Relationship between the redox state of P700 and variable thermal dissipation in isolated photosystem I particles

The relationship between the redox state of P700 monitored using absorbance changes at 830 nm and photochemical energy storage in PSI reaction centers assayed with the photoacoustic method (PA) was studied in isolated PSI submembrane particles aspirated onto nitrocellulose filters. Several donors have been used to support the electron transport through PSI. NADPH and NADH demonstrated low rates of electron donation to PSI, while ascorbate and ascorbate plus 2,6-dichlorophenolindophenol (DCIP) couple have been found more effective in both P700+ reduction and stimulation of the variable component of the PA signal. A linear relationship was found in isolated PSI particles between the (A830, max - A830, steady)/A830, max and (PAmax - PAsteady)/PAmax ratios, which characterized the relative amount of P700 in the reduced state and the relative magnitude of the variable PA component, respectively. That linear relationship was independent on the nature of electron donor used for the reduction of P700+. Such linear relationship were obtained at various wavelength of modulated light used for the measurements of the PA signal in the range of 660 to 720 nm. Only the slope of the linear fits varied with wavelength. It is concluded that in isolated submembrane particles the photochemical energy storage in PSI reaction centers linearly depends on the amount of reduced P700.