Figure 5.2. Pulse sequence for HMQC experiments. The 90° (narrow bars) and 180° (wider bars) are x phase except as indicated. (a) Conventional HMQC with the coherence transfer pathways shown below the pulse sequence. The open 180° pulse is a 13Ca - or -selective pulse, which can be turned on for decoupling 13 Ca-13C^ coupling in the fi dimension since the frequency bands are well-separated with the exception of Gly, Ser, and Thr residues (McCoy, 1995; Matsuo et al., 1996). Phase = x, —x, + States-TPPI and 0rec = x, —x. Delay t = 1 /2/ch. Gradients are applied with duration of 0.5 ms and amplitude of g1 = 10 G cm-1. (b) Constant-time element (Ernst et al., 1987).

The 180° 1H pulse in the middle of f1 refocuses the evolution of the 1H chemical shift during the f1 period and during the t period. Therefore, the magnetization of 1H does not evolve. In addition, the heteronuclear scalar coupling does not affect the evolution of the multiple-quantum coherence Hx Cy during f1. As a result, the only evolution occurring during f1 is at the 13C chemical shift frequency. The multiple-quantum coherence is converted into observable single-quantum magnetization by the 13C 90° pulse combining with delay t, while the zero-quantum term HxCx will not produce observable coherence:

—2HxCy cos(^Ct1)-> —2HxCz cos(^Ct1) — —Hy cos(^Ci1)

In summary,

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