HECADE: HMQC- and HSQC-based 2D NMR experiments for accurate and sensitive determination of heteronuclear coupling constants from TOCSY-type cross peaks

Two-dimensional pulse sequences for the determination of heteronuclear long-range coupling constants are presented. The sequences are based on the HMQC/HMBC or HSQC technique with subsequent optional homonuclear I-spin transfer. However, they yield tilted cross-peak patterns displaying antiphase heteronuclear coupling constants in the projections of both dimensions, which allow accurate determination of the couplings even in cases where the linewidth is of comparable magnitude. Two characteristic pulse-sequence elements were implemented to shape the F-1 domain: the first element allows an arbitrary scaling of the heteronuclear coupling splittings relative to S-spin chemical-shift differences, whereas the second element achieves homonuclear broadband decoupling among the I spins in the HMQC/HMBC experiments and thus allows purely absorptive representations of such spectra. In comparison with established (omega(1)) X-half-filtered TOCSY spectra, the signal dispersion in F-1 is significantly improved and largely under experimental control. Furthermore, heteronuclear couplings of (I-1, S) pairs where S is either quaternary or carries one or more I spins that do not belong to the same I-coupling network as I-1 can also be measured, The implementation of pulsed field gradients results in good suppression of spectral artifacts. (C) 1997 Academic Press.