Molecular rearrangements of thylakoids after heavy metal poisoning, as seen by Fourier transform infrared (FTIR) and electron spin resonance (ESR) spectroscopy

The specific effects exerted by different heavy metals on both the function and the structure of the photosynthetic apparatus were addressed. The func tional analysis performed via the fluorescence induction kinetics revealed that the applied toxic heavy metals can be classified into two groups: Cd a nd Ni had no significant effect on the photosynthetic electron transport, w hile Cu, Pb and Zn strongly inhibited the Photosystem II (PS II) activity, as evidenced by the dramatic decreases in both the variable (F-v) and the m aximal (F-m) fluorescence. The structural effects of the heavy metal ions o n the thylakoid membranes were considered in three relations: (1) lipids, ( 2) proteins - studied by Fourier transform infrared (FTIR) spectroscopy, an d (3) lipid-protein interactions - investigated by electron spin resonance (ESR) spectroscopy using spin-labeled probe molecules. The studied heavy me tal ions had only a non-specific rigidifying effect on the thylakoid lipids . As regards proteins, Cd and Ni had no effect on the course of their heat denaturation. The heat denaturation of the proteins was accompanied by a de crease in the alpha-helix content (1656 cm(-1)), a parallel increase in the disordered segments (1651 cm(-1)), a decrease in the intramolecular beta-s heet (1636 cm(-1)) content and the concomitant appearance of an intermolecu lar beta-structure (1621 cm(-1)). In contrast with Cd and Ni, Cu and Zn blo cked the appearance of the intermolecular beta-structure. Pb represented an intermediate case. It seems that these heavy metals alter the native membr ane structure in such a way that heat-induced aggregation becomes more limi ted. The ESR data revealed that certain heavy metals also affect the lipid- protein interactions. While Cd and Ni had hardly any effect on the solvatio n fraction of thylakoid lipids, Cu, Pb and Zn increased the fraction of lip ids solvating the proteins. On the basis of the FTIR and ESR data, it seems that Cu, Pb, and Zn increase the surfaces available for lipid-protein inte ractions by dissociating membrane protein complexes, and that these 'lipida ted' proteins have a smaller chance to aggregate upon heat denaturation. Th e data presented here indicate that the damaging effects of poisonous heavy metals are element-specific, Cu, Pb and Zn interact directly with the thyl akoid membranes of the photosynthetic apparatus, while Cd and Ni interfere rather with other metabolic processes of plants.