DISTAL AND PROXIMAL LIGAND INTERACTIONS IN HEME-PROTEINS - CORRELATIONS BETWEEN C-O AND FE-C VIBRATIONAL FREQUENCIES, O-17 AND C-13 NUCLEAR-MAGNETIC-RESONANCE CHEMICAL-SHIFTS, AND O-17 NUCLEAR-QUADRUPOLE COUPLING-CONSTANTS IN (CO)-O-17-LABELED AND (CO)-C-13-LABELED SPECIES

We have obtained the oxygen-17 nuclear magnetic resonance (NMR) spectra of a variety of C-17 O-labeled heme proteins, including sperm whale (Physeter catodon) myoglobin, two synthetic sperm whale myoglobin mutants (His E7 --> Val E7; His E7 --> Phe E7), adult human hemoglobin, rabbit (Oryctolagus cuniculus) hemoglobin, horseradish (Cochlearia armoracia) peroxidase (E.C. 1.11.1.7) isoenzymes A and C, and Caldariomyces fumago chloroperoxidase (E.C. 1.11.1.10), in some cases as a function of pH, and have determined their isotropic O-17 NMR chemical shifts, delta-i, and spin-lattice relaxation times, T1. We have also obtained similar results on a picket fence porphyrin, [5,10,15,20-tetrakis(alpha,alpha,alpha,alpha-pivalamidophenyl)porphyrinato]iron(II)(1-MeIm)CO, both in solution and in the solid state. Our results show an excellent correlation between the infrared C-O vibrational frequencies, nu-(C-O), and delta-i, between nu(C-O) and the O-17 nuclear quadrupole coupling constant (e2qQ/h, derived from T1), and as expected between e2qQ/h and delta-i. Taken together with the work of others on the C-13 NMR of (CO)-C-13-labeled proteins, where we find an excellent correlation between delta-i(C-13) and nu(Fe-C), our results suggest that IR and NMR measurements reflect the same interaction, which is thought to be primarily the degree of pi-back-bonding from Fe d to CO-pi-* orbitals, as outlined previously [Li, X.-Y., & Spiro, T. G. (1988) J. Am. Chem. Soc. 110, 6024]. The modulation of this interaction by the local charge field of the distal heme residue (histidine, glutamine, arginine, and possibly lysine) in a variety of species and mutants, as reflected in the NMR and IR measurements, is discussed, as is the effect of cysteine as the proximal heme ligand.