Microstructure of aggregated and nonaggregated κ-carrageenan helices visualized by atomic force microscopy

Gelation Of K-carrageenan is believed to involve two steps: helix formation on cooling and a further specific cation (salt) induced side-by-side aggregation of helices. Samples that should contain aggregated and also nonaggregated helices Of K-carrageenan were prepared in aqueous solutions, spread onto freshly cleaved mica surfaces, and visualized under butanol using atomic force microscopy. In the presence of an excessive amount of a gel-promoting salt, KCI, K-carrageenan appeared to form rigid rodlike structures considered as large aggregates of double helices. Even when the side-by-side interhelical aggregation was suppressed by diluting random coiled solutions prior to cooling, by adding an aggregation-impeding salt, NaI, or by transforming K-carrageenan into the tetramethylammonium. (TMA) salt, branched rodlike structures were still evident, suggesting that the side-by-side aggregation of helices is not a prerequisite for K-carrageenan to form a network structure, at least locally. Even in the absence of factors that promote side-by-side aggregation, K-carrageenan helices appeared to be capable of associating and forming gel networks.