Nuclear magnetic resonance shifts in paramagnetic metalloporphyrins and metalloproteins

We report the first detailed investigation of the ¹H, ¹³C, ¹⁵N and ¹⁹F nuclear magnetic resonance (NMR) spectroscopic shifts in paramagnetic metalloprotein and metalloporphyrin systems. The > 3500 ppm range in experimentally observed hyperfine shifts can be well predicted by using density functional theory (DFT) methods. Using spin-unrestricted methods together with large, locally dense basis sets, we obtain very good correlations between experimental and theoretical results: R² = 0.941 (N = 37, p < 0.0001) when using the pure BPW91 functional and R² = 0.981 (N = 37, p < 0.0001) when using the hybrid functional, B3LYP. The correlations are even better for C^α and C^β shifts alone: Cα, R² = 0.996 (N = 8, p < 0.0001, B3LYP); C^β, R² = 0.995 (N = 8, p < 0.0001, B3LYP), but are worse for C^meso, in part because of the small range in C^meso shifts. The results of these theoretical calculations also lead to a revision of previous heme and proximal histidine residue ¹³CNMR assignments in deoxymyoglobin which are confirmed by new quantitative NMR measurements. Molecular orbital (MO) analyses of the resulting wave functions provide a graphical representation of the spin density distribution in the [Fe(TPP)(CN)₂]^- (TPP = 5,10,15,20-tetraphenylporphyrinato) system (S = 1/2), where the spin density is shown to be localized primarily in the d_xz (or d_yz) orbital, together with an analysis of the frontier MOs Fe(TPP)CI (S = 5/2), Mn(TPP)CI (S = 2), and a deoxymyoglobin model (S = 2). The ability to now begin to predict essentially all heavy atom NMR hyperfine shifts in paramagnetic metalloporphyrins and metalloproteins using quantum chemical methods should open up new areas of research aimed at structure prediction and refinement in paramagnetic systems much the same way that DFT methods have been used successfully in the past to predict/refine elements of diamagnetic heme protein structures.