Polyelectrolyte Behaviour of Phosvitin

The results of spectroscopic and microcalorimetric measurements on dilute aqueous solutions of phosvitin are described. Our infrared and circular dichroism spectra show that the phosphoprotein chains may assume either random‐coiled or partially helical or β‐structures depending upon the pH. The transition to the low pH β‐form occurs with a distinct cooperative character. The strongly interdependent ionization of fixed charges densely spaced along phosvitin chains, typical of highly charged polyelectrolytes, is well illustrated by the calorimetric enthalpy of protonation data. The interaction of phosvitin with acridine orange in dilute aqueous solution (pH ∼5) has been studied by means of spectral and equilibrium dialysis experiments. For relatively low polymer to dye concentration ratios, bound acridine orange exhibits γ‐type spectrum thus suggesting aggregation of dye molecules onto the protein. Gradual decrease in the extent of bound acridine orange aggregation is produced by increasing the concentration of the protein; the monomerically bound form is hardly obtained, however, even at relatively high phosvitin concentrations. Binding curves reveal a cooperativity character for the dye‐protein interaction. It is suggested that about 60% only of phosphomonoester side groups of phosvitin are available for acridine orange binding, these being characterized by a high “stacking” coefficient. The results are tentatively discussed on the basis of present information on phosvitin chemical composition and conformation.