Equilibrium studies of galactomannan of Cassia fastuosa and Leucaena leucocephala and Cu2+ using potentiometry and EPR spectroscopy

The binding constants of Cu2+ and galactomannans composed of sugar units of mannose and galactose from the seeds of Cassia fastuosa and Leucaena leucocephala were determined by potentiometric titrations acid inspected by EPR spectroscopy. Two complexed species were found in both systems, one moiety being the metal ion (M) and the other moiety a sugar monomer of the polysaccharide (L), non-protonated, ML, and protonated, MHL. The values for the logarithms of the binding constants are in the range of 15 units for ML for both employed galactomannans, and 8.0 for MHL for Man:Gal medium average ratio 4:1 galactomannan (C. fastuosa) and 6.8 for Man:Gal medium average ratio 2.6:1 one (L. leucocephala). The EPR spectra have shown one octahedral complex species linked through the oxygen atoms of the hydroxyl groups of the monomer with tetragonal deformation. The A and g obtained values were A(parallel to) = 178G, g(parallel to) = 2.260, and g(perpendicular to) = 2.050 and A(perpendicular to) = 30G for CU2+ with the galactomannans. This was assigned to be ML2 complex which was only detected in the solid state due to the formation of insoluble products in solution, above pH value of 7.0. The extension of formation of complexed species of galactomannans with Cu2+ is probably much more affected by the pH variation than by the degree of substitution of monomers of the chain of the biopolymers studied so far. (C) 1998 Elsevier Science Limited, All rights reserved.