A LA-139 NMR-STUDY OF THE INTERACTIONS BETWEEN THE LA(III) CATION AND D-RIBOSE IN AQUEOUS-SOLUTION

The interactions of the La(III) cation with D-ribose and with D-arabinose in aqueous solution were investigated by (139)LaNMR spectroscopy. In the case of D-ribose, the formation of a La(LII)-sugar complex was indicated by variations of the La-139 chemical shift and linewidth with an increase of the sugar concentration in solution. In contrast, the complexation of La(III) by arabinose is very weak and almost undetectable by La-139 NMR. On the basis of a 1:1 stoichiometry, the stability constant for the complex of La(m) with D-ribose was calculated from the observed La-139 chemical shift values. A similar treatment was done for the viscosity corrected La-139 linewidths using arabinose as an uninteractive reference. The stability constants, K, obtained independently from La-139 chemical shifts and linewidths are in good agreement, 2.8 +/- 0.5 and 2.2 +/- 0.6 M(-1) respectively at 299.0 +/- 0.5 K. The thermodynamic parameters for the complexation of La(m) by D-ribose could also be obtained: Delta H-0 = -12 +/- 2 kJ mol(-1)and Delta S-0 = -31 +/- 5 J K-1 mol(-1) mol(-1). These values are in very good agreement with those obtained by calorimetric measurements.