An X-ray absorption spectroscopic investigation of the nature of the zinc complex accumulated in Datura innoxia plant tissue culture

X-Ray absorption spectroscopy has proven to be a useful technique to investigate the nature of heavy metal complexes accumulated in metal tolerant and hyperaccumulating plant species. Zn K-edge X-ray absorption spectroscopy has been used to determine the nature of the zinc center in Datura innoxia plant tissue culture exposed to 250 muM, 500 muM, and 1000 muM Zn2+ for 4 days. Both edge and EXAFS data suggest that the zinc complex accumulated in D. innoxia cells is distinctly different from the types of zinc complexes expected to be associated with the accumulation, transport and storage of Zn2+ in plants. In D. innoxia at the levels of Zn2+ investigated, the X-ray absorption edges of Zn in the cell samples are similar to each other and occur at a similar energy to that of a complex between Zn2+ and free histidine, however, the shape and intensity of the edge suggests that the zinc center in the cells is not identical to that of this model solution. EXAFS analysis suggests that the zinc center is 4 coordinate with mixed N/O ligation at an average distance of approximately 1.98 Angstrom for the cells exposed to 250 muM and 500 muM Zn2+. For cells exposed to 1000 muM Zn2+, a 5 coordinate N/O zinc center with an average first shell bond length of 2.02 Angstrom was indicated. These bond lengths are significantly shorter than the -2.06/2.07 Angstrom bond length expected for a zinc-histidine or zinc-organic acid complexes and the similar to2.35 Angstrom bond length expected for a zinc complex with cysteine (errors in bond length determined by EXAFS are 0.02 Angstrom). Some possible interpretations of these results are that under the conditions of study, a zinc metalloenzyme is over expressed in the cells, or that a novel zinc-binding protein is produced by the cells. (C) 2002 Elsevier Science B.V. All rights reserved.