Molecular association and the viscosity of hemoglobin solutions

Assuming that hemoglobin in solution associates isodesmically, it is possible to calculate the concentrations of the different oligomeric species at each concentration and to estimate the effect they have on the viscosity of the solution. The calculations show an increasing viscosity with concentration, and suggest a change from flexible chain to rigid rod at the higher concentrations. This could come about if higher oligomers of normal hemoglobin form coils as a result of head to tail interactions in the same chain. The association properties of human sickle hemoglobin may, therefore, be a special case of association, where the strength of the subunit interactions and the geometry of the resulting structures strongly favor the associated forms.