SULFUR SPECIATION IN PLANT LEAVES/LICHENS, AND STUDIES OF METAL-CYS COMPLEXES US

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Sulfur is an essential element for plants, being converted into cysteine and then to other sulfur metabolites. Current research on sulfur metabolism in plants is severely restricted by the present "wet chemistry" methods for identifying sulfur species, which require digestion/heating/destruction of the sample and only provide rather inaccurate "total organic" vs. "total inorganic" amounts of sulfur. By means of sulfur K-edge X-ray absorption spectroscopy (XAS) we will perform detailed speciation analyses of the major sulfur species present in intact plant leaves to clarify the key steps in the sulfur metabolism. We will also study samples from areas in Alberta, Canada, exposed to well-documented extensive sour gas emissions, to investigate the effects on the sulfur metabolism. Lichens, which only take up sulfur from the air, are great natural monitors for sour gas effects. Cysteine and its derivatives are medically used for heavy metal detoxification. Cysteine, one of 21 amino acids found in living organisms, contains three coordinating sites: the amino, thiol and carboxylate groups. The thiol group can form strong bonds and accumulate ions of soft, heavy metals. Coordination chemical studies have so far been focused on metal complexes in the solid state, mostly by elemental analysis, IR and in a few cases, X-ray crystallography. Little is known about the structure and coordination in solution, near to physiological conditions. We will use metal and sulfur XAS to study the local structure around the metal center, i.e. the metal-ligand bond distances, number and type of coordinating atoms, as well as the electronic environment around the sulfur atom in complexes of heavy metals, e.g. Pb, Cd, Hg and Ag, with cysteine and its derivatives.