| Abstract
| - The technique of femtosecond coherence spectroscopy is applied to a variety of photostable and photochemicallyactive heme protein samples. With the exception of cobalt-substituted myoglobin, strong oscillations are detectednear 40 cm-1 in all of the samples studied. Additional modes near 80, 120, and 160 cm-1 are observed in thephotochemically active samples. The amplitude and phase behavior of the low-frequency modes are studiedby tuning the pump/probe carrier wavelength across the Soret absorption spectrum. A simple harmonic modelis not able to account for the observed relative intensities of these modes or the carrier wavelength dependenceof their frequency and phase. As a result, we develop an anharmonic model where the oscillatory signal isdamped as the result of heterogeneity in the potential surface. The underlying source of the heterogeneity inthe anharmonic potential surface is found to be correlated with the inhomogeneous broadening of the Soretband. The presence of the higher harmonics in the photochemically active samples demonstrates that theanharmonic mode is strongly coupled to the ligand photodissociation reaction (i.e., upon photolysis it isdisplaced 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 thismode is associated with nuclear motion of the core of the porphyrin macrocycle. Since normal mode calculationsand prior kinetic models predict the frequency of the heme “doming mode” to be near 50 cm-1, we suggestthat the reaction coupled oscillations at ∼40 and ∼80 cm-1 are a direct reflection of anharmonic heme domingdynamics. Evidence for coupling between the heme doming dynamics and the Fe−His stretching mode isalso presented.
|
