Chemical shift-dependent apparent scalar couplings: an alternative concept of chemical shift monitoring in multi-dimensional NMR experiments.

The paper presents an alternative technique for chemical shift monitoring in a multi-dimensional NMR experiment. The monitored chemical shift is coded in the line-shape of a cross-peak through an apparent residual scalar coupling active during an established evolution period or acquisition. The size of the apparent scalar coupling is manipulated with an off-resonance radio-frequency pulse in order to correlate the size of the coupling with the position of the additional chemical shift. The strength of this concept is that chemical shift information is added without an additional evolution period and accompanying polarization transfer periods. This concept was incorporated into the three-dimensional triple-resonance experiment HNCA, adding the information of (1)H(alpha) chemical shifts. The experiment is called HNCA(coded)HA, since the chemical shift of (1)H(alpha) is coded in the line-shape of the cross-peak along the (13)C(alpha) dimension.