Conclusions

Direct calorimetric determinations ofboth the heat of transition and of its partial derivative have been proved to be irreplaceable methods to quantify the cooperative character of the polymeric helical chain. Evidence is in favour of a true temperature-driven phase transition occurring in polysaccharides under given circumstances. Besides the formal presentation of the thermodynamic analysis of the well-known process of helix-coil transition in linear biopolymers, accurate DSC data (even alone) can give the structural information of the cooperativity parameter (and therefore of N°) from the evaluation of calorimetric AH and AHvH. Reference to the theoretical approaches shows that N° « ct"1/2, at least for chains with a degree of polymerisation m larger that N°, which becomes an effective number of monomer units energetically (and topologically) correlated. Therefore, the larger N°, the larger is the topo logical constraint in an ordered biopolymer chain.

Several investigations on the rheological properties of concentrated solutions of polysaccharides have produced data which can be interpreted on the basis of the polymer conformation and chain rigidity. In fact, worm-like polysaccharides with low flexibility show a remarkable deviation from the behaviour described for flexible statistically disordered chains. The viscoelastic spectrum of the former resembles that of a typical gel system, indicating that non-transient supramolecular structures also occur, although these weak-gel systems flow upon increasing shear. This behaviour, typical of so-called weak gels, has been claimed to reflect the occurrence in the polymer network of weak non-covalent intermolecular forces. Persistence of these weak interactions over an extended length of the chain is conceivable within a fraction of regular structures. It is the authors' intention to claim that the energetics and the cooperativity of the phase transition are just another aspect of the macroscopic solution behaviour of the gelling ordered polysaccharides. Molecular description of these structures is not yet achieved and other experimental data are necessary to substantiate these hypotheses.

Acknowledgements

The paper has been prepared with financial support of M.U.R.S.T. and of University of Trieste. F. S. is grateful to INSTM (Florence) for research grant.

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