Femtosecond Coherence Spectroscopic Study of the Onset of Chemical Denaturation of Myoglobin upon Addition of Minor Amounts of Urea

The interaction of urea with myoglobin, as a benchmark system for heme-containing proteins, is studied via femtosecond coherence spectroscopy. The work focuses on the effect of urea on the appearance of low-wavenumber oscillations, which are a measure of the geometrical structure of the heme group and its interaction with the polypeptide chain. Pursuing this approach, structural alterations (i.e. changes in the vibrational dynamics of the heme group) are detected at denaturant concentrations below the full denaturation limit of 6M urea for myoglobin. In particular, the low-wavenumber oscillation associated with the heme-doming (i.e. the out-off-plane vibration of the propyrin macrocycle) is found to appear spectrally shifted with a concentration of only 3M urea. These results suggest that the local environment around the heme is already altered despite the fact that macroscopic unfolding as manifested in the thermodynamic properties of the polypeptide chain is not complete at these urea concentrations.