Recording a molecular energy machine in action

In all living things, structural changes in proteins are responsible for many biochemically controlled functions, for example energy production at cell membranes. The protein bacteriorhodopsin occurs in microorganisms that live on the surface of lakes, streams, and other bodies of water. Activated by sunlight, this molecule pumps positively charged particles, protons, from the inside to the outside through the cell membrane. While doing this, it is constantly changing its structure.

PSI researchers were already able to elucidate one part of this process at free-electron X-ray lasers (FELs) such as SwissFEL. Now they have also managed to record the still unknown part of the process in a kind of molecular movie. For this they took a method that had previously been usable only at FELs and further developed it for use at the Swiss Light Source SLS. The study underlines the synergy between the analytical options at these two large-scale research facilities at PSI. "With the new method at SLS, we can now follow the last part of the movement of bacteriorhodopsin, where the steps are in the millisecond range", explains Tobias Weinert, first author of the paper. "With measurements at FELs in the USA and Japan, we had already measured the first two sub-processes before SwissFEL was commissioned", Weinert says. "These take place very fast, within femtoseconds to microseconds." A femtosecond is one-trillionth of a second.



To be able to observe such processes, the researchers use so-called "pump-probe" crystallography. With this method, they can take snapshots of protein movements that can then be assembled into movies. For the experiments, proteins are brought into crystal form. A laser beam, imitating sunlight, triggers the sequence of movements in the protein. X-rays that hit the sample afterwards produce diffraction images, which are recorded by a high-resolution detector. From these, computers generate an image of the protein structure at each point in time.

The movie created from the measurements at SLS shows how the structure of the bacteriorhodopsin molecule changes in the next 200 milliseconds after it is activated by light. With that, a complete so-called "photocycle" of the molecule has now been elucidated.

More details on PSI News, MirageNews,

Reference: Weinert, T., P. Skopintsev, D. James, F. Dworkowski, E. Panepucci, D. Kekilli, A. Furrer, S. Brünle, S. Mous, D. Ozerov, P. Nogly, M. Wang and J. Standfuss (2019). Proton uptake mechanism in bacteriorhodopsin captured by serial synchrotron crystallography. Science 365: 61 (10.1126/science.aaw8634)