4.4. J-correlation experiments

The third set of experiments for measuring couplings relies on the principle referred to as quantitative J-correlation (Blake et al. 1992; Vuister & Bax 1993). In this case, the coupling constants are extracted from either (i) the relative intensities of two cross-peaks in two separate experiments, or (ii) the intensity ratio of diagonal peak and cross-peak in a single experiment. The former can be obtained by allowing the evolution of the coupling of interest during a fixed delay in the first experiment (referred to as the J-modulated experiment), whereas the coupling is effectively decoupled in the second experiment (referred to as the reference experiment). There is a simple trigonometric relation between the cross-peak intensities in the J-modulated and reference experiments, i.e. the reference experiment has the intensity of 1, whereas the J-modulated experiment has a cos(πJT) dependence on the signal intensity. In the latter case (ii), a reference experiment is not necessary since the method is based on the coherence transfer efficiency during a finite delay in an out-and-back-type experiment. The intensity of the diagonal peak has a cosinusoidal dependence, and that of the cross-peak a sinusoidal dependence on the coherence transfer efficiency. Hence, the coupling constant of interest can be calculated from this intensity ratio of cosine- and sine-modulated magnetization (see Section 5.1.4).