Investigations of coherent vibrational oscillations in myoglobin

The technique of femtosecond coherence spectroscopy (FCS) is applied to the heme protein myoglobin. Photostable samples of deoxy myoglobin (Mb) and photochemically active samples of the nitric oxide adduct (MbNO) are investigated. The pump-induced change in the probe transmittance for both samples displays coherent oscillations that, when transformed into the frequency domain, are in agreement with the resonance Raman spectrum of deoxy Mb. This indicates that the coherences associated with the photoreactive sample (MbNO) arise from the rapidly changing forces appearing in the crossing region(s) between the reactant and product state potential energy surfaces. The relative phase and amplitude of the Fe-His vibration, associated with the sole covalent linkage between the heme and the protein, are analyzed as a function of sample state and pump/probe carrier frequency. The dependence of the phase on carrier frequency is found to be significantly different for the "field driven" coherence in Mb and the "reaction driven" coherence in MbNO. In MbNO we observe a dip in amplitude and a phase flip near 439 nm for the Fe-His mode, whereas in deoxy Mb we observe a nearly constant phase and amplitude for this mode across the Soret absorption band. These observations are shown to be in good agreement with a simple theoretical model of the pump-probe experiment. Finally, we present recent observations of strong low-frequency oscillations, occurring near 40 cm(-1) in both species and near 80 cm(-1) for MbNO.