PH Dependence Of Photosynthetic Characteristics Of Plant Photosystem I Particles

The PSI particles were extracted from the fresh leaves of high plant spinach, and some photosynthetic characteristics of PSI particles and PSI particles recombinated with PG molecules were studied. The main results were concluded as follows: 1.The absorbance spectra of PSI particles were measured under different pH conditions. It was found that strong acid treatments (pH1-2) caused evident blue-shift of the absorbance peaks in the blue region, and weak red-shift in the red band. In contrast, severe basic incubations (pH13-14) led to clear blue-shift of peaks both in the blue and red regions.The results indicated the modification of the microenvironment surrounding Chl molecules both under strong acidification and basification conditions. In addition, strong acid and alkali treatments mainly caused the damage of those Chl a molecules with longer wavelength at about 683nm. Furthermore, severe basic treatments led to the disappearance of the absorption of 473nm Chl b moleules, and the appearance of the absorbance peak of 647nm Chl b molecules. pH3-12 incubations showed no evident change of the absorbance spectra. 2.The 77K fluorescence emission spectra of PSI particles were recorded. The results showed that strong acid and alkali treatments caused the decline of the low temperature fluorescence yields, the decrease of the ratio of F720-740/F680, and the blue-shift of the characteristic long-wavelength fluorescence peak, indicating the inhibition of energy transfer process from 440nm Chl a molecules to the long-wavelength Chl a molecules. In addition, a new fluorescence peak of 680nm appeared at pH14, demonstrating the dissociation of some LHCI-680 components from the PSI core complexes, which resulting in the inhibition of energy transfer from LHCI-680 to LHCI-730 and to P700 reaction center. When PG was added into PSI solutions, more evident blue-shift occured to the 731nm fluorescence peak of PSI particles. Furthermore, the new peak of 680nm appeared under either strong alkali or strong acid conditions, indicating that the addition of PG aggravated the damage of LHCI-680 caused by H+ and OH- ions, and more LHCI-680 components broke away from the PSI core complexes. 3.The relative oxygen-uptake rates of PSI were measured under different pH conditions, and the results demonstrated that pH9-treated PSI particles exhibited a maximal electron transfer activities, while the lower or higher pH caused the decline of PSI photochemical activities, indicating the dependence of PSI activities on the pH values. In contrast, the existence of PG led to some changes such as the optimal pH value was at pH10 rather than at pH9. At the same time, the increase of PSI activities was found under all kinds of pH condtions after the addition of PG. 4.The results of changes in absorbance at 830nm measurements showed the increase of the redox potentials of PSI reaction center P700 upon pH8-12 incubations, and the decline of the redox capacities of P700 under acid (pH1-6) and strong alkali (pH13-14) conditions, demonstrating that P700 reaction center was stable under mild alkali conditions, but was sensitive to acid treatments, and small amount of H+ ions affected clearly the redox capacity of P700. After the addition of PG, the absorbance at 830nm of PSI incresed under all kinds of pH condtions, indicating the improvement of P700 redox capacity caused by the existence of PG. 5.The results of the far-ultraviolet (200-250nm) CD spectra measurements demonstrated that PSI particles recombinated with PG had stronger negative signal of about 226nm under a certain pH range (i.e.pH4-11) than PSI particles without PG, indicating that PG could improve the content of α-helix, and make the PSI particles stay in a more folding state. 6.The outcomes of SDS-PAGE analysis showed that non-severe acid and alkali (pH3-12) treatments caused no evident damage of PSI protein subunits.