Sulfur XANES studies of mercury(II) interactions with sulfur in biological compounds and microorganisms

Mercury L-edge XAFS provides information about the atoms in the first and second coordination shells, including internuclear distances and coordination numbers. Proof that S is present in the first atomic shell can be gained using Hg L-edge XAFS but limited information on which S oxidation states participate in Hg binding is obtained. Sulfur K-edge XANES presents us with the opportunity to detect S ligand interactions with and changes in S oxidation state caused by Hg(II). Our hypothesis is that additions of Hg(II) to biological compounds containing reduced S will alter the position and possibly the fine structure of the S absorption edge. Our results confirm our hypothesis. Furthermore, we found evidence that adding Hg(II) leads to sulfur oxidation in certain cases. Hg(II) is judged to act as either an S oxidation catalyst or oxidizing agent in these cases. Sulfur K-edge XANES represents a supplementary method to detect Hg(II) binding by S in biological samples.; Sulfur containing compounds in microorganisms are known to play a role in protecting microbes from oxidative damage. Amino acids, sulfamates and iron-sulfur proteins are among the S forms identified in microbial cells. Using S K-edge XANES we found that a fraction ranging from 7--35% of the S was oxidized by air to an oxidation state of 2. Addition of Hg(II) resulted in enhanced oxidation of the S in 50% of the samples. The appearance of highly oxidized (oxidation state 4--6) S was detected in the S XANES spectra.; We grew bacteria in the presence of Hg(II), and saw two different growth patterns, one where the lag time increased linearly with Hg(II) but the final cell density did not change and another where we saw an increase in lag time and a decrease in the final cell density. The only effect detected in the S XANES was the oxidation of S in some of the insoluble cell fractions. We also determine the consequences of exposing resting-stage cells to Hg(II) after withholding Hg(II) during growth. In some cases we see that Hg(II) addition affects the sulfur speciation in the organisms studied; determined by S XANES analysis.