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 studiedwith a variety of experimental, theoretical and computationalmethods [1]. The results of these investigations have beenrationalized in terms of a model that implies a protein quake-like motion [2], i.e. the propagation of the strain releasedupon 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 onspectroscopic measurements or did not have sufficient time-resolution to measure the timescale of such “proteinquake”.We have obtained direct experimental evidences of myoglobinproteinquake through femtosecond X-ray solution scatteringmeasurements performed at the LCLS X-ray free electronlaser [3]. Our data show that the structural changes inducedon heme upon photolysis propagate through the polypeptidechain in the picosecond timescale and that an underdampedprotein 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).