No abstract

Aqueous solutions of L-histidine have been analysed in parallel by infrared (IR) and Raman spectroscopy over the pH range 0-14 with increments of one pH unit. The vibrational spectra in the region 2000-500 cm À1 have been interpreted and band positions have been assigned tentatively, taking into account assignments from literature after critical evaluation. As a result, a complete and complementary set of vibrational data has been obtained that can be used to determine all possible states of protonation of histidine, i.e. H 4 His 2+ , H 3 His + , H 2 His 0 , HHis À and His 2À . In addition, IR and Raman bands have been proposed as markers for the presence of the imidazole N p -or N t -protonated tautomeric forms of H 2 His 0 and HHis À .

Aqueous solutions of Cu 2+ /histidine (his) (1:2) have been analyzed in parallel with infrared, Raman, ultraviolet/ visible/near-infrared, electron spin resonance, and X-ray absorption spectroscopy in the pH range from 0 to 10. Comprehensive interpretation of the data has been used to extract complementary structural information in order to determine the relative abundance of the different complexes. O c ,N am ,N im )] 2 is the major species with the N atoms in the equatorial plane and the O atoms in the axial position. This complex decomposes at pH > 10 into a copper oxide/hydroxide precipitate. The overall results provide a consistent picture of the mechanism that drives the coordination and complex formation of the Cu 2+ /his system.

UV-vis spectroscopy was used in a combined in-situ UV-vis/XAFS spectroscopic setup to study the synchrotron radiation effect on aqueous homogeneous copper solutions. Two different systems were studied. In the first study, the focus was on a copper bipyridine-catalyzed oxidation of benzyl alcohol to benzaldehyde with 2,2,6,6-tetramethylpiperidinyl-1-oxy and base as cocatalysts. It was found that when the reaction mixture is exposed to the X-ray beam, the features present in the in-situ UV-vis spectrum develop differently compared to the situation when the reaction mixture is not exposed to the X-ray beam. Besides a temperature effect of the X-ray beam, both the UV-vis analysis and the XAFS analysis showed a reducing influence of the X-ray beam on the sample. To investigate this in more detail, we studied a series of dilute aqueous copper solutions from different precursor salts, viz., Cu(NO 3 ) 2 ‚3H 2 O, CuSO 4 ‚5H 2 O, CuCl 2 , and CuBr 2 . It was found that the different aqueous copper solutions have different stabilities under the influence of the X-ray beam. Especially the solution from the CuCl 2 precursor salt was found to be unstable and to be subjected to reduction. These examples illustrate the need for a second technique, such as in-situ UV-vis spectroscopy, to evaluate the effect of synchrotron radiation used to measure in-situ XAFS on catalytic systems.

Time-resolved in situ video monitoring and ultraviolet-visible spectroscopy in combination with X-ray absorption near-edge spectroscopy (XANES) have been used for the first time in a combined manner to study the effect of synchrotron radiation on a series of homogeneous aqueous copper solutions in a microreactor. This series included both non biologically relevant (pyridine, bipyridine, neocuproine, terpyridine, dimethylpyridine, ammonia, ethylenediamine, and 1,10-phenanthroline) and biologically relevant (histidine, glycine, and imidazole) ligands. It was found that when water is present as solvent, gas bubbles are formed under the influence of the X-ray beam. At the liquid-gas interface of these bubbles, in particular cases colloidal copper nanoparticles are formed. This reduction process was found to be influenced by the type of copper precursor salt (SO 4 2-, NO 3 -, and Cl -), the ligands surrounding the copper cation, and the redox potential of the copper complexes (ranging between +594 and -360 mV). In other words, in some cases, no reduction was encountered (e.g., ammonia in the presence of SO 4 2-and NO 3 -), whereas in other cases reduction to either Cu + (neocuproine with SO 4 2-) or Cu 0 (e.g., histidine and imidazole both with SO 4 2-, NO 3 -, and Cl -) was observed. These results illustrate the added value of video spectroscopy for the interpretation of in situ XANES studies. Not only do the results give an illustration of the parameters that are important in the redox processes that occur in biological systems, they also show the potential problems associated with studying catalytic processes in aqueous solutions by XANES spectroscopy.