| Abstract
| - We describe three-dimensional magic angle spinning NMR experiments that enable simultaneousband-selective measurement of the multiple distance constraints between carbonyl and side chain carbonsin uniformly 13C,15N-labeled peptides. The approaches are designed to circumvent the dipolar truncationand to allow experimental separation of the multiple quantum (MQ) relaxation and dipolar effects. Thepulse sequences employ the double quantum (DQ) rotational resonance in the tilted frame (R2TR) to performselective polarization transfers that reintroduce the 13C‘−13Cγ,δ dipolar interactions. The scheme avoidsrecoupling of the strongly coupled C‘−Cα and C‘−Cβ spin pairs, therefore minimizing dipolar truncationeffects. The experiment is performed in a constant time fashion as a function of the radio frequency irradiationintensity and measures the line shape of the DQ transition. The width and the intensity of this line shapeare analyzed in terms of the DQ relaxation and dipolar coupling. The attenuation of the multispin effects inthe presence of relaxation enables a two-spin approximation to be employed for the analysis of theexperimental data. The systematic error introduced by this approximation is estimated by comparing theresults with a three-spin simulation. The contributions of B1-inhomogeneity, CSA orientation effects, andthe effects of inhomogeneous line broadening are also estimated. The experiments are demonstrated inmodel U-13C,15N-labeled peptides, N-acetyl-l-Val-l-Leu and N-formyl-l-Met-l-Leu-l-Phe, where 10 and 6distances, ranging between 3 and 6 Å, were measured, respectively.
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