SOLID STATE NMR MEASURE OF PINPEPTIDES AN NCCN2Q HETERONUCLEAR LOCAL FIELD

本文描述了一种固态核磁共振实验

A solid-state NMR experiment has been described for measuring P torsion angles in (15N-13C~-13C 0-15N)-labeled peptides. Creation of 13Cu-13C o double-quantum coherence (DQC) is followed by selective evolution under '5N- 3C dipolar interactions (Costa, 1997; Feng, 1997). The time-course of '3Cc~- 3C o DQC dephasing is sensitive to the P torsion angle, particularly for values associated with B-sheet secondary structure (l20o~l80o). We apply the technique to glycylglycineHCl (GG) and extract a value for P that matches the diffraction-determined value (P= 162.10) with a precision of q5g. Feng, et. al., have recently proposed a novel, magic angle spinning (MAS)-based method for measuring torsion angles about 13CH1 3CH bonds (denoted 2Q-Heteronuclear Local Field (HLF)) by examining the relative orientation of the two directly-bonded 13C-1H dipolar interactions (Feng, 1996). A similar proposal has been made for 15NH-13CH torsion angles (Hong, 1997). These experiments are examples of a general class of measurements which correlate spatially anisotropic spin interactions such as the dipolar coupling and the chemical shift anisotropy (CSA) to extract structural information. Because these techniques correlate large spin interactions, they are more easily extended to multiply-labeled samples, whereas the weak internuclear dipolar interactions whose magnitudes provide useful distance information may be obscured by strong couplings between directly-bonded nuclei. Here we describe a modification of the 2Q-HLF concept applied to a 15N-1 3C-1 3C-1 5N (NCCN) spin quartet. Creation of 13C2 DQC, followed by dephasing solely under the influence of the 15N-13C heteronuclear couplings, will yield a DQC dephasing curve whose shape provides information about the relative orientation of the 15N-13C couplings, and hence the '3C-13C torsion angle. This corresponds to the '~P angle along a peptide backbone when the 13C nuclei are directly-bonded cx- and carbonyl carbons. By recoupling the '5N-13C interactions during the dephasing period, rather than observing their effects during a single rotor cycle at (very) slow spinning speed, we are able to perform the experiment effectively at much higher spinning speeds and with finer time resolution. The result is an experiment that, at least in certain torsion angle regimes, is extremely sensitive to conformation. Because the 13C=O lacks a bound 1H, this represents a unique approach (short of 17O NMR) of applying the 2Q-HLF concept to measuring peptide backbone ~P angles. This provides a general framework for measuring the peptide backbone torsion angle tP in solids that involves 13C-15N dipolar dephasing of 13Ccx-13C o DQ coherence. The specific implementation described here is designed to function effectively at relatively high spinning speeds. The region of high sensitivity to conformation (~P= 1 2O~- 1800) corresponds roughly to the B-sheet structural regime. The extended nature of this type of secondary structure has made it difficult to accurately define the structural details of peptides in which it occurs using distance measuring techniques with outer ranges of 5-6 A. Application of the NCCN 2Q-HLF technique to these systems should be particularly useful in delineating the precise nature of their B-sheet conformation.