Boosting the NMR Assignment of Carbohydrates with Clean In-Phase Correlation Experiments

Novel CLIP-COSY based homo- and heteronuclear correlation experiments are reported for the rapid, semi-automated NMR assignment of small to medium-sized molecules. The homonuclear CLIP-COSY and corresponding relayed experiments employ the perfect-echo based mixing sequence for in-phase coherence transfer between directly and/or indirectly coupled proton spins. The combined analysis of the resulting CLIP-COSY and relayed spectra made it possible to easily track down, layer by layer, the proton-proton connectivity network. In larger molecules the narrow chemical shift range of protons may, however, compromise the efficacy of the homonuclear correlation based assignment strategy. To overcome this limitation, an HSQC variant of the CLIP-COSY experiment has now been devised. Combined treatment of HSQC-CLIP-COSY (relayed) and standard HSQC spectra facilitates simultaneous and semi-automatic assignment of ¹ H and ¹³ C resonances of medium-sized molecules, such as pentasaccharides. The recently introduced PSYCHE broadband homonuclear decoupling scheme has been also implemented into the devised homo- and heteronuclear CLIP-COSY based experiments, resulting in fully decoupled high-resolution pure-shift correlation spectra.