Heme methyl 1H and 13C resonances of met‐cyano form of myoglobin from the shark, Geleorhinus japonicus (GJMbCN), have been assigned via 1H‐13C heteronuclear shift correlated spectroscopy (COSY) connectivities and their hyperfine shifts were compared with those of the corresponding resonances of some hemoproteins. Variation of the heme methyl 1H hyperfine shift pattern correlates well with the angle (Φ) between the projection of the proximal histidyl imidazole plane onto the heme plane and the NII‐Fe‐Niv vector. The alteration of the interaction of the heme peripheral side‐chains and/or the iron‐bound ligand with the surrounding amino acid residues cannot account for large differences in the shifts of the corresponding heme methyl resonances between GJMbCN and sperm whale MbCN. Since the heme methyl 1H shifts for GJMbCN fall in between those of the corresponding resonances for sperm whale Mb and Aplysia limacma Mb in which the Φ values have been reported to be 19° and 29°, respectively, the Φ value in GJMb is estimated to be slightly larger than 19°.
The molecular structure of the active site of myoglobin from the shark, Galeorhinusjaponicus, has been studied by 'H-NMR. Some hyperfine-shifted amino acid proton resonances in the met-cyano form of G. japonicus myoglobin have been unambiguously assigned by the combined use of various two-dimensional NMR techniques; they were compared with the corresponding resonances in Physter catodon myoglobin. The orientations of ThrElO and IleFG.5 residues relative to the heme in G. japonicus met-cyano myoglobin were semiquantitatively estimated from the analysis of their shifts using the magnetic susceptibility tensor determined by a method called MATDUHM ( Mol. Biol. 153, 117-1241. In spite of a substantial difference in shift between the corresponding amino acid proton resonances for the two proteins, the orientations of these amino acid residues relative to the heme in the active site of both myoglobins were found to be highly alike.NMR of paramagnetic hemoproteins has provided a wealth of information about the structural properties of their active sites [l-61. 'H-NMR resonances arising from heme peripheral groups and amino acid residues near the heme have been effectively used as a useful structural probe in the NMR study of hemoprotein because these resonances are resolved well outside the poorly resolved diamagnetic envelope in the chemical shift region of 0 -10 ppm and they are quite sensitive to the heme molecular environments. Additionally, the observation of the nuclear Overhauser effect (NOE) [7], even in the presence of paramagnetic relaxation, allows not only the signal assignments of hyperfine-shifted resonances [3, 8 -101 but also the determination of internal mobility of specific groups in the heme pocket [ l l -141.As far as the structural determination of the active site in hemoprotein is concerned, resonances of non-coordinated amino acid protons are of particular importance because of their extremely high sensitivity to the spatial position of the nuclei relative to the heme [I5 -171. Their hyperfine shifts are purely pseudo-contact shifts (dPJ and hence can be interpreted quantitatively in terms of the dipolar interaction between Correspondence to Y. Yamamoto,
1H and 13C nuclear relaxation of paramagnetic iron(III) tetraphenylporphyrin–imidazole complexes has been investigated in order to gain insight into the relationship between the paramagnetic relaxation rate and the electron-nucleus distance. The analysis of the nuclear spin–lattice relaxation time (T1) in terms of the distance indicated that, for the iron porphyrin complexes used, the contribution of the electron-nucleus dipolar coupling no longer contributes significantly to the relaxation process of a proton located at more than 0.75 nm away from the iron. For the complexes studied, the electron spin–lattice relaxation time (T1e) of 6.4 ps was calculated from the T1 values measured at 26°C.
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