No abstract

To contribute to the comprehension of the metal ion sorption properties in mixed mineral-organic matter systems, interaction studies between Eu(III) and polymaleic acid (PMA)-coated alumina colloids were carried out at pH 5 in 0.1 M NaClO4. The studied parameters were the metal ion concentration (1 x 10(-10) to 1 x 10(-4) M) and PMA loading on alumina (gamma = 10-70 mg/g). The data were described by a surface complexation model. The constant capacitance model was used to account for electrostatic interactions. The results showed thattwo sites were necessary to explain the sorption data. At high Eu loading, Eu is surrounded by one carboxylate group and one aluminol group. The existence of this ternary surface site was in agreement with time-resolved laser-induced fluorescence spectroscopy measurements. At low metal ion concentrations, a surface site corresponding to a 1:1 Eu/COO-stoichiometry was deduced from modeling. Spectroscopic data did not corroborate the existence of this latter site. This discrepancy was explained by postadsorption kinetic phenomenon: a migration of the metal ion within the adsorbed organic layer.

Quantitative 2H NMR spectroscopy has been used to determine the natural abundance site-specific 2H isotopic content of 6,7-dihydrocapsaicin (1) and capsaicin (2). Prior to analysis, the fatty acyl moieties were released as methyl 8-methylnonanoate (3) and methyl E-8-methylnon-6-enoate (4), respectively. A marked and similar nonstatistical isotopic distribution of (2)H is observed for both fatty acids. Notably, it can be seen that: 1) the isobutyl portion of 3 is more impoverished in 2H than the methylenic portion; 2) the isobutyl portion of 4 is more impoverished than that of 3; 3) an alternating pattern occurs in the (2H/1H)i between the C3 to C7 positions; and 4) the ethylenic hydrogens at C6 and C7 of 4 are, respectively, impoverished and unchanged relative to these positions in 3. These observations are compatible with the proposed biosynthetic origins of the different parts of 1 and 2, and with the view that 1 is a proximal precursor of 2. Furthermore, it can be suggested that, firstly, the hydrogen atoms at C3 to C7 originate alternatively from the substrate and from the environment and, secondly, that the Delta6-E desaturation is introduced by a mechanism closely mimicking that of the Z desaturation of higher plants.