Proton magnetic relaxation dispersion in human fluoromethaemoglobin solutions.

The solvent proton spin-lattice relaxation time of high spin Fe3+ (S=5/2) human A fluoromethaemoglobin aqueous solutions was measured at 14 Larmor frequencies in the range from 2.2 to 96 MHz. The observed paramagnetic relaxation rates are analysed in terms of the Solomon-Bloembergen theory, with the g-tensor value of 2 based on the consideration of the protein tertiary structure. From the H2O (pH 6) haemoprotein solution relaxation data, tau(c) =(9.3+/-0.3) X 10(-10) sec. If the total relaxation rates are corrected for the "outer-sphere" paramagnetic contribution, tau(c)=(6.5+/-0.4) X 10(-10) sec. The latter correction is obtained from the p.m.r. of the non-exchangeable aliphatic protons of C2H4(OD)2 added to the D2O-solution of fluoromethaemoglobin. Assuming that single proton transfer is taking place through the protein channel along the axis normal to the haem (g=2), the protein "binding" site is at a distance of 3.93 to 3.98 A from the haem Fe3+ ion.