Structural studies on polygalacturonate gels: an EXAFS investigation combined with molecular modelling

Gels and powders of Cu(II) and Zn(II) polygalacturonate have been investigated using X-ray absorption spectroscopy to obtain coordination distances and coordination numbers. The analysis of the near edge zone of the spectra showed that, for a given cationic species, gels and powders exhibited the same coordination number. The analysis of the extended X-ray absorption fine structure (EXAFS zone) revealed that Cu(II) and Zn(II) complexes were characterised by different coordination numbers. Cu(II) ions resulted to be pentacoordinate with four coordination distances at 1.92Å and one further distance at 2.25Å, whilst Zn(II) was hexacoordinate exhibiting a coordination distance of 2.00Å for all the ion-to-oxygen bonds. Both experimental coordination numbers and coordination distances were used as constraints for molecular modelling calculations. Different polymeric chain-packing configurations and the possibility to include water molecules into the coordination shells were taken into account. Possible detailed molecular models of cation complexation sites between ordered segments of polygalacturonate chains are reported.