title: Metastable and stable states of xanthan polyelectrolyte complexes studied by atomic force microscopy.
abstract: The compaction of the semiflexible polysaccharide xanthan with selected multi- and polyvalent cations was studied. Polyelectrolyte complexes prepared at concentrations of 1-2 microg/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.
authors.affiliation: Biophysics and Medical Technology, Department of Physics, The Norwegian University of Science and Technology, NTNU, NO-7491 Trondheim, Norway.
authors.forename: Gjertrud
authors.initials: G
authors.lastname: Maurstad
authors.forename: Bjørn T
authors.initials: BT
authors.lastname: Stokke
mesh_terms: D002686:Chitin / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D048271:Chitosan; D002857:Chromium / Q000737:chemistry / Q000378:metabolism; D018625:Microscopy, Atomic Force; D011094:Polyethyleneimine / Q000737:chemistry / Q000378:metabolism; D011135:Polysaccharides, Bacterial / Q000737:chemistry* / Q000378:metabolism; D013696:Temperature; D013997:Time Factors
chemical_list: D011135:Polysaccharides, Bacterial; D002857:Chromium; D002686:Chitin; D011094:Polyethyleneimine; D048271:Chitosan; C002563:xanthan gum