Studies of Proteins in Solution by Natural-Abundance Carbon-13 Nuclear Magnetic Resonance. Spectral Resolution and Relaxation Behavior at High Magnetic Field Strengths

Natural-abundance 13C nuclear magnetic resonance spectra of hen egg-white lysozyme at 63.4 kG are compared with spectra at 14.2 kG. The increase in resolution when going from the low to the high field is much greater for aliphatic and methine aromatic carbon resonances than for carbonyl and nonprotonated aromatic carbon resonances. This result is consistent with experimental and calculated spin-lattice relaxation times, which demonstrate the dominance of chemical shift anisot-ropy as a relaxation mechanism for nonprotonated unsaturated carbons of a native protein at magnetic field strengths much above 40 kG. For the aliphatic and methine aromatic carbons, the 13C-1H dipolar relaxation mechanism should be dominant at all magnetic field strengths now available for NMR.