Abstract
A selective saturation pulse at fast magic angle spinning (MAS) frequencies (60+kHz) suppresses t1 noise in the indirect dimension of two-dimensional 1H MAS NMR spectra. The method is applied to a synthetic nucleoside with an intense methyl 1H signal due to triisopropylsilyl (TIPS) protecting groups. Enhanced performance in terms of suppressing the methyl signal while minimising the loss of signal intensity of nearby resonances of interest relies on reducing spin diffusion – this is quantified by comparing two-dimensional 1H NOESY-like spin diffusion spectra recorded at 30–70kHz MAS. For a saturation pulse centred at the methyl resonance, the effect of changing the nutation frequency at different MAS frequencies as well as the effect of changing the pulse duration is investigated. By applying a pulse of duration 30ms and nutation frequency 725Hz at 70kHz MAS, a good compromise of significant suppression of the methyl resonance combined with the signal intensity of resonances greater than 5ppm away from the methyl resonance being largely unaffected is achieved. The effectiveness of using a selective saturation pulse is demonstrated for both homonuclear 1H–1H double quantum (DQ)/single quantum (SQ) MAS and 14N–1H heteronuclear multiple quantum coherence (HMQC) two-dimensional solid-state NMR experiments.
Highlights
Two-dimensional homonuclear and heteronuclear 1H solidstate NMR spectroscopy is a powerful analytical tool for characterising non-covalent interactions [1,2,3,4], including hydrogen bonding [5] and p–p interactions [6], which direct the arrangement of small and moderately sized organic molecules in the solid state [7]
Nitrogen–proton interactions [14,15,16], and notably hydrogen bonding in, for example, guanosine self-assembly [17,18] and pharmaceuticals [19,20,21]. These and other two-dimensional 1H magic angle spinning (MAS) experiments e.g. NOESY-like SQ/SQ spin diffusion and 13C–1H heteronuclear correlation experiments, are finding increasing application [22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]. In such two-dimensional 1H MAS spectra, strong NMR signals due to alkyl protons can lead to excessive t1 noise, presenting as long trails emanating from that resonance in the solid-state spectra, considerably detracting from the appearance and viewability of such spectra as observed, for instance, for nucleic acid derivatives [17]
This phenomenon is caused by relaxation processes induced by the conformational flexibility of those functional groups. Such t1 noise is frequently observed in solution-state NMR, often as a result of residual proton signal from deuterated solvents, where it is known that the t1 noise is proportional to the signal strength [42]
Summary
Two-dimensional homonuclear and heteronuclear 1H solidstate NMR spectroscopy is a powerful analytical tool for characterising non-covalent interactions [1,2,3,4], including hydrogen bonding [5] and p–p interactions [6], which direct the arrangement of small and moderately sized organic molecules in the solid state [7]. In this paper, we show that by employing a simple selective pulse prior to the main pulse sequence, and in combination with fast MAS (60+ kHz), it is possible to dramatically improve the appearance of two-dimensional homo- and heteronuclear experiments in the solid state by essentially eliminating or severely reducing the intensity of a strong perturbing signal, minimising t1 noise The effectiveness of this method is demonstrated using the DNA base analogue 5-iodo-20-deoxy-30,50-di(trii sopropylsilyl[TIPS])cytidine [55] referred to as compound 1 (C27H52IN3O4Si2); such pyrimidine (and purine) derivatives have manifold applications in the rapidly evolving area of molecular self-assembly [56,57,58,59,60,61,62,63,64,65]
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