ROLE OF IONIC-STRENGTH ON HEMOGLOBIN INTERPARTICLE INTERACTIONS AND SUBUNIT DISSOCIATION FROM LIGHT-SCATTERING

The average diffusion coefficients of hemoglobin (bovine CO-Hb) in dil ute solutions (phi = 0.005), as obtained by cumulant analysis of the a utocorrelation functions of the scattered light, are found to depend u pon pH and ionic strength. A decrease of the diffusion coefficient is observed vs ionic strength for two salts, NaCl or CaCl2, in the range 10-300 mM, when the solution pH is away from that corresponding to the isolectric point of the protein. It is shown here that the results ca n be interpreted in terms of electrostatic effects with a model that s hould include both interparticle interactions and subunit dissociation of the protein. Global analyses of the first cumulant diffusion coeff icient vs ionic strength were then applied in order to obtain accurate values of the hemoglobin charge vs pH and of its single particle diff usion coefficient. A quantitative estimate of the dissociation was als o obtained, leading then to good descriptions both of the electrostati c and of the non-electrostatic contribution to the protein tetramer to dimer dissociation process.