CALCULATION OF MEAN-SQUARE DIPOLE MOMENT AND PROTON FLUCTUATION ANISOTROPY OF HEMOGLOBIN AT LOW IONIC STRENGTH

The mean-square dipole moment and proton fluctuation anisotropy of horse oxy hemoglobin at low ionic strength have been calculated from the Kirkwood-Shumaker and extended Tanford-Kirkwood theories. Site coordinates were taken from a model built according to Perutz' 1965 description. Results are presented for the pH range 4.5-9.0. The mean moment is considerably more sensitive to the parameters and approximations used than is the fluctuation moment, and the correlation averages were found to make a significant contribution to the fluctuation moment. The anisotropy of the fluctuations is appreciable and varies with pH. Calculations for met hemoglobin and for hypothetical models of deoxy and human oxy hemoglobin are also presented. Good agreement was obtained with data for horse oxy hemoglobin, and small but unexplained discrepancies with the available data were noted for the other modifications.