Suppression of spinning sidebands in magic-angle-spinning NMR spectroscopy

Abstract

In our group magic-angle-spinning (MAS) i3C NMR spectroscopy has been used to study tobacco mosaic virus (TMV), a well-known virus with a molecular weight of 42 X 106, that consists of 2200 identical coat protein subunits and a single RNA chain of 6600 nucleotides. Well-resolved MAS 13C NMR spectra have been obtained in which several carbon resonances of amino acids of the coat protein can be assigned, which are not observable in conventional high-resolution NMR spectroscopy (I). The appearance of spinning sidebands in MAS 13C NMR spectra of this large biomolecule is a severe problem since interesting resonances can be obscured by the spinning sidebands. This is especially the case at high NMR measuring frequencies (i.e., 75 MHz), even at the maximal achievable spinning frequency (i.e., ~4.5 kHz). In Fig. 1 a comparison is given of the MAS 13C NMR spectra of TMV at 45 and 75 MHz. Note that at 75 MHz the spinning sidebands of the carbonyl resonances strongly overlap with the aromatic resonances and vice versa. Also observe a dramatic increase of the intensity of the spinning sidebands in the 75-MHz spectrum as compared to the 45-MHz one. To make full use of the advantages of MAS NMR at high measuring frequencies (i.e., an increase of signal-to-noise ratio and resolution), a suitable technique for removing or suppressing the spinning sidebands would be extremely valuable to properly analyze complicated spectra as shown in Fig. 1. A number of methods have been proposed that identify or suppress spinning sidebands (2-6). For MAS NMR spectra of biosystems in which resonances appear over a broad range of chemical shift values, a method that shifts the phase of the spinning sidebands in combination with coaddition of spectra (6) seems to be the most promising one. Dixon (6) utilized a pulse sequence of four 180” pulses to alter the phase of the spinning sidebands. In this communication, we introduce a simple alternative in which a single 180” pulse is used to alter the phase of the spinning sidebands. In addition we demonstrate that the effects of a 180” pulse on spinning sidebands can easily be observed in a model system consisting of a spinning sample in a conventional high-resolution NMR spectrometer. We also show some calculations explaining these effects.