A test of the role of electrostatic interactions in determining the CO stretch frequency in carbonmonoxymyoglobin

The vibrational frequency of CO bound to myoglobin can be varied by up to 60 cm-1 by making site-specific mutations in the distal pocket. These changes may result from specific chemical interactions between distal amino acids and the CO or from changes in the electrostatic field of the distal pocket. In this paper, we separate the relative contributions of these two effects by comparing the IR spectra of the carbonmonoxy complexes of human myoglobin mutants V68N, V68D, and V68E. The effect of replacing valine with these polar amino acids on the electrostatic environment of the distal heme pocket has been independently determined earlier by measurements of the heme reduction potential and electronic absorption spectral band shifts. While all three mutations result in a negative dipole pointing towards the CO ligand, the CO stretch frequency shifts differently in each case. These differences are attributed to specific chemical interactions between the amino acids and the CO ligand.