X-ray absorption spectroscopic studies on model compounds for biological iodine and bromine

Abstract

We have measured the X-ray absorption spectra of a number of organic iodine and bromine compounds of biological relevance, as well as of a series of iodine compounds with different oxidation states. We find that the iodine K edge spectra (XANES) are relatively featureless but that the position of the edge is sensitive to formal valence (among other factors), and the edge shape to the number of bound oxygens. EXAFS spectra of organohalogen compounds (both iodine and bromine) can be used to discriminate between aliphatic and aromatic compounds. There are differences both in the distances from the halogens to the first shell of carbons, which are shorter for aromatic compounds, as well as in the patterns of shells in the Fourier transforms. This result is expected to be relevant to studies at these edges in biological systems.A b b r e v ia tio n s : EXAFS = extended X-ray absorption fine structure; XANES = X-ray absorption near-edge structure; XAS = X-ray absorption spectroscopy 1

Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry

“…We conclude that iodine is accumulated in Laminaria in its reduced form, iodide. Moreover, the strongest FT peak is at the same distance, Ϸ3.5 Å, as for hydrated iodide in NaI solution (22), where this peak represents oxygen atoms of the solvent, water. The reduced amplitude in the kelp FT compared with NaI solution is reminiscent of that found upon changing the solvation from small molecules (water) to larger ones (t-butanol, dimethyl formamide) (23).…”

“…Iodine K-edge XAS measurements (22) were carried out at the European Molecular Biology Laboratory, Hamburg Unit, Outstation Hamburg at Deutsches Elektronen Synchrotron, Germany, using a Si(311) order sorting monochromator, which was set at 50% of peak intensity to suppress harmonics. During data collection, the storage ring DORIS III was operated at 4.5 GeV with ring currents between 150 and 90 mA.…”

“…22. Simulations of the calibrated, averaged, and backgroundsubtracted EXAFS were carried out with the Windows XP version of the EXAFS simulation program EXCURVE (40), Release 9.272, using the Rehr and Albers expansion for the calculation of phase shifts (41).…”

Anion binding in biological systems

“…In the cases shown in Figure 1, the amorphous BN in which the compounds were diluted for the measurements does not appear to contribute to the EXAFS. It is possible to discriminate between halogens bound to sp 2 -and sp 3 -hybridized carbons using EXAFS, due to shortening of the carbon-halogen bond with increasing s-character of the bonding orbital [3].…”

“…Transition metal cations interact in a dynamic way with a number of electron-donating ligands in order to achieve a favourable coordination number (or secondary valence); halogens are subject to solvation and H-bonding in their anionic form, and can form single covalent bonds (primary valence) in systems where they can still be H-bond acceptors or themselves be part of a halogen bond. For the Cu spectrum, 2 oxygens of the weakly coordinating nitrate anion were also included in the simulation at 2.5 Å [2]; for the Br spectrum, the R group was not included, whereas for the I spectrum none of the other ring substituents was included [3]. Figure 1 shows that the complex of a transition metal ion like Cu with an aromatic ligand gives the same type of EXAFS spectrum as that of I and Br covalently bound to aromatic groups such as in the amino acids phenylalanine and tyrosine.…”