Iron-57 nuclear magnetic resonance spectroscopic study of alkyl isocyanide myoglobins and a comparison of the ⁵⁷Fe chemical-shift anisotropies of alkyl isocyanide myoglobins, carbonmonoxymyoglobin, ferrocytochrome c, and [⁵⁷Fe(bipy)₃]X₂ · 5H₂O (X Cl, Br, I)

The ⁵⁷Fe nuclear magnetic resonance spectra and spin-lattice relaxation times (T₁) of ethyl isocyanide (EtNC), isopropyl isocyanide (iPrNC), and n-butyl isocyanide (n-BuNC) ligated ferrous myoglobins (∼ 12 mM, pH 7.1, 22°C) at 8.45 T (corresponding to a ⁵⁷Fe Larmor frequency of 11.7 MHz) have been obtained. The isotropic chemical shifts are 9223, 9257, and 9238 ppm downfield from Fe(CO)₅, which yields chemical-shift anisotropies, ∥δ_∥ - δ_∥∥, of 1288, 1260, and 1205 ppm, for the EtNC, and n-BuNC species, respectively. The T₁ values (of about 140 ms) are very much longer than those found previously for carbonmonoxymyoglobin (∼ 17 ms) and are consistent with a change in sign of the chemical-shift tensor upon moving from carbonmonoxymyoglobin to ferrocytochrome c, as previously postulated by L. Baltzer, (J. Am. Chem. Soc. 109, 3479 (1987)). The solution- and solid-state ⁵⁷Fe NMR chemical shifts of [⁵⁷Fe(bipy)₃]X₂ · 5H₂O (X Cl, Br, I) complexes are also reported, and for the solids it is found that δ_∥ = δ_∥ = δ_i = 11,854, 11,721, and 11,635 ppm, for X Cl, Br, and I, respectively. T₁ values for the solids are in the range ∼ 1-5 s, and are probably dominated by electron exchange with paramagnetic impurities.