Improvement of carbon-detected proton-proton-TOCSY experiments by employing a DEPT transfer
H. Kessler, G. Gemmecker, M. Köck, R. Osowski, P. Schmieder
Magn. Reson. Chem. (1990) 28, 62-67
A new version of the 2D combination of TOCSY and H,C-COSY is proposed which uses the DEPT technique for the heteronuclear transfer. Hence the resulting 2D spectrum can be easily corrected to pure absorption mode for optimum resolution in both dimensions. In contrast to an earlier published version with a partially refocused INEPT sequence and delayed heteronuclear decoupling in F2, the new variant does not generate any base-line distortions interfering with interesting signals. In addition, experimental spectra obtained from cyclosporin A and a cyclic hexapeptide also show improved sensitivity compared with the INEPT version. In all these experiments, the information about direct H-C connectivities is lost owing to the TOCSY transfer preceding the heteronuclear transfer. This information can, however, be provided by a version with only partial heteronuclear decoupling in F1, which can be easily obtained by shifting the refocusing 180° 13C pulse out of the center of t1. Thus all couplings can be reduced to a fraction of their original value, as long as the large 1J coupling still causes a visible doublet splitting in the 2D spectrum. The reduced heteronuclear couplings lead to a spectrum which is easy to interpret, since the resulting small splittings of the direct cross-peaks are unlikely to interfere with other signals. In addition, all long-range couplings in F1 are also scaled down, thus causing less line broadening and therefore higher sensitivity.