Pure-shift NMR spectroscopy

NMR spectroscopy is one of the most powerful and most often used analytical techniques for the structural characterization of organic, inorganic and small to medium sized bio-molecules. The nucleus most often recorded is H-1 because of its high sensitivity and widespread occurrence. However, compared to other nuclei, proton NMR spectroscopy suffers from its poor signal dispersion, which results from broad multiplets due to homonuclear scalar coupling to other hydrogen nuclei in the vicinity. We have shown previously that homonuclear broadband decoupled NMR spectra can be obtained by a combination of frequency and spatially-selective pulses. This approach leads to proton spectra consisting of single lines, reminiscent of proton-decoupled C-13 spectra. The resulting resolution is comparable to regular H-1 spectra acquired at several GHz. However, this method, which is meanwhile referred to as pure-shift (or also Zangger-Sterk) method is very insensitive due to the spatially restricted excitation and the necessity to acquire a series of one-dimensional spectra which is then processed to result in one decoupled spectrum. Within this project we will establish two new techniques which are aimed at alleviating these shortcomings and will significantly increase the sensitivity of pure shift NMR spectra. These novel approaches aim at fast pulsing without the need for an interscan relaxation delay by continuous frequency shifting of the used selective pulses and using homonuclear decoupling during acquisition. In addition to homonuclear broadband decoupling, these methods will also be used for acquiring very fast series of NMR spectra (up to ~50 spectra per second) to monitor fast reaction kinetics and to acquire multidimensional homonuclear NMR spectra without diagonal peaks.

NMR spectroscopy is one of the most useful techniques for the structural characterization of organic and biomolecules. Due to its widespread occurrence and high sensitivity, hydrogen nuclei (protons) are most often used in NMR spectroscopy. However, the resolution of proton NMR is rather limited. The main reason for that are signal splitting by scalar coupling between neighboring hydrogen nuclei. The aim of pure shift NMR is the removal of all signal splittings to enhance the resolution of hydrogen NMR spectra. Within this project we developed a method for generally applicable real-time pure shift spectra. In contrast to previously used techniques, there is no special data processing needed anymore. Pure shift NMR spectra enable much better signal dispersion in the spectra, but all information about scalar coupling is lost. Often it is necessary to determine these coupling patterns with high accuracy. For this purpose we could acquire spectra with enlarged scalar coupling. This real-time J-upscaling experiment provides structurally relevant coupling information with higher resolution than regular NMR spectra. Using the methods developed within this project we are providing tools to record NMR spectra with higher resolution with respect to both signal separation (chemical shift) but also signal splittings (scalar coupling). Such resolution enhanced spectra can be used not only to acquire more accurate structures of organic molecules and compound mixtures, but should also prove useful for higher quality protein NMR spectra. This in turn should result in higher accuracy protein NMR structures, which are needed for example for structure guided drug design.