Salt dependence and ion specificity of the coil–helix transition of furcellaran

Abstract

AbstractThe conformational transition and the cation‐binding properties of aqueous furcellaran (a gel‐forming, low‐sulfated polysaccharide of the carrageenan family) in various salts and salt mixtures was studied by optical rotation and by 133Cs‐nmr. The results were compared with theoretical predictions based on the Poisson–Boltzmann cell model (PBCM). The conformational transition of furcellaran occurs in a single step, which implies a nonblocklike distribution of sulfate groups along the polymer chain. The chloride salts of sodium, lithium, and tetramethylammonium are equally potent in inducing helix formation of furcellaran, indicating that these ions act by nonspecific electrostatic interactions. In contrast, the potassium and cesium ions specifically promote helix formation and aggregation (gelation) of furcellaran. The divalent calcium and magnesium ions are nonspecific, but more potent than the nonspecific monovalent ions in inducing helices. Anions differ in their capacity to stabilize the furcellaran helix in the sequence Cl− < NO < Br− < SCN− < I−. The iodide and thiocyanate anions impede aggregation and gel formation. 133Cs‐nmr chemical shifts indicate specific binding of cesium ions to the furcellaran helix. Thus, with respect to its ion specificity and ion‐binding properties, furcellaran, with 0.6 sulfate group per repeating disaccharide, resembles κ‐carrageenan (1 sulfate/disaccharide) but differs from ι‐carrageenan (2 sulfates/disaccharide). The conformational transition temperatures of furcellaran are, however, generally higher than those of κ‐carrageenan under comparable conditions, and in mixtures of the two polysaccharides, separate transitions still occur, indicating that no mixed helices are formed. The observed ion sensitivity and cation‐binding properties of furcellaran agree with predictions, by the PBCM, for a K‐carrageenan with a reduced charge density.