EDS and µ-XRF mapping of amalgam degradation products in ancient mirrors.

An amalgam mirror is a mirror type, used from the fifteenth century until the end of the nineteenth century, where the reflective layer is constituted by a tin amalgam layer adhered to a glass sheet. In this work, two amalgam mirrors samples were studied by scanning electron microscopy with an energy dispersive spectrometer and by micro-X-ray fluorescence elemental mapping to go deeply into the understanding of the degradation mechanism of the amalgam layer of ancient mirrors. The investigation has been focused for the first time on the reflective surface of the amalgam layer adherent to the glass sheet to better understand the processes of amalgam corrosion. The two amalgam degradation compounds, romarchite and cassiterite, has been spatially differentiated by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) maps. SEM images and micro-X-ray fluorescence and EDS maps showed that the amalgam degradation products grow up to form hemispherical stratified calottes. This structure is probably due to a mechanism involves cyclic phases and oxygen radial diffusion from a superficial oxidation nucleus.

Comparative kinetic study between native and chemically modified Cu,Zn superoxide dismutases.

The kinetic behaviour of native bovine erythrocyte Cu,Zn superoxide dismutase (N-SOD) and of its derivatives by reaction with polyethylene glycol, acetic and succinic anhydrides has been investigated here in detail. Their responses to changes of pH and ionic strength (I) have been used as a probe for quantitatively displaying the relevance to kinetic rate constant of superficial positive charges driving the superoxide ion (O2-) toward the enzyme's active site. Overall kinetic trends indicate that this long-range O2- electrostatic guidance is essentially due to the positive charges of the amino-acid residues Lys-120 and Lys-134 which are strategically located around the active site. The comparison between the kinetic data obtained from N-SOD and those from polyethylene-glycolated SOD (PEG-SOD) enabled us to state that in PEG-SOD an O2(-)-steering positive electrostatic force, halved in comparison with N-SOD, is still operating, and that only Lys-120 is linked in the reaction of N-SOD with PEG. Elimination of the electrostatic driving force, carried out either by deprotonation of lysine amino groups at high pH, or by their neutralization with succinic anhydride and acetic anhydride, or by ionic screening at high ionic strength, always lowered the kinetic rate constant to a value of approx. 3 x 10(8) M-1.s-1. This value is about 15 times smaller than that measured in the presence of the reactant-steering mechanism and represents the k value of the reaction limited by pure diffusion. Finally, the kinetic behaviour of acetylated SOD and succinylated SOD demonstrated the inhibitor effect of OH- at strongly alkaline pH.

The active site of manganese-containing superoxide dismutase from Bacillus stearothermophilus studied by 1H and 19F magnetic relaxation.

The dependence of the magnetic relaxation rates of 1H and 19F- on temperature, frequency, pH and N-3 concentration, were measured in solutions of Manganese-containing superoxide dismutase of Bacillus stearothermophilus, and were compared to activity measurements, in order to obtain some information on the structure and dynamics at Mn(III) present in the active site of the enzyme. The experimental data lead us to hypothesize the presence of two binding sites in the coordination sphere of the enzyme bound Mn(III), which are accessible to water and anions and have different chemical and spectroscopic properties. NMR measurements carried out in the presence of competitive inhibitors and the pH dependence of both NMR relaxation rates suggest that F-, N-3 and OH- ions bind to one site, while a water molecule binds to the other one. The stability constant values of the complexes between these anions and the enzyme are reported. The influence of the anions on activity and the pH dependence of NMR parameters are discussed.

The 1H-NMR relaxation of Rhus laccase: assignment to different types of copper.

1H-NMR relaxation measurements of Rhus laccase showed that a portion of the relaxivity was specifically abolished by less than stoichiometric EDTA. Another portion of relaxivity was removed by addition of N3(-) to the EDTA saturated enzyme. This treatment or selective removal of the Type 2 Cu left a large residual paramagnetic relaxivity (1700 M-1s-1) which was assigned to the Type 1 Cu. It is concluded that only a portion of the laccase relaxivity can be assigned to the Type 1 Cu and that this copper type does not behave homogeneously: the two fractions have different relaxivity, 5200 and less than or equal to 2400 M-1s-1 respectively.