Figure 3.

[¹⁵N,¹H] correlation experiments using CRINEPT-based polarization transfer. (a) Fully relaxation-optimized [¹⁵N,¹H]-CRINEPT-TROSY. (b) [¹⁵N,¹H]-CRINEPT-HMQC-[¹H]-TROSY. Narrow and wide black bars indicate nonselective 90 and 180° pulses applied at the proton (¹H) or nitrogen (¹⁵N) frequency. Sine-bell shapes on the line marked ¹H indicate selective 90° pulses on the water resonance to keep the water magnetization along +z during the whole sequence (17, 30). The optimal length of the period T, as set according to Eq. 7, is typically between 3 and 5.4 ms. On the line marked PFG, rectangles indicate duration and strength of pulsed magnetic field gradients applied along the z axis. (a) G₀: 400 μs, 15 G/cm; G₁: 400 μs, 15 G/cm. (b) G₁: 1,000 μs, 30 G/cm. The phase cycles for both experiments are φ₁ = {x, −x}, ψ₁ = {x, −x}. All other radio-frequency pulses are applied with phase x, unless labeled differently. In a, two free induction decays are recorded for each t₁ increment, with ψ₂ = {x, x} and ψ₂ = {−x, −x}, respectively, and are added with a 90° phase correction in both dimensions. Quadrature detection in the ¹⁵N(t₁) dimension is achieved by the States-TPPI method (31) applied to the phase ψ₁. The water-selective pulses have a Gaussian shape and a length of 1.3 ms.