Modeling of photosynthetic water-splitting by chlorophyll a dihydrate

In this work the photocatalytic properties of chlorophyll a dihydrate in polar aqueous solution have been studied. It has been verified that (Chl a $\cdot$ 2H$\sb2$O)$\sb2$ is the most probable species, rather than (Chl a $\cdot$ 2H$\sb2$O)$\sb{\rm n\u3e2}$ oligomer, as the P680 model based on data obtained from optical absorbance, fluorescence, ESR, FTIR and photoelectrochemical measurements. The discrepancies between (Chl a $\cdot$ 2H$\sb2$O)$\sb2$ dimer and (Chl a $\cdot$ 2H$\sb2$O)$\sb{\rm n}$ oligomer can be explained by the fact that the dihydrate oligomer possesses p-type semiconduction properties, but the dimer does not. We have identified a 695 nm absorbing Gaussian component, attributable to (Chl a $\cdot$ 2H$\sb2$O)$\sb2$. Stabilization of (Chl a $\cdot$ 2H$\sb2$O)$\sb2$ as the predominant species in water/acetone mixtures has been achieved. The action spectrum, obtained upon monochromatic light excitation of Pt/Chl a, containing (Chl a $\cdot$ 2H$\sb2$O)$\sb2$ predominantly, exhibits a maximum photoresponse at about 680 nm. This is in accordance with those results, shown in the in vivo absorption difference spectra of oxidized Chl a$\sb{\rm II}\sp{+\cdot}$, located at about 690 nm and 682 nm for whole plant cell and PS II subchloroplast, respectively. A photoanodic current has been observed on light excitation of Pt/Chl a electrode, containing predominantly the (Chl a $\cdot$ 2H$\sb2$O)$\sb2$. This is also in agreement with in vivo photogalvanic experimental results obtained from illumination of PS II subchloroplast deposited on platinized platinum electrodes. A reversible light-induced ESR signal (ESR line width $\simeq$ 1G, g = 2.0025), having an absorbance maximum at 740-760 nm, has been observed in 1: 1 water/acetone only when O$\sb2$ is present, which indicates O$\sb2$ is the primary electron acceptor. The intermediate, superoxide ion O$\sb2\sp{-\cdot}$ (or HO$\sb2$), has been detected by using DMPO as a spin trap. These side reactions reduce the photosynthetic efficiency. Quantitative ESR studies have revealed the side reaction mechanism between (Chl a $\cdot$ 2H$\sb2$O)$\sb{\rm n\u3e2}$ and O$\sb2$. The dark decay of the narrow ESR signal, attributed to (Chl a $\cdot$ 2H$\sb2$O)$\sb{\rm n\simeq100}\sp{+\cdot}$, exhibited second order kinetics with a half-life of the order of 13.6 seconds. An exponential flux dependence for the steady-state ESR signal has been observed. The least-squares fit to this curve indicated an alternative photoexcitation mechanism of (Chl a $\cdot$ 2H$\sb2$O)$\sb{\rm n}$ and electron-transfer pathway in the presence of oxygen