Solid-State 17O NMR of Amino Acids

Abstract

17O solid-state NMR from 14 amino acids is reported here, greatly increasing the number investigated. In most cases well-separated resonances from carbonyl and hydroxyl oxygens with distinct second-order quadrupolar line shapes are observed using a 600 MHz spectrometer with fast magic angle spinning (MAS). This is in contrast to the motionally averaged resonances usually seen from amino acids in solution. For amino acids double-angle rotation (DOR) produces a decrease in the line width by more than a factor of 40, providing very high resolution, ∼1 ppm, spectra. The oxygen lines in alanine and the carbonyl oxygens in l-glutamic acid hydrochloride are assigned using 1H-decoupled DOR. The NMR interaction parameters for amino acids show a wide variation of χQ, from 6.4 to 8.6 MHz, η, from 0.0 to 0.9, and δiso, from 83 to 353 ppm. The high quality of the MAS NMR line shapes obtained at 14.1 T means that even small changes in parameters can be very accurately deduced, offering the possibility of 17O NMR as a sensitive probe of structural changes in these and related compounds. The d- and l-forms of glutamic acid hydrochloride are shown to have the same NMR parameters to within error, which are very different from those reported in the literature for the d,l-form. A strong correlation (∼−1200 ppm/Å) is found between δiso and the C−O bond length of the carbonyl oxygens. On the basis of these data, enriching specific amino acids in more complex polypeptides and proteins could provide site-selective information about the bonding and functionality of different sites in biomolecules. An estimate is made of the possible detection limit for such species.

Solid-State 17O NMR of Amino Acids

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