Pressure effect on the chain shrinkage of poly(vinylpyrrolidone) induced by complexation with a hydrophobic compound in aqueous solution

The effects of pressure on the binding of a hydrophobic compound (8-anilina -1-naphthalenesulfonate (ANS)) to poly(vinylpyrrolidone) (PVP) in aqueous s olution were followed by the fluorescence enhancement over the pressure ran ge 0.1-400 MPa. An elevation in pressure induced the dissociation of ANS fr om an oligomer PVP (molecular weight <6000) but favored its binding to a la rger PVP molecule (MW > 10 000). We calculated the binding constant between ANS and PVP (K-b) and estimated the apparent volume changes accompanying t he ANS binding to PVP (Delta V-b) from the pressure dependence of K-b. The volume changes for ANS binding to high-MW PVP's were dependent on the press ure, and negative values were obtained in the pressure region from 100 to 4 00 MPa. SAXS measurements demonstrated that the ANS binding to the high-MW PVP chain led to the reduction of radii of gyration, suggesting that ANS bi nding induced local cluster formation in the PVP chain around the binding s ite. The Kratky plot indicated that global shrinkage of the chain occurred, and the negative volume change for ANS binding to high-MW PVP is explained by the fact that the cluster formation around the ANS is followed by an in clusion of the cluster around ANS into the random coil for PVP.