Light Scattering Investigation of Charge Fluctuations in Isoionic Serum Albumin Solutions

Light scattering investigations have been carried out on Armour bovine plasma albumin (BSA), bovine serum mercaptalbumin (BMA) and human serum mercaptalbumin (HMA) in their isoelectric region. All three proteins were deionized by passing over an ion-exchange column. In the case of salt free aqueous solutions it was found that the function H(C₂/Δτ) is close to linear in the square root of protein concentration for all three proteins, as predicted by the fluctuating charge theory of Kirkwood and Shumaker. Values of the fluctuating charge were found to be 3.58, 3.51 and 3.98 protonic units for BSA, BMA and HMA, respectively. These are in good agreement with values calculated from other types of measurements. An analysis is made of the contribution of the progressive ionization of the three proteins in the isoionic state to the derivative of the excess chemical potential of the protein with respect to its concentration, showing this effect to be not large under the experimental conditions used. It may, however, ′ca ise the light scattering plot in the absence of salt to pass through a maximum at low concentrations of protein. An analysis in terms of multicomponent light scattering theory of the data obtained in the presence of sodium chloride revealed that the contribution to the intercept of the thermodynamic interaction term between protein and salt is small in the present case.