The technique of femtosecond coherence spectroscopy is applied to a variety of photostable and photochemically active heme protein samples. With the exception of cobalt-substituted myoglobin, strong oscillations are detected near 40 cm-1 in all of the samples studied. Additional modes near 80, 120, and 160 cm-1 are observed in the photochemically active samples. The amplitude and phase behavior of the low-frequency modes are studied by tuning the pump/probe carrier wavelength across the Soret absorption spectrum. A simple harmonic model is not able to account for the observed relative intensities of these modes or the carrier wavelength dependence of their frequency and phase. As a result, we develop an anharmonic model where the oscillatory signal is damped as the result of heterogeneity in the potential surface. The underlying source of the heterogeneity in the anharmonic potential surface is found to be correlated with the inhomogeneous broadening of the Soret band. The presence of the higher harmonics in the photochemically active samples demonstrates that the anharmonic mode is strongly coupled to the ligand photodissociation reaction (i.e., upon photolysis it is displaced far from equilibrium). Moreover, the observation of the ∼40 cm-1 oscillations in all of the iron-based heme protein samples, including porphine and protoporphyrin IX model compounds, suggests that this mode is associated with nuclear motion of the core of the porphyrin macrocycle. Since normal mode calculations and prior kinetic models predict the frequency of the heme "doming mode" to be near 50 cm-1, we suggest that the reaction coupled oscillations at ∼40 and ∼80 cm-1 are a direct reflection of anharmonic heme doming dynamics. Evidence for coupling between the heme doming dynamics and the Fe-His stretching mode is also presented.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry