Degradation and inactivation of photosystem I complexes during linear heating

The heat effect on photosytem 1 (PSI) was characterized by visible and fluorescence spectroscopy, oxygen electrode polarography, sodium dodecyl sulfate-polyacryamide gel electrophoresis (SDS-PAGE) and Fourier transform infrared (FTIR). During linear heating, the absorption maximum at 680 run of PSI decreased and blue shifted, of which pigments and polypeptides in PSI complexes degraded at high temperature. The chlorophyll a (Chla) molecules with maximum absorption at 683 run in the fourth derivative absorption spectra, which are mainly attributed to light-harvesting complex of PSI (LHCI), were more sensitive to high temperature. So it was suggested that Chla (absorbing at 683 nm) was destroyed firstly by elevated temperature. The characteristic peak at 728 run and the ratio of F728-F720 to F680 in 77 K fluorescence spectra decreased apparently when temperature increased, showing that high temperature blocked energy transfer in LHCI and the energy transfer from LHCI 680 to LHCI 730 and PSI reaction center was inhibited. SDS-PAGE displayed that PsaA/B subunits and LHCI subunits were degraded and aggregated at different levels upon linear heat treatment. However, at high temperature especially at 90-100degreesC, the PsaA/B subunits disappeared completely while LHCI Subunits remained, indicating that the core complex Subunits were more sensitive to heat treatment than LHCI Subunits. The probable reason for the above results is that during linear heating the disconnection of the antenna front the reaction center occurred and then followed by the inhibition of the photochemical charge separation in reaction center. The oxygen uptake rate decreased with the increase of temperature, and it was almost completely lost at around 70degreesC, demonstrating that the PSI complexes were destroyed seriously when the temperature reached 70degreesC. FTIR analysis indicated that a major conformational transition of PSI complexes occurred above 60degreesC, which was accompanied by unfolding of alpha-helical to the beta-sheet structures. (C) 2004 Elsevier Ireland Ltd. All rights reserved.