Etude de l'acide polyxanthylique et de ses complexes

Abstract

Poly(X) displays two kinds of thermal dissociations, corresponding to two different states of the polymer. At low temperatures, and for values of pH which are not too high, the polymer is organized in a multi‐stranded structure, in which the xanthylic residues are partially ionized, i.e. carry negative charges; this structure is called poly(X α− ) n , where α is the degree of ionization, and n the number of strands. α is a variable parameter. The thermal dissociation of this structure is cooperative, and its transition temperature ( T mX ) is maximum at pH max . pH max depends upon the saline concentration. In alkaline solutions, the xanthylic residues are totally ionized: poly(X − ) shows another type of thermal transition. Even though this transition is not cooperative, it differs from purely non‐cooperative transitions in that its melting temperature is dependent upon salt concentration (Δ T m = 13 °C for a factor of 10), and the Δ H value is extremely high (109 kJ/mol, or 26 kcal/mol). Poly(X) forms a double‐stranded complex with poly(U) in which the xanthylic residues are negatively charged, poly(X − ) · poly(U). With poly(I) we find two complexes. The pH range of stability of these complexes implies that the degree of ionisation of the xanthylic groups is variable, as we found in the structure poly(X α− ) n : These two‐ and three‐stranded complexes, poly(X α− ) · poly(I) and poly(X α− ) · 2 poly(I) are differently affected by ionic strength. We describe the stability diagrams (temperature, pH, ionic strenght) for all these structures, and we present possible hydrogen bonding schemes. It appears that the negative charges of the xanthylic residues help strengthen the hydrogen bonds, so that the stability of these structures depends upon this ionization (α): the balance between this factor and electrostatic repulsion would then explain the occurrence of an optimum pH value for the stability of the structure.