As synchrotron light sources and optics deliver greater photon flux on samples, X-ray-induced photo-chemistry is increasingly encountered in X-ray absorption spectroscopy (XAS) experiments. The resulting problems are particularly pronounced for biological XAS experiments. This is because biological samples are very often quite dilute and therefore require signal averaging to achieve adequate signal-to-noise ratios, with correspondingly greater exposures to the X-ray beam. This paper reviews the origins of photo-reduction and photooxidation, the impact that they can have on active site structure, and the methods that can be used to provide relief from X-ray-induced photo-chemical artifacts.

Copper(II) coordination by bis(cyclohexanone)oxalyldihydrazone (also known as cuprizone), resulting in the formation of an intensely coloured blue complex, was first reported over 70 years ago. The cuprizone reaction has been...

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Cycloalkylphenyl derivatives Cycloalkylphenyl derivatives Q 0760Cytotoxic Analogues of 2,6-Bis (arylidene)cyclohexanones. -Various cytotoxic molecules, e.g. (IIIa), (IIIb) and (IIId), serve as prototypes for systematic molecular modification. -(DIMMOCK*, J. R.; PADMANILAYAM, M. P.; ZELLO, G. A.; NIENABER, K. H.; ALLEN, T. M.; SANTOS, C. L.; DE CLERCQ, E.; BALZARINI, J.; MANAVATHU, E. K.; STABLES, J. P.; Eur. J. Med. Chem. 38 (2003) 2, 169-177; Coll. Pharm. Nutr., Univ. Saskatchewan, Saskatoon, Sask. S7N 5C9, Can.; Eng.) -M. Paetzel 24-083

The metal coordinating properties of the prion protein (PrP) have been the subject of intense focus and debate since the first reports of copper interaction with PrP just before the turn of the century. The picture of metal coordination to PrP has been improved and refined over the past decade, and yet the structural details of the various metal coordination modes have not been fully elucidated in some cases. Herein we employ X-ray absorption near edge spectroscopy as well as extended X-ray absorption fine structure (EXAFS) spectroscopy to structurally characterize the dominant 1:1 coordination modes for CuII, CuI and ZnII with an N-terminal fragment of PrP. The PrP fragment constitutes four tandem repeats representative of the mammalian octarepeat domain, designated OR4, which is also the most studied PrP fragment for metal interactions, making our findings applicable to a large body of previous work. Density functional theory (DFT) calculations provide additional structural and thermodynamic data, and candidate structures are used to inform EXAFS data analysis. The optimized geometries from DFT calculations are used to identify potential coordination complexes for multi-histidine coordination of CuII, CuI and ZnII in an aqueous medium, modeled using 4-methylimidazole to represent the histidine side chain. Through a combination of in silico coordination chemistry as well as rigorous EXAFS curve fitting, using full multiple scattering on candidate structures from DFT calculations, we have characterized the predominant coordination modes for the 1:1 complexes of CuII, CuI and ZnII with the OR4 peptide at pH 7.4 at atomic resolution, which are best represented as a square planar [CuII(His)4]2+, digonal [CuI(His)2]+ and tetrahedral [ZnII(His)3(OH2)]2+, respectively.