13C-coupled relaxation studies of a leucine zipper peptide using polarization-transfer pulse sequences

Three new pulse sequences are described for perturbing magnetization modes in 13C-coupled relaxation experiments via polarization-transfer techniques. Relative to non-polarization-transfer pulse sequences, these new pulse sequences result in carbon NMR spectra with significantly improved signal-to-noise ratios, a condition required for coupled relaxation studies of larger biomolecules. These pulse sequences are used to study molecular tumbling of a 13C-labeled leucine zipper peptide, GCN4-p1, in aqueous solution. Experimental data obtained for the AX2 spin system associated with the 13CH2 moiety of the peptide are fitted to the Favro diffusion model via nonlinear least-squares minimization. The least-squares fits provide values for rotational diffusion coefficients. Diffusion coefficients from relaxation studies performed at different temperatures yield enthalpies for the diffusional motion. Deficiencies in the fits of the relaxation data suggest the need for expanded relaxation models that account for proximate protons in the vicinity of the 13CH2 moiety.