Many organisms use sunlight to fuel cellular functions. But exactly how does this conversion of solar energy into chemical energy unfold?

In a recent experiment, an international team of scientists, including several research groups from the BioXFEL Science and Technology Center, used  time-resolved serial femtosecond crystallography (TR-SFX) to watch a light reactive membrane protein found in of some marine bacteria as it was exposed to sunlight.

For this experiment, the researchers documented, for the first time, the dynamics of a chloride ion pumping rhodopsin (CIR), a molecular “pump,” which is jump-started by sunlight and moves chloride ions unidirectionally into the bacterial cells.  

These pumps may also serve as a kind of molecular solar cell for energy conversion. 

Once inside, the chloride ions create a negatively charged environment, while the outside of the cell remains neutral. This results in an electric field that the bacteria could use to move, grow and maintain other vital functions.

With TR-SFX sequential 3D snapshots show the molecular changes of biological macromolecules at work. How the ClR accomplishes the chloride pumping and transport processes has been elusive, said Marius Schmidt, UWM professor of physics who is one of the corresponding authors on the paper, that appeared in the journal Proceedings of the National Academy of Sciences (

“We intended to find out how sunlight drives the pump,” Schmidt said. “The rhodopsin’s response to light starts the mechanism. The response is extremely fast – at picoseconds, a trillionth of a second. So, we needed to make a molecular movie on extremely fast time scales to follow the ‘pumping’ action.”

The movie shows the mechanics of the rhodopsin pump and the motion of a central “piston”, which rapidly displaces the chloride ions. The size of this piston, however, is one billion times smaller than a piston in the macroscopic world. The movie shows in atomic detail how the piston pushes the chloride ions into the bacterial cell.

Imaging was done at the Linac Coherent Light Source in California, an X-ray Free Electron Laser (XFEL) that uses ultra-short and intense X-ray pulses that repeat rapidly to capture the frames of the molecular movie. Haiguang Liu, a professor from Beijing Computational Science Research Center, said, “It is not possible for the experiment to work without close collaboration from multidisciplinary experts. The pump-probe experiments require large volume of crystal samples that were grown in the lipidic cubic phase. Prof. Weontae Lee and Dr. Jihye Yun from Yonsei University prepared the samples both on site and in advance to ensure the experiments can be carried out as planned. The optimal wavelength and power density of the pumping laser were obtained using transient absorption spectroscopy approach prior to the SFX experiments by Professor Wenkai Zhang and his team.” Approximately 40 Terabyte of raw data were processed to obtain the detailed dynamics within 100 ps after illumination. The joint team is aiming to reveal dynamics at longer time scales using the same approach using X-ray Lasers.

The work is also relatable to human biology, Schmidt said, because the structure of the chloride pump is very similar to the rhodopsin, also called “visual purple,” and the photopsins in the human eye. While it doesn’t pump chloride, rhodopsin enables the human eye to absorb light and convert it into the generation of electrical signals that communicate with the brain.

The structure of this chloride-ion pump also resembles that of other G-protein coupled receptors (GPCRs) in humans, which regulate functions such as blood pressure and hormone management. GPRCs are of interest to the pharmaceutical industry because they are prime drug targets.

Although the imaging is being used to advance fundamental biological science, this mechanism could also be applied in the future, to design light-sensitive molecular pumps. The chloride pump could be inserted into other organisms and one would be able to manipulate their behavior with light through a method called optogenetics.

This research was conducted through the combined expertise of high-profile researchers mainly from Asia and the United States. Members of the scientific teams on the paper include researchers from Yonsei University in South Korea, Beijing Computational Science Research Center in China, Tsinghua University in China, Beijing Normal University in China, Arizona State University, University of Wisconsin-Milwaukee, Linac Coherent Light Source, La Trobe University in Australia, and Yokohama City University in Japan.

Full reference:

J.-H. Yun, X. Li, J. Yue, J.-H. Park, Z. Jin, C. Li, H. Hu, Y. Shi, S. Pandey, S. Carbajo, S. Boutet, M. S. Hunter, M. Liang, R. G. Sierra, T. J. Lane, L. Zhou, U. Weierstall, N. A. Zatsepin, M. Ohki, J. R. H. Tame, S.-Y. Park, J. C. H. Spence, W. Zhang, M. Schmidt, W. Lee, H. Liu(2021) Early stage dynamics of chloride ion-pumping rhodopsins revealed by femtosecond X-ray laser, Proceedings of the National Academy of Sciences, published online March 22, 2021,