Carbon-13 nuclear magnetic resonance studies of proteins.

Abstract

Nuclear magnetic resonance (NMR) methods, in principle, are capable of resolving resonances of single atoms in protein molecules. In practice, most of the resonances from hydrogen nuclei (lH NMR) in proteins overlap to produce a broad unresolved envelope (1). Carbon-13 NMR (13C NMR) has an intrinsically higher resolving power than proton NMR by a factor of about 20 because of the greater range of electronic configurations experienced by carbon atoms (2). Consequently, initial results on amino acids were interpreted to indicate that I3C NMR would be a potentially valuable tool in studies of proteins in solution (3). Initially, sensitivity problems made this potential unrealizable. The sensitivity of carbon-13 relative to proton NMR at the same magnetic field strength is 1:62.9. If the fact that I3C has a natural abundance of 1.1 % is taken into account (12C has no resonance condition), this leads to a relative sensitivity of 1:5716. This sensitivity problem in l3C NMR, however, has to a large extent been overcome by the introduc­ tion of the pulse Fourier transform method (4). Further losses of signal intensity in BC NMR spectra arise as a result of coupling between 13C and IH nuclear spins. These couplings can be removed by the applica­ tion of a second radiofrequency field at the resonance value for protons. If this frequency has sufficient power and is spread over the entire range of proton absorp­ tions, a process often accomplished by noise modulation, then complete decoupling of the proton nuclear spins from the 13C nuclear spins results. Each carbon atom in the molecule then gives rise to a single resonance, and the resultant proton­ decoupled I3C spectra are relatively simplified. In addition, applying a IH decou­ piing frequency leads to changes in the relative populations of the energy levels for the DC nuclear spins; this phenomenon, known as the nuclear Overhauser effect