Characterization of P700 As a Photochemical Quencher in Isolated Photosystem I Particles Using Simultaneous Measurements of Absorbance Changes at 830 Nm and Photoacoustic Signal

The relationship between the redox state of P700, the primary donor of PS I, monitored using absorbance changes at 830 nm and photochemical energy storage in PS I reaction centers assayed with the photoacoustic method (PA) was studied in isolated PS I submembrane particles aspirated onto nitrocellulose filters. Several donors have been used to support the electron transport through PS I. NADPH and NADH demonstrated low rates of electron donation to PS I, 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 PS I particles between the (A(830,max) - A(830,steady))/A(830,max) and (PA(max) - PA(steady))/PA(max) 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 obtained independently from the nature of electron donor used for the reduction of P700(+). Such linear relationship was also obtained at various wavelengths of modulated light in the range of 660 to 720 nm, only the slope of the linear fits varied with wavelength. It is concluded that reduced P700 act as a photochemical quencher of absorbed energy. Variable thermal dissipation in PS I reaction centers of isolated submembrane particles linearly depends on the amount of reduced P700 and thus constitutes an appropriate indicator of the redox pressure applied to PS I.