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Double‐stranded helix of xanthan: Rigidity in 0.01M aqueous sodium chloride containing 0.01 N hydrochloric acid

Six samples of Na xanthan in 0.01M aqueous NaCl containing 0.01 N HCl (pH = 2) were studied by light scattering and viscosity. This study was motivated by the finding that the intrinsic viscosity [η] fairly sharply decreased when the pH of the solvent was lowered from about 6 to 2 by adding HCl to 0.01M aqueous NaCl in which Na xanthan dissolves as rigid dimers having a double‐helical structure. The data for weight‐average molecular weight, radius of gyration, and [η] showed that Na xanthan at pH = 2 remains a dimer behaving as a semiflexible chain. Data analysis in terms of known theories for unperturbed wormlike chains yielded 0.47 ± 0.02, 2.0 ± 0.6, and 68 ± 7 nm for the contour length h per main‐chain residue, diameter d, and persistence length q of the dimer, respectively. these h and d values agreed with the pitch per main‐chain residue and the diameter of the double helix of Na xanthan in 0.01 or 0.1M aqueous NaCl. However, the q value, which was close to the intrinsic persistence length q0 ( = q in the absence of electrostatic interaction) of Na xanthan at pH = 2, was much smaller than the q0 (106 nm) of this helix. We concluded that the xanthan dimer at pH = 2 assumes a double‐helical structure, which is geometrically the same as, but is more flexible than, that at neutral pH.