Simultaneous recoupling of chemical shift tensors of two nuclei by R-symmetry sequences

Abstract

Chemical shift tensors (CSTs) are sensitive probes of structure and dynamics. R-symmetry pulse sequences (RNCSA) can efficiently recouple CSTs of varying magnitudes in magic angle spinning (MAS) NMR experiments, for a broad range of conditions and MAS frequencies. Herein, we introduce dual-channel R-symmetry pulse sequences for simultaneously recording CSTs of two different nuclei in a single experiment (DORNE-CSA). We demonstrate the performance of DORNE-CSA sequences for simultaneous measurement of ¹³C and ¹⁵N CSTs, on a U-¹³C,¹⁵N-labeled microcrystalline l-histidine. We show that the DORNE-CSA method is robust, provides accurate CST parameters, and takes only half of the measurement time compared to a pair of RNCSA experiments otherwise required for recording the CSTs of individual nuclei. DORNE-CSA approach is broadly applicable to a wide range of biological and inorganic systems.

Keywords: Chemical shift anisotropy; MAS NMR; R-symmetry; Simultaneous acquisition; Symmetry based recoupling.

Figure 1.

CSA space-spin selection diagrams for the first-order symmetry selection rules. a) Symmetry allowed CSA terms in R14₂³. Homonuclear dipolar coupling terms are suppressed [33]. b) Symmetry allowed CSA terms in R14₂⁵. Homonuclear dipolar coupling terms are symmetry allowed [33]. Space-spin selection diagrams of space components m<0 are omitted for simplicity. Every line represents a possible value of mn-μν, and the vertical line on the right represents the symmetry-allowed values with NZ_λ/2 indicated on the right and shown as holes. Red lines are first-order symmetry-allowed CSA terms for a specific R-symmetry sequence, with the corresponding term depicted on the right. Terms possessing a space component m=0 are averaged out by MAS.

Figure 2.

2D R-symmetry based CSA recoupling pulse sequences. a) 2D RNCSA (top) and $R{N}_n^v$ recoupling sequence on channel I with heteronuclear decoupling on channel S (bottom). b) 2D DORNE-CSA (top) and $R{N}_n^{v_I,v_S}$ recoupling sequence on channels I and S. φ₀=x-x; φ₁=xx-x-x; φ_rec=x-x-xx.

Figure 3.

Space-spin selection diagrams for the first-order symmetry selection rules for heteronuclear dipolar couplings. a) Symmetry-allowed heteronuclear dipolar coupling terms in R14₂^3,3. b) Symmetry-allowed heteronuclear dipolar coupling terms in R14₂^3,5. c) Symmetry-allowed heteronuclear dipolar coupling terms in R14₂^5,5. Homonuclear dipolar coupling terms are either symmetry forbidden (for ν=3) or allowed (for ν=5) [33]. Space-spin selection diagrams of space components m<0 are omitted for simplicity. Every line represents a possible value of mn-μν, and the vertical line on the right represents the symmetry-allowed values with NZ_λ/2 indicated on the right and shown as holes. Red lines are first-order symmetry-allowed homonuclear dipolar coupling terms for a specific R-symmetry sequence, with the corresponding term depicted on the right. Terms possessing a space component m=0 are averaged out by MAS.

Figure 4.

Heteronuclear dipolar coupling scaling factor in DORNE-CSA sequences for σ₁-CSA recoupling on one and two channels (squares and circles, respectively), as a function of ω_RF/ω_r, the ratio of the rf amplitude and MAS frequency. |κ| is the absolute value of the scaling factor for heteronuclear dipolar coupling during the R-symmetry pulse sequence. The numerical values are presented in Table 1.

Figure 5.

¹³C and ¹⁵N R14₂³ RNCSA and R14₂^3,3 DORNE-CSA of crystalline L-histidine. a) Chemical structures of bi-protonated (left) and Nε2-protonated (right) L-histidine. b,c) ¹³C and ¹⁵N CPMAS spectra with chemical shift assignments indicated. d-h) ¹³C and ¹⁵N RNCSA and DORNE-CSA line shapes of representative sites shown as dashed black and solid magenta traces, respectively. i) Correlation between ¹³C and ¹⁵N δ_σ extracted from DORNE-CSA and RNCSA experiments for all imidazole ring atoms in crystalline L-histidine, as shown in panels (b) and (c). ¹³C and ¹⁵N |δ_σ| are plotted as circles and triangles, respectively.