Observing myoglobin proteinquake with an X-ray free-electron laser

The events following the photodissociation of the bond be- tween myoglobin and its ligand have been extensively studied with a variety of experimental, theoretical and computational methods [1]. The results of these investigations have been rationalized in terms of a model that implies a protein quake- like motion [2], i.e. the propagation of the strain released upon photoexcitation through the protein similar to the prop- agation of acoustic waves during an earthquake. The exper- imental investigations performed so far have been based on spectroscopic measurements or did not have sufficient time- resolution to measure the timescale of such “proteinquake”. We have obtained direct experimental evidences of myoglobin proteinquake through femtosecond X-ray solution scattering measurements performed at the LCLS X-ray free electron laser [3]. Our data show that the structural changes induced on heme upon photolysis propagate through the polypeptide chain in the picosecond timescale and that an underdamped protein collective vibration with a ∼ 3.6 ps period is activated. [1] H. Frauenfelder, et al. PNAS , 100, 8615 (2003). [2] A. Ansari, et al. PNAS , 85, 5000 (1985). [3]M.Levantino,etal. Nat. Commun. , 6, 6772 (2015).