Circular dichroism studies on the interactions between tris (L-alaninato)cobalt(III) complexes and basic polyelectrolytes in solution

Abstract

The circular dichroism spectra of the poly-L-lysine complexes with abc(–), abd(–), and abd(+)Co(L-ala-O)₃ in water and isopropyl alcohol–water (1 : 1), at different pH values and over a range of complex to polymer molar ratios, have been measured. Evidence of a specific site binding is presented for abc(–)Co(L-ala-O)₃ in the mixed solvent medium. Of the diastereoisomers used, only in the presence of this material does the macroion assume an α-helical conformation at pH values where the coil form normally predominates. The greater the complex to polymer ratio, the greater the proportion of material of α-helical conformation there is present. Directional modes in the binding, very likely involving hydrogen bonding interactions, are indicated by the changes in the visible c.d. spectra of abc(–)-Co(L-ala-O)₃ in PLL-50% isopropyl alcohol solutions. In contrast, the same complex destabilizes the α-helix structure of poly-L-ornithine. All these features are examined in the light of the structural characteristics of the interacting species. The influence of solvent composition is also considered. Implications of the different stereochemical features of abd-diastereoisomers on the association process with the polypeptides are discussed.