Comparison of Maximum Quantum Filtered NMR Spectroscopy MAXY NMR and Other TwoDimensional NMR Approaches for Resonance Assignment of Peptides

The 1H and 13C NMR spectra of the tridecapeptide neurotensin were assigned in trifluoroethanol solution with the help of two-dimensional 1H-detected maximum quantum filtered NMR spectroscopy (MAXY NMR), which allows the separate detection of CH, CH2 and CH3 1H NMR resonances. In particular, the selective detection of CH protons from leucine and isoleucine residues, normally obscured by CH2 resonances, was facilitated by the use of the MAXY–TOCSY experiment, as was the identification of individual methylene protons in amino acids with overlapping methylene resonances. The approach is compared with the use of conventional TOCSY spectroscopy. A new pulse sequence, gem-TOCSY, is demonstrated and its relative usefulness is also discussed in the context of peptide resonance assignment. The NMR results demonstrated conformational interchange, which was slow on the NMR time-scale, between cis and trans conformers of proline-10. Conformation-dependent data, including coupling constants, amide proton chemical shift temperature coefficients, NOEs obtained from a ROESY experiment and circular dichroism spectra indicated that neurotensin in trifluoroethanol solution is flexible with no single fixed conformation predominating. Nevertheless, this work demonstrates clearly the potential for the general application of the MAXY NMR approach to the assignment of highly overlapped resonances in 1H NMR spectra of peptides and proteins.