Efficient β-Sheet Identification in Proteins by Solid-State NMR Spectroscopy

An efficient two-dimensional magic-angle-spinning (MAS) NMR technique to selectively detect β-sheet residues in proteins is demonstrated. The method exploits the relative orientation between N−H and Cα−Hα dipolar couplings, that is, the φ torsion angle, in each amino acid residue. The different φ angles between the α-helical and β-sheet conformations give rise to distinctly different dipolar modulations. After one-half of a rotation period of dipolar evolution, β-sheet residues retain significant intensities (20−50%), while α-helical signals vanish for suitable spinning speeds. Thus, a distinction between sheet and helix geometries can be achieved without lengthy measurements of the entire dipolar modulation curve. Combining this constant-time dipolar modulation with 2D 15N−13C correlation, we selected the β-sheet signals and removed the α-helical resonances in mixtures of model amino acids N-acetyl valine and N-tBoc alanine. To demonstrate the suitability of this technique for solid proteins, β-sheet filtration was applied to 13C- and 15N-labeled ubiquitin. Two pulse sequences that are suitable for slow (∼4 kHz) and faster spinning speeds (∼7 kHz) are compared. The technique represents a complementary approach to the recently introduced α-helix filter experiment (Hong, J. Am. Chem. Soc. 122 (2000) 3762−3770) and is expected to further enhance the efficiency and confidence of protein structure determination by solid-state NMR.