Structural studies on polygalacturonate gels: an EXAFS investigation combined with molecular modelling M. Bosco a , S. Miertus b , S. Turchini c , T. Prosperi c , I. Ascone d , R. Rizzo e, * a POLYtech S.C.ar.l., Area Science Park, Padriciano 99, 34012 Trieste, Italy. b International Centre for Science and High Technology, Area Science Park, Padriciano 99, 34012 Trieste, Italy. c ICMAT-CNR, Area della Ricerca, C.P. 10, 00016 Monterotondo Stazione, Italy. d LURE, Universite  Paris-Sud, Bat. 209, 91045 Orsay Cedex, France. e Dipartimento di Biochimica, Bio®sica e Chimica de Macromolecole, Universita di Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy. Received 24 April 2000; revised 10 October 2000; accepted 20 November 2000 Abstract Gels and powders of CuII) and ZnII) polygalacturonate have been investigated using X-ray absorption spectroscopy to obtain coordina- tion 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 ®ne 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 1.92 A Ê and one further distance at 2.25 A Ê , whilst ZnII) was hexacoordinate exhibiting a coordination distance of 2.00 A Ê 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 con®gurations 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. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Polygalacturonate gels; Divalent cation complexation; EXAFS spectroscopy; Molecular modelling 1. Introduction The widely accepted molecular model for polysac- charidic aqueous gels is based on the following two main features. On one hand, both secondary structure stretches and conformationally disordered segments are present along the polymeric chains. On the second hand, ordered domains, belonging to different chains, interact with each other through van der Waals interactions and hydrogen bonding, as occurring in neutral polysaccharides e.g. agar). Moreover, in ionic macromolecules e.g. alginate and polygalacturonate) ion-bridges further stabilise the ordered domains. The above features give rise to an extended multichain network where the solvent can occupy the resulting empty spaces Rees, Morris, Thom & Madden, 1982). The so-called ªegg-boxº model Grant, Morris, Rees, Smith & Thom, 1973) for the Ca 11 -induced gelation of alginates and polygalacturonate exhibits the above depicted structural features and, in addition takes into account conformational and con®gurational characteristics speci®c for these carboxylate polysaccharides. Up to now, structural details of ordered domains of gelling-networks were obtained from solid state X-ray ®bre diffraction studies see Millane, 1990; Walkinshaw & Arnott, 1981, for polygalacturonate gels) and circular dichroism of solutions and ®lms Morris, Rees, Thom & Boyd, 1978; Ravanat & Rinaudo, 1980; Thom, Grant, Morris & Rees, 1982). The possibility of performing Extended X-ray Absorption Fine Structure EXAFS) experi- ments directly on gels using synchrotron radiation, prompted us to use this technique for the investigation of the geometrical features of the binding sites of calcium atoms in polygalacturonate gels Alagna, Prosperi, Tomlinson, & Rizzo, 1986). In this paper the complexation geometryofCuII)andZnII)wasinvestigated.Forthesake of comparison, EXAFS spectra of both gels and powders, the latter obtained from gels after freeze-drying, were acquired. Finally, the structural data obtained were used as the boundary conditions in molecular mechanics calcula- tions in order to obtain detailed three-dimensional mol- ecular models for the ordered domains of the gel structure. Carbohydrate Polymers 47 2002) 15±26 0144-8617/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0144-861700)00339-8 www.elsevier.com/locate/carbpol * Corresponding author. Fax: 1390-406-763-691. E-mail address: rizzor@bbcm.univ.trieste.it R. Rizzo).