Abstract
The growing number of data on heme proteins in the ultrafast time domain have revealed complicated photophysics of the heme that are sensitive both to the structure of the protein and to the ligand species. In this work, ultrafast time-resolved pump–probe in visible spectral range was performed using an ultrashort visible laser pulse. Broad spectral width of visible laser pulse enabled us to observe the pump–probe signal with a broadband range. A broadband multi-channel detector array coupled to a multi-channel lock-in amplifier was used to obtain the pump–probe signals at all of the probe frequencies simultaneously. In the simultaneous measurement at many probe wavelengths, we could obtain ultrafast spectral change after the photo-excitation of oxy-hemoglobin only with a short measurement time, avoiding laser damage on the sample. The time constant of the primary process was determined for the first time. Ultrafast spectral change observed in the early delay time region directly shows ultrafast photo-dissociation process of oxy-hemoglobin.
Highlights
Ultrafast laser experiment using ultra-short laser pulses can reveal the transient existence of intermediate species in the chemical reactions [22]
The method can be used for the measurements of amorphous and liquid-phase materials, which may be difficult to be measured by X-ray or electron diffraction
The probe wavelength dependency of Fourier intensity spectra could be discussed in the future work when vibrational phases of molecular vibrations are observed with good signal-to-noise ratio
Summary
Ultrafast laser experiment using ultra-short laser pulses can reveal the transient existence of intermediate species in the chemical reactions [22]. Mentioned multiple electronic intermediate state model, which is based on single wavelength measurements [20], was considered not to adequately explain the observed broadband spectral dynamics by later experiment by Ye et al in their careful study of photo-dissociation process of myoglobin [26,27]. Observed spectral shift on the blue side of the Soret band by Ye et al was considered to be better explained (unless even further more electronic states are postulated) by another model invoking hot photoproduct in the ground state. They demonstrated how their new model fully describes the ultrafast optical signals that occur in ferrous heme proteins. Ultrafast spectral change found in the measurement result is thought to be reflecting ultrafast photo-dissociation process of oxyhemoglobin
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