Nuclear magnetic resonance of heme protein crystals. General aspects

A new technique capable of determining the static and dynamic structures of heme protein crystals is reported. It is shown that microcrystals of a variety of paramagnetic heme proteins, suspended in approximately 90% saturated (NH4)2SO4, may be perfectly aligned by an intense static external magnetic field, H0, due to the large anisotropy in the magnetic susceptibility of the protein caused by the paramagnetic center. Myoglobin from sperm whale (Physeter catodon) was isotopically enriched at the C epsilon methyl groups of methionine residues 55 and 131 with either 13C or 2H and studied in the crystalline solid state by 2H-quadrupole echo and 13C-Fourier transform nuclear magnetic resonance spectroscopy. It was found that suspensions of both high (S = 5/2) and low (S = 1/2) spin ferric forms of the labeled protein were ordered, the axis of ordering being approximately perpendicular to the low temperature minimum g tensor valve, even though upper Kramers levels are populated at room temperature. The paramagnetic CoII derivative "coboglobin" showed similar ordering behavior, but the diamagnetic carboxymyoglobin was unaffected. The magnetic ordering method permits the recording of "single crystal" NMR spectra from microcrystalline arrays of proteins which cannot be prepared in large enough form (approximately 1 cm3) for single crystal NMR spectroscopy and thereby allows the resolution and assignment of numerous single atom sites in the crystalline solid state. The information from a "single crystal" NMR spectrum combined with that obtained on the crystal powder allows for the direct determination of (i) the spatial orientation of the particular labeled residue within the protein crystal and (ii) the rates and types of side chain motion. Resonances were assigned by spin label broadening experiments and by use of existing x-ray data to predict 2H-NMR spectra. This new technique opens up the possibility of determining directly the dynamic structure of protein crystals and of comparing the structures of proteins in the crystalline solid state with that in solution and is applicable to other heme proteins, e.g. catalase.