Solution NMR Determination of the Anisotropy and Orientation of the Paramagnetic Susceptibility Tensor as a Function of Temperature for Metmyoglobin Cyanide: Implications for the Population of Excited Electronic States

Comprehensive 1H NMR assignments of the heme cavity proton resonances of sperm whale metmyoglobin cyanide have provided the dipolar shifts for nonligated residues which, together with the crystal coordinates of carbonyl myoglobin, allow accurate determination of both the anisotropies and orientation of the paramagnetic susceptibility tensor, χ, in the molecular framework. The resulting axial, Δχax = 2.48 × 10-8 m3/mol, and rhombic anisotropy, Δχrh = −0.58 × 10-8 m3/mol, values at 25 °C determined from the most complete set of dipolar shifts are determined to 2% and 6% uncertainty, respectively, and agree well with theoretical estimates (Horrocks, W. D., Jr. and Greenberg, E. S. Mol. Phys. 1974, 27, 993−999). Numerically and spatially restricted input data sets lead to larger uncertainties in Δχax and Δχrh, but do not systematically bias the orientation of the tensor. Determination of the anisotropies and orientation over the temperature range 5−50 °C shows that the susceptibility tensor orientation is min...