Solid-state <sup>17</sup> O NMR of amino acids
Pike KJ., Lemaitre V., Kukol A., Anupõld T., Samoson A., Howes AP., Watts A., Smith ME., Dupree R.
17 O 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 1 H-decoupled DOR. The NMR interaction parameters for amino acids show a wide variation of XQ, from 6.4 to 8.6 MHz, η from 0.0 to 0.9, and δ iso 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 17 O 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 δ iso 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.