Free energy calculations on stiff chain constituents of polysaccharide gels

Summary

The free energy, as a function of end-to-end length, is estimated for some of the polysaccharide constituents of pectate and alginate gels, for chains with degrees of polymerisation ranging from 10 to 1000. The energy is obtained from the distribution of end-to-end lengths estimated by a computer simulation method. Chains are generated in free space assuming the probability of a particular orientation occurring between adjacent monomers is related to the appropriate dimer energy by a Boltzmann function not; but, the dimer energy having been previously calculated by well established procedures.

It is found that the 1–4 linked polymers of α-D-galacturonic and α-L-guluronic acids are appreciably stiffer than β-D-mannuronic acid. However, for all three polymers, for degrees of polymerisation comparable to the shortest distances between crosslinks in alginate and pectate gels the chains are found to be constrained in an energy minimum close to the fully extended length. As the degree of polymerisation increases the shape of the free energy curve changes, though Gaussian behaviour is only observed in the case of polymannuronic chains with a degree of polymerisation of 1000.

It is suggested that some of the properties of alginate and pectate gels can be understood in terms of the flexibilities of the polymer chains connecting adjacent crosslinks.