Proton assisted insensitive nuclei cross polarization

Abstract

This communication presents a solid-state NMR ¹⁵N-¹³C polarization transfer scheme applicable at high B₀ and high MAS frequencies, requiring moderate r.f. powers (~50 kHz ¹³C/¹⁵N) and mixing time (1–6 ms). The sequence, PAIN-CP, involves the abundant nearby protons in the heteronuclear recoupling dynamics, and provides a new tool for obtaining long distance ¹⁵N-¹³C contacts. It should be of major interest for biomolecular structural studies.

Figure 1

PAIN-CP ¹⁵N-¹³C correlation pulse sequence. The proper combination of ¹⁵N, ¹³C and ¹H r.f. power results in enhanced rates and efficiency of ¹H mediated ¹⁵N-¹³C polarization transfer.

Figure 2

Comparison of ¹⁵N-¹³C two-bond polarization transfer for PAIN-CP, DCP, TEDOR, GATE AC sequences at ω_r/2π=20 kHz. Note that variants of DCP such as RFDRCP, SPICP, and iDCP are not considered here as they mainly improve the recoupling bandwidth, which is not the major concern in this simulation.

Figure 3

Aliphatic region of 2D ¹⁵N-¹³C correlation spectra obtained at 750 MHz with 20 kHz MAS: (a) DCP with 3 ms mixing, (b) PAIN-CP with 4 ms mixing. The ¹H r.f. field strength was 112 and 62 kHz for (a) and (b) respectively. In (a) the n=1 ZQ Hartmann-Hahn condition was matched with 45 kHz ¹³C r.f. and 25 kHz ¹⁵N r.f.. In (b) ω₁/2π = 50 kHz for both ¹³C and ¹⁵N. All spectra were acquired and processed in exactly the same manner. The contour levels are set to the same value.