Hee Cheon Lee

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)

Abstract 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∥ = δi = 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.