Metastable and stable states of xanthan polyelectrolyte complexes studied by atomic force microscopy

The compaction of the semiflexible polysaccharide xanthan with selected multi‐ and polyvalent cations was studied. Polyelectrolyte complexes prepared at concentrations of 1–2 μg/ml were observed by tapping mode atomic force microscopy. High‐molecular‐weight xanthan compacted with chitosan yields a blend of mainly toroidal and metastable structures and a small fraction of rod‐like species. Polyelectrolyte complexes of xanthan with polyethylenimine and trivalent chromium yielded similar structures or alternatively less well packed species. Racquet‐type morphologies were identified as kinetically trapped states occurring on the folding path toward the energetically stable state of the toroids. Thermal annealing yielded a shift of the distribution of xanthan–chitosan morphologies toward this stable state. Ensembles of toroidal and rod‐like morphologies of the xanthan–chitosan structures, collected using an asphericity index, were analyzed. The mean height of the toroids increased upon heating, with a selective increase in the height range above 2 nm. It is suggested that the observed metastable structures are formed from the high‐molecular‐weight fraction of xanthan and that these are driven toward the toroidal state, being a low‐energy state, following annealing. Considered a model system for condensation of semiflexible polymers, the compaction of xanthan by chitosan captures the system at various stages in the folding toward a low‐energy state and thus allows experimental analyses of these intermediates and their evolution. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004