Backbone conformational constraints in a microcrystalline U- ¹⁵N-labeled protein by 3D dipolar-shift solid-state NMR spectroscopy

Structural studies of uniformly labeled proteins by magic-angle spinning NMR spectroscopy have rapidly matured in recent years. Site-specific chemical shifts of several proteins have been assigned and structures determined from 2D or 3D data sets containing internuclear distance information. Here we demonstrate the application of a complementary technique for constraining protein backbone geometry using a site-resolved 3D dipolar-shift pulse sequence. The dipolar line shapes report on the relative orientations of ¹H-¹⁵N[i] to ¹H-¹⁵N[i+1] dipole vectors, constraining the torsion angles φ[i] and ψ[i]. In addition, from the same 3D data set, several ¹H-¹⁵N[i] to¹H-¹⁵N[i+2] line shapes are extracted to constrain the torsion angles φ[i], ψ[i], φ[i+1], and ψ[i+1]. We report results for the majority of sites in the 56-residue β1 immunoglobulin binding domain of protein G (GB1), using 3D experiments at 600 MHz ¹H frequency. Excellent agreement between the SSNMR results and a new 1.14 Å crystal structure illustrate the general potential of this technique for high-resolution structural refinement of solid proteins.