Solution deuterium nmr quadrupolar relaxation study of heme mobility in myoglobin

Abstract

NMR Spectroscopy has been used to monitor the quadrupolar relaxation and motional dynamics of 2H selectively incorporated into skeletal and side chain positions of the heme in sperm whale myoglobin. The hyperfine shifts of the heme resonances in paramagnetic states of myoglobin allow resolution of the signals of interest, and paramagnetic contributions to the observed line widths are shown to be insignificant. The 2H line widths for the skeletal positions of deuterohemin-reconstituted myoglobin yield a correlation time identical with that of overall protein tumbling (9 ns at 30 °C) and hence reflect an immobile heme group. The 2H NMR line widths of heme methyl groups exhibit motional narrowing indicative of very rapid internal rotation. Hence the methyl rotation is effectively decoupled from the overall protein tumbling, and the residual quadrupolar line width can be used directly to determine the protein tumbling rate. The 2H NMR lines from heme vinyl groups were found narrower than those from the heme skeleton. However, the range of quadrupolar coupling constants for sp2 hybridized C-2H bonds does not permit an unequivocal interpretation in terms of mobility. © 1989, American Chemical Society. All rights reserved.