Fig. 2.

The ¹H R₂ weighted ¹⁵N HSQC-AP pulse experiment. a Pulse sequence used for the experiment. Hard 90° and 180° pulses are used for both ¹H and ¹⁵N channels, using phase x when undefined. Phase cycling: φ₁ = x, −x, y, −y; φ₂ = 2(y), 2(x); φ₃ = 2(x), 2(−x); φ₄ = 4(x), 4(−x); φ_rec = x, −x, −x, x, −x, x, x, −x. PFG gradients of 200 μs were used, with 100 μs for gradient recovery. Standard parameters to measure fast relaxing nuclei as follows: aq = 47 ms, T = ranging from 200 μs to 10 ms, recycle delay = 150 ms. b Comparison between different R₂ relaxation building blocks. Considering the relaxation building block shown on the left panel (Donaldson 2001; Iwahara et al. 2007), using τ_a = 2.65 ms and T/4 = 1.8 ms as shortest possible delay, the exponential decay can be measured from about 12.5 ms. Signals with R₂ values over 70 s⁻¹ will be lost or detected at very weak intensities for few points. In the R₂-weighted building block shown in the right panel, they could be easily measured for a sufficient number of time delays in the range 0–10 ms, provided signals have enough S/N. The time scales of the two curves highlight the complementarity of the two experiments