ON THE PH-INDUCED CONFORMATIONAL TRANSITION OF THE EXOCELLULAR POLYSACCHARIDE FROM RHIZOBIUM-TRIFOLII STRAIN TA-1

The pH-induced conformational transition of the exocellular polysaccharide from Rhizobium trifolii strain TA-1 (TA1EPS) has been studied by chiroptical, calorimetric and potentiometric methods. By lowering the pH of neutral aqueous TA1EPS solutions, the specific rotation shows an abrupt change from positive to negative values of [α]. The same trend is also observed in the molar ellipticity. No evidence of a pH-induced change in the chain conformation of TA1EPS has been noticed using aqueous 40 mM NaCl as solvent. From isothermal calorimetric and differential scanning calorimetric experiments, the value of the enthalpy change for the pH-induced ordering process is -6 kJ equiv.-1. Potentiometric titration data suggest that the pH-induced transition involves only a small change in the overall charge density. The possible differences between the ordered form at low pH and that previously found in salt solution are discussed. © 1990.