Determination of the sequence of peptides using paramagnetic probes with NMR spectroscopy

The paramagnetic ion Gd3+ binds to the α-carboxyl group of a peptide and broadens the resonances from the 13C and 1H nuclei of the backbone sequentially along the chain. The sequential broadening of both carbonyl and Cα nuclei of PheAspAlaSerVal have been observed in 6 M guanidine hydrochloride at pH 4.0. Sequential broadening from the N-terminus is also possible using Cu2+ at pH ≥ 8. Broadening experiments using triglycine and either Gd3+ or Cu2+ show that it is more difficult to broaden 13C than 1H resonances on the same amino acid residue. The ratio of [Gd3+]CMR/[Gd3+]PMR is 12 ± 5, which agrees reasonably well with the theoretical value of 16. The ratio [Cu2+]CMR/[Cu2+]PMR is 3 ± 2. The relative advantages of proton magnetic resonance (PMR) and carbon-13 magnetic resonance (CMR) spectroscopy for this application are discussed. It is concluded that sequence determinations of 20 mg samples of peptides, ranging in size up to 7–9 residues, should be possible using CMR spectroscopy. The main advantage of PMR spectroscopy is the much smaller amounts of material required, but other problems preclude its use at this time except for very simple peptides.

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