In vitro preparation and characterization of a 700 nm absorbing chlorophyll-water adduct according to the proposed primary molecular unit in photosynthesis.

1. This study characterized chlorophyll a-H2O adducts in vitro in order to establish their generic relationship to the recently proposed [15, 18-20, 31] primary molecular adduct in photosynthesis. The effects of water titration and temperature on the absorption, fluorescence, excitation, and redox properties of the various in vitro chlorophyll a aggregate species are investigated. 2. From fluorescence measurements, we conclude that the driest chlorophyll a sample contains an equimolar amount of water. This conclusion is consistent with earlier experimental work [2, 3, 14, 17, 31], and clarifies the origin of the controversial [15] Katz model [14] of chlorophyll a-H2O interactions. 3. With increasing water concentration or as the temperature is lowered below room temperature, the A-663 monohydrate chlorophyll a-H2O (species absorbing at 663 nm) is favored at the expense of the A-678 anhydrous aggregate according to the equilibrium 2H2O+chlorophyll a2in equilibrium2 chlorophyll a-H2O. Under excess water conditions, A-663 is converted to A-743 (chlorophyll a-2H2O)n. 4. On slow sample cooling to T less than or approximately 200 degrees K, we observe the growth of A-700 at the expense of A-663. There is a direct correspondence between the increasing (decreasing) absorption by A-700 (A-633) and increasing (decreasing) fluorescence at 720 nm (664 nm). 5. It is concluded that A-700 is most probably the dimer participating in the equilibrium 2 chlorophyll a-H2O in equilibrium (chlorophyll a-H2O)2. The A-700 band consists of two exciton components (separated by approximately 280 cm1) that are interpretable in terms of the dimeric origin of A-700. 6. The deconvoluted A-700 absorption spectrum and the excitation spectrum of the 720 nm fluorescence are compared with the light-minus-dark spectra of P-700. 7. It is found that A-700 is reversibly bleached by I2 (E0 equals 0.54 V). The significance of this observation is discussed in terms of the redox properties of monomeric chlorophyll a and P-700.