Low Frequency Modes in Heme Proteins

Abstract

Abstract The technique of femtosecond coherence spectroscopy is applied to heme protein samples. 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. 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 source of the heterogeneity in the anharmonic potential surface is correlated with the inhomogeneous broadening of the Soret band. 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. As a result, we suggest that the reaction coupled oscillations at ∼40 cm-1 and ∼80 cm-1 are a direct reflection of anharmonic heme doming dynamics.