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

The application of high-resolution solid-state C-13 NMR spectroscopy t o investigate the bending between carbon monoxide and myoglobin is exp lored. Selective pulse sequences (non-quaternary suppression and SELDO M) significantly reduce the problem of (CO)-C-13 peaks overlapping wit h those arising from the natural C-13 abundance myoglobin molecule. Th is enables the (CO)-C-13 spinning sideband manifold to be measured and , hence, the principal components of the (CO)-C-13 chemical shift tens er 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 li near Fe-C-O moieties. FTIR measurements of both myoglobin samples show that the major nu(co) stretching frequency is due to the A(3) conform er. 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 histi dine.