Abstract
Spectral resolution in proton NMR spectroscopy is reduced by the splitting of resonances into multiplets due to the effect of homonuclear scalar couplings. Although these effects are often hidden in protein NMR spectroscopy by low digital resolution and routine apodization, behind the scenes homonuclear scalar couplings increase spectral overcrowding. The possibilities for biomolecular NMR offered by new pure shift NMR methods are illustrated here. Both resolution and sensitivity are improved, without any increase in experiment time. In these experiments, free induction decays are collected in short bursts of data acquisition, with durations short on the timescale of J-evolution, interspersed with suitable refocusing elements. The net effect is real-time (t 2) broadband homodecoupling, suppressing the multiplet structure caused by proton–proton interactions. The key feature of the refocusing elements is that they discriminate between the resonances of active (observed) and passive (coupling partner) spins. This can be achieved either by using band-selective refocusing or by the BIRD element, in both cases accompanied by a nonselective 180° proton pulse. The latter method selects the active spins based on their one-bond heteronuclear J-coupling to 15N, while the former selects a region of the 1H spectrum. Several novel pure shift experiments are presented, and the improvements in resolution and sensitivity they provide are evaluated for representative samples: the N-terminal domain of PGK; ubiquitin; and two mutants of the small antifungal protein PAF. These new experiments, delivering improved sensitivity and resolution, have the potential to replace the current standard HSQC experiments.Electronic supplementary materialThe online version of this article (doi:10.1007/s10858-015-9913-z) contains supplementary material, which is available to authorized users.
Highlights
To understand relationships between biological structure and function, we need tools for the study of complex systems, such as proteins and oligonucleotides, that have large numbers of constitutionally very similar elements
In the real-time pure shift spectra, all doublets have collapsed to singlets
The results shown clearly demonstrate that the pure shift methods proposed here are suitable for the study of small proteins at low (15N labelled) and standard concentrations
Summary
To understand relationships between biological structure and function, we need tools for the study of complex systems, such as proteins and oligonucleotides, that have large numbers of constitutionally very similar elements. Recent improvements in non-uniform sampling (Mayzel et al 2014; Mobli and Hoch 2014) can shorten the overall durations of multidimensional NMR experiments, and can be used to increase resolution in indirect dimensions. Improvements in the basic resolution of standard protein 2D NMR experiments can translate directly into the more complex nD experiments that share building blocks, so improving the resolution of the basic HSQC experiment is a priority. This can generally only be achieved by using higher magnetic fields. While adequate sensitivity is usually available from cryogenically cooled NMR probes at high magnetic fields (Styles and Soffe 1984; Kovacs et al 2005) spectral resolution in the direct proton dimension remains a fundamental limiting factor in current biomolecular NMR applications
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