Characterization of colominic acid by circular dichroism and viscosity analysis

Conformations of oligo- and poly-(alpha(2 --> 8)-D-Neu pNAc) (colominic acid) and its derivatives were studied by circular dichroism (CD) spectroscopy and viscometry to understand the molecular basis of their unusual antigenic properties. No temperature-dependent conformational transition between 5 and 70 degrees C or divalent salt effect of Ca2+ or Mg2+ was observed in colominic acid or its N-deacetylated form by CD spectroscopy. However, CD spectroscopy indicated that the distribution of conformers in oligocolominic acid changes continuously from n=2 to octamer, and there was no further change of the conformer distribution for n>9. Colominic acid exhibited a much lower intrinsic viscosity compared with the values for other polyelectrolytes of similar linear charge density, such as polynucleic acids. The apparent absence of induced conformational transition by salt or temperature, and the high flexibility indicated that the binding of colominic acid to its antibodies may not contain a significant amount of specific conformationally controlled determinant. Instead, our data suggest that more than nine saccharide units are needed for a cooperative binding process.