SOLID-STATE C-13 NMR EVIDENCE FOR A LARGE DEVIATION FROM LINEARITY OF THE FE-C-O UNIT IN THE CO COMPLEX WITH MYOGLOBIN

The application of high-resolution solid-state C-13 NMR spectroscopy to investigate the bending between carbon monoxide and myoglobin is explored. Selective pulse sequences (non-quaternary suppression and SELDOM) significantly reduce the problem of (CO)-C-13 peaks overlapping with those arising from the natural C-13 abundance myoglobin molecule. This enables the (CO)-C-13 spinning sideband manifold to be measured and, hence, the principal components of the (CO)-C-13 chemical shift tenser to be obtained. Results were obtained on two samples of myoglobin: one a dry powder and the other carefully prepared needle-like crystals containing water of crystallization. The spectra show that there is a large increase in the asymmetry of the C-13 Shielding tenser in (CO)-C-13-myoglobin compared to heme model compounds containing close to linear Fe-C-O moieties. FTIR measurements of both myoglobin samples show that the major nu(co) stretching frequency is due to the A(3) conformer. It can be concluded that in this particular CO-myoglobin substate there must be substantial deviation from linearity of the Fe-C-O unit, probably due to a significant polar interaction with the distal histidine.