PROTON MAGNETIC RELAXATION DISPERSION IN HUMAN FLUOROMETHAEMOGLOBIN SOLUTIONS

Abstract

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,τ _c = (9.3±0.3) × 10^-10sec. If the total relaxation rates are corrected for the “outer-sphere” paramagnetic contribution, τ_c = (6.5±0.4) × 10^-10sec. 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 proton “binding” site is at a distance of 3.93 to 3.98 Å from the haem Fe³⁺ion.