PSYCHE NMR: High-resolution not all in the mind
- Published: Jun 15, 2014
- Author: David Bradley
- Channels: NMR Knowledge Base
Researchers at the University of Manchester, UK, and colleagues have developed an ultra-high-resolution NMR spectroscopy technique, which they have given the wonderful acronym PSYCHE, for pure shift yielded by chirp excitation.
The new technique is flexible and general and is much more sensitive than most other broadband homonuclear decoupling approaches. It also gives clean spectra and has a tolerance for strong coupling. Manchester's Mohammadali Foroozandeh, Ralph Adams, Nicola Meharry and Gareth Morris together with Mathias Nilsson (University of Copenhagen, Denmark) and Damien Jeannerat (University of Geneva, Switzerland) demonstrated proof of principle by comparing the partial spectra of the sex hormone estradiol in deuterated dimethyl sulfoxide obtained by normal proton NMR spectroscopy and PSYCHE.
Far better than adequate
Morris and colleagues explain that adequate spectral resolution is critical to a successful NMR experiment, but resolution is limited by the instrumentation available. There have been many developments recently in the area of pure shift pulse-sequences that give resolution a boost, but the downside is that they greatly reduce the sensitivity of an already relatively insensitive spectroscopic technique. PSYCHE largely avoids this disheartening compromise.
Soon after the invention of NMR spectroscopy spectroscopists realised that resolution might be boosted if the effects of homonuclear spin-spin couplings could be suppressed. Unfortunately, simple as that sounds it has proved to be a continuing frustration in the quest for clearer spectra. The pure shift approach essentially gives spectra consisting of a single signal for each chemically distinct site in the molecule being analysed; but much sensitivity is lost. Morris's team has now demonstrated that their PSYCHE technique can produce pure shift spectra with ten times the sensitivity of competing approaches.
Their approach uses a pair of low flip angle swept-frequency pulses and a weak magnetic field gradient. The method is related to anti-z-COSY, but does not suffer from the long minimum acquisition times and complicated processing that experiment requires. In effect, the two chirp pulses refocus one spin per molecule at a time, allowing both the signals from, and the couplings to, the remaining spins to be suppressed. By adjusting the pulse flip angle, the spectroscopist can balance sensitivity and spectral purity for a given sample to obtain the cleanest spectrum.
The team has pitted PSYCHE NMR against the rather problematic sex hormone estradiol structure, a molecule that normally produces crowded spectra that are difficult to decipher. They have demonstrated close to perfect decoupling for their spectra, generating a single, discrete signal for each chemical shift. Similarly, testing PSYCHE's powers against the cyclic peptide immunosuppressant drug cyclosporin A showed that a significant enhancement is possible even with that complex structure. The team also demonstrated proof of principle with the regulatory protein ubiquitin, which as its name suggests is ubiquitous in our body's tissues, and the macrolide antibiotic azithromycin.
"The PSYCHE method is flexible, general, and offers almost an order of magnitude improvement in performance over existing methods for broadband homonuclear decoupling. It has the potential to find wide application in the NMR spectroscopy of small molecules, NMR-based metabolomics, and biomolecular NMR spectroscopy," the team says.
"The next step with PSYCHE is to integrate the method into the other class of NMR techniques commonly used to gain resolution, multidimensional NMR," Morris told SpectroscopyNOW. "By combining PSYCHE with 2D NMR experiments such as TOCSY and NOESY it should be possible to tackle significantly more complex structural problems than is currently the case; combining it with DOSY, which is very demanding of both signal-to-noise ratio and spectral resolution, should significantly improve our ability to analyse mixtures by NMR." He adds that, "First-generation pure shift NMR experiments are already beginning to be implemented as standard in commercial spectrometer systems, and we expect PSYCHE to become widely available." Morris admits that, "The ultimate goal of a general pure shift approach that simultaneously improves both resolution and sensitivity is probably unachievable, although there is already one technique (Morris et al, Angew Chem, Int. Edn, 2013, 52, 11616) that achieves this for the special case of the HSQC experiment."
Angew Chem Int Edn, 2014, online: "Ultrahigh-Resolution NMR Spectroscopy"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.