Fig 1

(a) Pulse sequence for zero-quantum homonuclear correlation spectroscopy under fast or ultrafast MAS conditions; no 1H decoupling was applied during the mixing period. Various RNN1 symmetry sequences can be applied during the mixing time, including (b) basic RNnv (named RN), super phase cycled (c) RNnv RNn?v (named RNi), (d) [RNnv RNn?v]0[RNnv RNn?v]180 (named RNixix), (e) [RNnv RNn?v]0[RNnv RNn?v]120[RNnv RNn?v]240 (named RNixy3), (f) [RNnv RNn?v]0[RNnv RNn?v]90[RNnv RNn?v]180[RNnv RNn?v]270 (named RNixy4) symmetry sequences. Phase cycling for pulses shown in Fig. 1a: Φ1 = 1, 3; Φ2 = 0; Φ3 = 0, 0, 2, 2; Φ4 = 1, 1, 1, 1, 3, 3, 3, 3; Φ5 = 0, 0, 0, 0, 0, 0, 0, 0, 2, 2, 2, 2, 2, 2, 2, 2; Φrec = 1, 3, 3, 1, 3, 1, 1, 3, 3, 1, 1, 3, 1, 3, 3, 1"

Table 1

Numbers of cross terms recoupled in the second-order average Hamiltonian for basic RNN1 sequences."

Fig 2

Transfer efficiency plots of various RN-type symmetry sequences for ZQ recoupling as a function of frequency difference between spin pairs, Δδiso, at the MAS frequencies of 40 and 20 kHz. The mixing time is 2.4 ms for (a) R4, (b) R6, (c) R8-based sequences at the MAS frequency of 40 kHz and 4.8 ms for sequences (d) R4, (e) R6, (f) R8-based sequences at the MAS frequency of 20 kHz. No proton decoupling was applied during the mixing period. 13C RF field strength for π pulses applied in RN symmetry sequences was 60 kHz for the MAS frequencies of 40 and 20 kHz.00Fig. 2 Transfer efficiency plots of various RN-type symmetry sequences for ZQ recoupling as a function of frequency difference between spin pairs, Δδiso, at the MAS frequencies of 40 and 20 kHz. The mixing time is 2.4 ms for (a) R4, (b) R6, (c) R8-based sequences at the MAS frequency of 40 kHz and 4.8 ms for sequences (d) R4, (e) R6, (f) R8-based sequences at the MAS frequency of 20 kHz. No proton decoupling was applied during the mixing period. 13C RF field strength for π pulses applied in RN symmetry sequences was 60 kHz for the MAS frequencies of 40 and 20 kHz."

Fig 3

Simulated dependence of homonuclear polarization transfer efficiency on RF field strength and the frequency difference between spin pairs at the MAS frequency of 40 kHz, for the following sequences: (a)–(c) basic R4, R6 and R8; (d)–(f) R4i, R6i and R8i; (g)–(i) R4ixy3, R6ixy3, and R8ixy3; (j)–(l) R4ixy4, R6ixy4 and R8ixy4. The mixing time was 2.4 ms. Labels A and B shown in (g) correspond to regions with relatively low transfer efficiency. 66 REPULSION angles (α, β) and 12 γ were used to get the powder average."

Fig 4

Transfer efficiency plots of R4-based (a) and R6-based (b) symmetry sequences for ZQ homonuclear recoupling as a function of Δδiso, corresponding to the traces extracted from Fig. 3g and Fig. 3h, respectively. Black, red, green, blue and purple lines correspond to the RF field strengths of 20, 60, 80, 100 and 200 kHz, respectively."

Fig 5

Transfer efficiency plots for various R4 and R6-type symmetry sequences with or without 1H-decoupling as a function of frequency difference between spin pairs, Δδiso, at MAS frequencies of 40 kHz (a)–(d) and 20 kHz (e)–(h). High power proton decoupling with RF field strength of 250 kHz was applied during the mixing period wherever indicated; the other simulation parameters are same as in Fig. 2."

Fig 6

Simulated dependencies of transfer efficiencies on RF mismatch and frequency difference between spin pairs at MAS frequency of 40 kHz, for R4-based schemes: (a) R4, (b) R4xy3, (c) R4i, (d) R4ixix, (e) R4ixy3, and (f) R4ixy4. The standard RF filed strength was 60 kHz, and was varied from 51 to 69 kHz, corresponding to RF mismatch up to ±15%. The mixing time was 2.4ms. 66 REPULSION angles (α, β) and 12 γ were used to get the powder average."

Fig 7

Simulated dependencies of transfer efficiencies on RF mismatch and frequency difference between spin pairs at MAS frequency of 40 kHz, for R6-based schemes: (a) R6, (b) R6xy3, (c) R6i, (d) R6ixix, (e) R6ixy3, and (f) R6ixy4. All other NMR simulation parameters are same as those in Fig. 6."

Fig 8

2D 13C–13C correlation NMR spectra of U-13C, 15N-enriched histidine recorded at the MAS frequency of 40 kHz by (a) R4, (b) R6, (c) R4ixy3 and (d) R4ixy4 recoupling sequences with the mixing time of 2.4 ms. 1D traces for representative carbon atoms are labeled. The first contour in all spectra is set at 4 × σ (σ is the noise rmsd (root-mean-square deviation))."

Fig 9

1D traces extracted from R6 -based 2D 13C–13C correlation NMR spectra of U-13C, 15N-enriched histidine at the MAS frequency of 40 kHz, corresponding to (a) Cα, (b) Cβ and (c) Cγ in ω1 dimension. The mixing time was 2.4 ms."

Fig 10

1D traces extracted from R6 -based 2D 13C–13C correlation NMR spectra of U-13C, 15N-enriched histidine at the MAS frequency of 40 kHz, corresponding to (a) Cα, (b) Cβ and (c) Cγ in ω1 dimension. The mixing time was 2.4 ms. Slices extracted from R4ixy3 and R4ixy4 are also included for comparison."

Fig 11

1D traces extracted from R4 and R6-based 2D 13C–13C correlation NMR spectra of U-13C, 15N-enriched histidine with or without high-power CW 1H decoupling at the MAS frequency of 25 kHz, corresponding to (a) Cα, (b) Cβ and (c) Cγ in ω1 dimension. The mixing time was 3.84 ms."

Fig 12

13C–13C correlation spectra of microcrystalline U-13C, 15N-enriched LC8 recorded at the MAS frequency of 40 kHz using (a) R4, (b) R6, (c) R4i, (d) R6i and (e) R4ixy3 recoupling sequences. The mixing time was 2.4 ms. The first contour in all spectra is set at 4 × σ (σ is the noise rmsd)."

Fig 13

13C-13C correlation spectra recorded on LC8 protein at 40 kHz MAS rate for R4ixy3 recoupling sequences with (a) 2.4 ms and (b) 9.6 ms mixing time. The first contour in all spectra is set at 4 × σ (σ is the noise rmsd)."