Electron spin resonance observation of the photooxidation of hydrated chlorophyll a dimers by water. In vitro photochemical characterization of reaction centers in photosynthesis

Theoretical Study of Chlorophyll a Hydrates Formation in Aqueous Organic Solvents

“…The monohydrate/dihydrate equilibrium will be strongly shifted toward the formation of the dihydrate, so that, roughly speaking, monohydrates can be expected only for relative concentrations of water below the 1:2 stoichiometric ratio. These results agree with the experimental findings that, in nonrigorously dried nonpolar solvents, Chl a can easily form a five-coordinated complex, ,,, as well as with the fact that, in the presence of small amounts of water, there is formation of mono- and dihydrated Chl a species, as well as their corresponding dimers. ,, Not surprisingly, upon further addition of water to a solution inducing Chl a aggregation, the equilibrium mixture is greatly shifted in favor of the dihydrated polymeric Chl a form (Chl a −2H 2 O) n . ,,, Note that the structure of this aggregate has been the subject of extensive debate in the literature. Both a cylindrical micelle and a planar sheetlike structure have been proposed. ,,, Evidence for the presence of neutral structural water molecules forming a hydrogen-bonded network in an arrangement similar to that of crystals of the ethyl chlorophyllide has been reported. ,,, Other experimental studies have suggested the possibility that polymeric (Chl a −2H 2 O) aggregates are stabilized in hexameric units …”

“…These results agree with the experimental findings that, in nonrigorously dried nonpolar solvents, Chla can easily form a five-coordinated complex, 8,17,19,21 as well as with the fact that, in the presence of small amounts of water, there is formation of mono-and dihydrated Chla species, as well as their corresponding dimers. 21,23,24 Not surprisingly, upon further addition of water to a solution inducing Chla aggregation, the equilibrium mixture is greatly shifted in favor of the dihydrated polymeric Chla form (Chla-2H 2 O) n . 21,22,24,60 Note that the structure of this aggregate has been the subject of extensive debate in the literature.…”

“…21,23,24 Not surprisingly, upon further addition of water to a solution inducing Chla aggregation, the equilibrium mixture is greatly shifted in favor of the dihydrated polymeric Chla form (Chla-2H 2 O) n . 21,22,24,60 Note that the structure of this aggregate has been the subject of extensive debate in the literature. Both a cylindrical micelle and a planar sheetlike structure have been proposed.…”

“…Since these pioneering works, the role of water in Chl a aggregation has been the subject of many publications. Most of these works have regarded the aggregation mechanisms of Chl a (or other chlorophylls) in aqueous organic solvents as a function of water concentration. ,– The nature of the solvent has been shown to be decisive. A seminal study using IR spectroscopy was carried out by Ballschmiter and Katz in the 1960s.…”

Elementary reconstitution of the water splitting light reaction in photosynthesis. 1. Time-resolved fluorescence and electron spin resonance studies of chlorophyll a dihydrate photoreaction with water in nonpolar solutions

“…It appears that the alkali metals plus Mg and Fe fall on a line, whereas the group 3 metals fall on a much steeper line. The first group of atoms (34) The calculations are carried out by using data from Table III and ref 31 in eq 5-7, except for Cs and Tl, for which wave functions are not available. For these cases, valence-orbital exponents and quadrupole moments are obtained by extrapolating from the lower members of their respective groups in the periodic table. is spherically symmetric (the small quadrupole moment of Fe is unimportant for the energetics) and interact with water chiefly through the 3a[ (lone pair) orbital.…”