Experimental scheme for TROSY-type two-dimensional ^{1}H,^{15}N correlation spectroscopy. In the rows marked ^{1}H and ^{15}N, narrow and wide bars stand for nonselective 90° and 180° rf-pulses, respectively. Water suppression is achieved by watergate (34), using the two off-resonance rf-pulses indicated by curved shapes. The ^{1}H and ^{15}N carrier frequencies are placed at 9 and 127 ppm, respectively. The delay τ_{1} corresponds to 1/(4^{1}*J*(^{1}H,^{15}N)) = 2.7 ms. Phases used are ψ_{1} = {y,−y,−x,x,y,−y,−x,x}; ψ_{2} = {4(x),4(-x)}; φ_{1} = {4(y),4(-y)}; φ_{2} (receiver) = {x,−x,−y,y,x,−x,y,−y}; x on all other pulses. The row marked PFG (pulsed field gradient) indicates the applied magnetic field gradients along the *z*-axis: G_{1}, amplitude = 30 G/cm, duration = 0.4 ms; G_{2}, −60 G/cm, 1 ms; G_{3}, 50 G/cm, 0.4 ms; G_{4}, 48 G/cm, 0.6 ms. Two free induction decays are recorded per *t*_{1} delay, with ψ_{1} incremented by 90° in between and stored as the real and imaginary parts of the interferogram in *t*_{1}. The Fourier transformation results in a two-dimensional ^{1}H,^{15}N correlation spectrum that contains only the component of the four-line ^{15}N–^{1}H multiplet that has the slowest *T*_{2} relaxation rates for both nuclei. With this scheme, DD/CSA relaxation interference, which has been known for many years (35, 36), can be used to extend the limits of protein NMR.

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