Frequency dependence of the proton magnetic relaxation in aqueous solutions of haemoproteins

The longitudinal proton magnetic relaxation times T 1 were measured for ferri (met)‐and carbonmonoxy‐bovine haemoglobin and equine myoglobin in 0.1 M KH 2 PO 4 aqueous solutions near pH 6 at 5°C and 35°C from 1.5‐ to 60‐MHz Larmor frequencies. It is concluded that the correlation time τ C for the dipole–dipole interaction of electron and nuclear spins is in fact the electron (ferric) spin relaxation time τ S being close to 1.5 × 10 −10 sec for both metHb and metMb at 5°C. At 35°C the paramagnetic relaxation rates are not determined solely by the relaxation of protons exchanging from the haem pocket with bulk solvent. Hence, τ C at 35°C cannot be calculated from the dispersion data obtained at this temperature. The relevance of this for the determination of interspin distances r is discussed.