Structure-volume relationships in hemoglobin. A densitometric and dilatometric study of the oxy leads to deoxy transformation.

Partial volume measurements have been performed for human hemoglobin, both on the oxygenated and deoxygenated forms. Density measurements (by pycnometry) give vHbO2 = 0.752 +/- 0.002 and vHb =0.753 +/- 0.006 for the partial specific volume and do not distinguish between the two different structures. Differential measurements, by dilatometry, however, show a signigicantly higher molal volume (of about 50 cm3/mol hemoglobin tetramer) for the deoxy over the oxygenated from at pH 7. The same reaction, at pH 9, gives a much smaller increase or even a decrease of volume. The different volume changes at pH 7 and at pH 9 are not due to the so-called Bohr ionization but to the weakening, at pH 9 compared to pH 7, of stabilising salt linkages in the deoxy structure.