We report the complete vibrational spectrum of the probe nucleus ${}^{57}\mathrm{Fe}$ at the oxygen-binding site of the protein myoglobin. The Fe-pyrrole nitrogen stretching modes of the heme group, identified here, probe asymmetric interactions with the protein environment. Collective oscillations of the polypeptide, rather than localized heme vibrations, dominate the low frequency region. We conclude that the heme “doming” mode is significantly delocalized, so that distant sites respond to oxygen binding on vibrational time scales. This has ramifications for understanding long-range interactions in biomolecules, such as those that mediate cooperativity in allosteric proteins.

• Received 30 August 1999

DOI:https://doi.org/10.1103/PhysRevLett.86.4966