Myoglobin dynamics: evidence for a hybrid solid/fluid state of matter

The dynamics of carboxy-myoglobin (MbCO) in water are studied from photodissociation of the ligand to bimolecular recombination over 12 decades in time to fully characterize the protein functions. Heterodyne-detected transient grating spectroscopy is used to resolve the dynamics of the protein from 10 ns to a few ms and provides a direct observation of the ligand escape. The process is well described by a bi-exponential function with decay rates of 50 and 725 ns at 20°C, suggesting that ligand escape occurs via a well defined pathway. Transient absorption in the Q-band (550–630 nm) also reveals the sensitivity of the electronic transition to the ligand motions. The dynamic range is extended by 10 6 through femtosecond coherence spectroscopy with 7 fs pulses to enable the observation of vibrational modes of energies >1500 cm −1 . The power spectrum is calculated by singular value decomposition and vibrational modes involved in the photodissociation are directly observed. The picture that is emerging is that proteins couple solid-like domains to fluid regions to facilitate functions and transport of ligands in and out of the protein to the active site.