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The toxicity of heavy metals (Hg2+, Zn2+) and pesticides has been investigated by comparing the physiological properties in wild and tolerant strains of Synechococcus cedrorum 1191. The differential pattern of growth, absorption spectra of pigments and nutrient uptake was observed in tolerant strain.
Light-dependent absorption change at 325 nm, ascribed to QA activity, was strongly reduced in the presence of Cu(II) in oxygen-evolving core complex. This change was much less affected in the presence of the herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), indicating that the Cu(II)-binding site is different from that of the DCMU and that Cu(II) blocks QA reduction. Cu(II) did not eliminate the absorption change at 545 nm, ascribed to pheophytin reduction, in Na2S2O4-treated oxygen-evolving core and D1-D2-cytochrome b559 complexes. This indicates that Cu(II) does not affect the electron transport between P680 and pheophytin. Moreover, the activity of the bacterial reaction center probed by the absorption change at 790 nm was inhibited by Cu(II), but the signal at 530 nm, associated to the reduction of bacteriopheophytin in Na2S2O4-treated reaction center, was not inhibited. We conclude that Cu(II) impaired the photosynthetic electron transport between pheophytin and QA in both higher plants and photosynthetic bacteria. Cu(II) would bind to an amino acid(s) highly conserved in non-oxygenic and oxygenic reaction centers, which is(are) necessary for the electron transfer between pheophytin and QA. Based on the atomic structure of the bacterial reaction center several schemes of possible Cu(II) binding are shown.
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The preferred binding orientations for the herbicide DCMU within the QB-binding of the D1 protein model from a photosystem II reaction centre have been determined. Calculation of the intermolecular energy between the herbicide and the binding site has been instrumental in obtaining optimum positions reinforced by experimental results from mutation studies and herbicide binding to analogous bacterial reaction centres. We have shown that two binding sites are possible, one involving a hydrogen bond to the His 215 residue of the QB-binding site and the other to the Ser 264 residue. In both cases, it appears to be the van der Waals forces which are more important for the stabilization of the interactions.
The study on the co-ordination chemistry of the complexes with thiolic heterocyclic ligands is based on the ion metals binding precisely to the sulphur atom. Studies were consequently carried out on the complexes with some thiolic ligands, such as: 2-imidazolidine-thione (IMT), hydantoin (ID), 2-thiohydantoin (TIOID), rhodanine (RD), 2-mercaptoimidazole (MI), 2-mercapto-1-methylimidazole (MMI) and 2-mercaptopyridine (MPYR), supposing that the co-ordination bond between sulphur and metal is stronger than the possible bond between nitrogen or oxygen and metal due to the minor difference in electronegativity existing between sulphur and metal compared with that existing between nitrogen or oxygen and metal. Complexes of the kind we prepared generally present relatively low constants of instability, marking a strong bond between the metal and the ligand. This would seem to be in direct correlation with the anti-bacterial and herbicidal activity. The activity of the complex in fact proves to be more marked than that of the pure ligand, and the increase appears to be in proportion to the strength of the bond between the metal and the ligand, as seen by the low values of the relative constants of instability. We may imagine that the decrease in the constants of instability corresponds to an increase in the strength of the co-ordination bond and to a parallel increase in the activity and the hypothesis is in accordance with the premised correlation between the strength of the metal-ligand bond and the biological activity.
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Ultrafast time-gated fluorescence imaging of optically trapped single motile cells is presented. The biflagellar green microalga Haematococcus pluvialis was confined with picoNewton trapping forces in the focal volume of a high numerical aperture objective by near infrared multitraps. Trapping radiation of 100 mW power at the sample was provided by a Nd:YLF laser (1047 nm) operating in the cw mode. Simultaneously, cellular autofluorescence was excited with a 633 nm picosecond 80 MHz laser diode. An ultrafast gated intensified slow scan CCD camera system with a tunable gate width (200 ps-1 ms) and tunable time-delay (0-20 ns) between excitation and detection was used as fluorescence detector. We demonstrate fluorescence imaging of high temporal (sub-ns) and high spatial (sub-microm) resolution and fluorescence lifetime determination of intracellular autofluorescence based on chlorophyll excitation. Exposure to the herbicide DCMU resulted in an increase of fluorescence intensity and lifetime by 250% and 150%, respectively.