregate. We have been unable to observe anhydrous chlorophyll, but have characterized both Chl a·H2O and Chl a·2H2O. The latter occurs in a crystalline form of Chl a that absorbs light at 743 nm. For pheophytin a, however, water-free films are readily prepared. Since the Mg in chlorophyll a is replaced by two hydrogen atoms in pheophytin a, we conclude that the water in monohydrate Chl a is bound directly to the Mg atom. The probable role of the Chi a Mg atom in plant photosynthesis is discussed in terms of current and earlier considerations of the apparently symbiotic functions of Mg and the ring V cyclopentanone ring in the Chl a molecule.