Presence Of Cr3 Research Articles

Investigations of the metal ion-binding sites of yeast inorganic pyrophosphatase.

Yeast inorganic pyrophosphatase was found to bind two Mn2+ per subunit in the absence of phosphate and three Mn2+ per subunit in the presence of phosphate. Kinetic studies of the pyrophosphatase-catalyzed hydrolysis of Cr(NH3)4PP and Cr(H2O)4PP were carried out with Mn2+ and with Mg2+ as activators. The results from these studies suggest that three divalent cations per pyrophosphatase active site are required for catalysis. NMR and EPR studies were conducted to evaluate the relative location of the metal ion binding sites on the enzyme. The two Mn2+ ions bound to the free enzyme are in close enough proximity to magnetically interact. Analysis of the NMR and EPR data in terms of a dipolar relaxation mechanism between Mn2+ ions provides an estimate of the distance between them of 10-14 A. When the diamagnetic substrate analog [Co(NH3)4PNP]- or intermediate analog [Co(NH3)4 (P)2]- are bound to pyrophosphatase, two Mn2+ ions still bind to the enzyme and their magnetic interaction increases. In the presence of these Co3+ complexes, the Mn2+--Mn2+ separation decreases to 7-9 A. Several NMR and EPR experiments were conducted at low Mn2+ to pyrophosphatase ratios (approximately 0.3), where only one Mn2+ ion binds per subunit, in the presence of Cr3+ or Co3+ complexes of PNP or PP. Analysis of the Mn2+--Cr3+ dipolar relaxation evident in proton NMR and EPR data provided for the calculation of Mn2+--Cr3+ distances. When the substrate analog CrPNP was present, the Mn2+--Cr3+ distance was congruent to 7 A whereas, when Cr(P)2 was bound to pyrophosphatase, the Mn2+--Cr3+ distance was congruent to 5 A. These results strongly support a model for the catalytic site of pyrophosphatase that involves three metal ion cofactors.

Hydrogenation of ethylene over chromia–alumina catalysts. Electron spin resonance and kinetic studies

The hydrogenation of ethylene over chromia–alumina catalysts containing known amounts of Cr2+ and Cr3+ has been investigated at temperatures between – 78 and 200 °C. The activity of the catalyst depends strongly on the presence of Cr3+(determined by e.s.r.), in as much as increasing Cr3+ concentration is accompanied by a considerable increase in the rate of hydrogenation. Preliminary kinetic studies show that the reaction rate is zeroth order in hydrogen at –78 °C and first order at higher temperatures (> 0 °C). The e.s.r. signal ascribed to Cr3+(g= 1.986) undergoes some changes under the reaction conditions when exposed to a 1 : 1 mixture of hydrogen and ethylene at 100 °C.

A Mössbauer study of the ferrimagnetic Fe1−xCrxBO3 system

Mössbauer effect measurements were made on solid solution Fe1−xCrxBO3, with 0⩽x⩽0.60, at room and at liquid nitrogen temperatures and on Fe0.50Cr0.50BO3 at temperatures from 82.5 to 150 K. The hyperfine field strength at Fe3+ sites decreases with increased Cr content. The isomer shift increases appreciably in samples containing Cr at low concentrations. The Mössbauer linewidth increases with increasing Cr content and exhibits a shoulder at 0.40⩽x⩽0.60. The data suggest that the presence of Cr3+ ions disrupts the magnetic superexchange interaction. Variations in the isomer shift, magnetic hyperfine splitting and linewidth as a function of increasing temperature in samples of Fe0.50Cr0.50BO3 suggest that a significant antiferromagnetic component is present in samples with Cr concentrations 0.40⩽x⩽0.60.