A proton magnetic relaxation study of human ferrihaemoglobin in aqueous salt solutions

The proton magnetic relaxation time, T1, has been measured at 29 MHz in 0.1M KH2PO4 and 0.1M NaCI (both pH 6) aqueous solutions of human ferrihaemoglobin, the protein concentrations ranging from 0.5 to 5 mM per haem. The linear dependence on protein concentration of the enhancement in relaxation rates, Δ(1/T1), due to the presence of the paramagnetic iron in haemoglobin was confirmed at 34°C and at ∼10°C. In the middle temperature range there is a thermally activated process, whose energy of activation depends on protein concentration. This dependence is different for the two salt solutions; Ea increases with cHb for 0.1M KH2PO4 and decreases for 0.1M NaCI. The model of water-proton exchange between the bulk solvent and the sixth coordination site of the haem iron was used to calculate the distance from the “liganded” water protons to the haem iron. This yields distances much larger than those determined by X-ray crystal structure analysis. A model is proposed which reconciliates both types of data. The low-temperature relaxation rates cannot be used in deriving quantitative stereochemical data for the haem pocket because of its special shape. Irrespective of the molecular model adopted, the experimental results show clearly that, both at low (∼10°C) and higher (>34°C) temperatures, the interaction of paramagnetic haem iron with water protons is practically the same for the two aqueous solutions. The dynamic state of the haemoglobin molecule, as indicated by the middle-temperature range, is completely different in 0.1M KH2PO4 and 0.1M NaCl, pH 6.