X-ray scattering study on hemoglobin solution with synchrotron radiation: a simple analysis of scattering profile at moderate angles in terms of arrangement of subunits.

X-ray scattering profiles in moderate-angle regions were recorded from carbon-monoxy-, oxy-, and deoxyhemoglobin solutions, using synchrotron radiation. They all display four distinct scattering peaks at R = 0.030, 0.055, 0.078, and 0.102 A-1 up to 2 theta approximately 10 degrees in addition to the main scattering around R approximately equal to 0. Contrast variation experiments, in which sucrose was used to change the electron density level of the solvent, revealed that the outer two scattering peaks are attributable to the variation of electron density within subunits in hemoglobin. The inner two were assigned as peaks due to the whole molecule and interpreted in terms of an interference function that is calculated from the inter-subunit distances in a molecule. This result is important in connection with evaluating the arrangement of constituent subunits in allosteric proteins and oligomeric proteins. The scattering profiles indicate that there is no difference in electron density variation within subunits between oxy- and deoxyhemoglobin. However, the arrangement of subunits is different between oxy- and deoxyhemoglobin molecules, as the scattering peaks at R = 0.030 and 0.055 A-1 shift toward smaller angles for deoxyhemoglobin.