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

✍ Scribed by Lina Zhang; Wei Liu; Takashi Norisuye; Hiroshi Fujita

• Publisher: Wiley (John Wiley & Sons)
• Year: 1987
• Tongue: English
• Weight: 467 KB
• Volume: 26
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360260303

✦ Synopsis

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 [ q ] fairly sharply decreased when the pH of the solvent was lowered from about 6 to 2 by adding HC1 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 [ q ] 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 i-0.6, and 68 i-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 qo (= q in the absence of electrostatic interaction) of Na xanthan at pH = 2, was much smaller than the qo (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.