Effects of chemical exchange and dipole-dipole interactions on the proton relaxation rates of surface histidyl residues in human hemoglobins

Abstract

Proton spin-spin and spin-lattice relaxation rates of several C2 protons of the histidyl residues located on the surfaces of human normal adult and sickle cell hemoglobins have been measured as a function of the hemoglobin concentration (10 to 16% (wt/ vol)), temperature ( 17 to 37°C), pH (5.7 to 9.0), and resonance frequency (300 to 600 MHz). The relaxation rates are compared to those calculated from the crystal coordinates of the hemoglobin molecule, assuming dipole-dipole interactions modulated by Brownian tumbling. We have found that the initial recoveries of the longitudinal magnetization of several of the C2 proton resonances of the surface histidyl residues investigated are in good agreement with the spin-lattice relaxation rates predicted by this model. On the other hand, the spin-spin relaxation rates are found to increase with the square of the resonance frequency and appear to be affected by the chemical exchange process between the ionized and unionized forms of the imidazole ring of the histidyl residue. From the experimental results, the lifetime of the exchange process is estimated to be 8 ± 3 μs for all the resonance lines studied in 10% deoxyhemoglobin at pH 7.0 and at 25°C.