3D HSQC-HSQMBC--increasing the resolution of long-range proton-carbon correlation experiments

A 3D HSQC-HSQMBC experiment is proposed for increasing the separation of proton-carbon long-range correlation cross peaks, the lack of which is occasionally seen in corresponding 2D experiments. It is aimed at complex molecules with many protonated carbons exhibiting a narrow spread of 13C chemical shifts e.g., complex carbohydrates. It does not yield long-range correlation of quaternary carbons. An extra indirectly detected 1H dimension of this experiment provides additional separation of long-range correlation cross peaks by utilising the chemical shifts of protons directly attached to 13C. Evolution of single-quantum coherences throughout the entire pulse sequence ensures that the cross peaks are inphase pure absorption singlets in both indirectly detected dimensions, thus maximising the resolution and sensitivity of the experiment. Partial signal cancellation can be expected due to the antiphase character of peaks in the directly detected dimension. The intensity of cross peaks depends on the length of a single long-range evolution interval and values of both active and passive long-range coupling constants of each carbon. The 3D HSQC-HSQMBC experiment provided high quality long-range correlation spectra of a 2 mg pentasaccharide sample in 27 h. The technique can also be used for measurement of long-range heteronuclear coupling constants from pure antiphase multiplets in the directly detected dimension.