The first step in our ability to see is the microscopic change of a protein called rhodopsin found in the retina. This protein is responsible for converting light into signals that our brain can interpret to form images. The process occurs within an incredibly small time frame, taking only a picosecond or one-trillionth of a second, and is the only light-dependent step in our visual perception.

At the heart of this process is a small kinked molecule called retinal, which is a derivative of vitamin A. When light hits the protein, retinal absorbs part of the energy and changes its three-dimensional form from 11-cis to all-trans. This transformation triggers a cascade of reactions that lead to the perception of a flash of light.

For years, scientists have known about the starting point and end product of the retinal transformation, but no one has been able to observe in real-time exactly how the change occurs in the sight pigment rhodopsin. However, researchers at PSI have used the SwissFEL X-ray free-electron laser to observe what happens in real-time when retinal transforms.

As the protein absorbs light energy, it briefly inflates, losing most of its contact with retinal and allowing it to rotate freely. During this “breathing in” stage, the retinal transformation occurs as it turns into its elongated all-trans form. Shortly afterward, the protein contracts again and has the retinal firmly back in its grasp, except now in a different more elongated form. The transformation only takes a picosecond, making it one of the fastest processes in all of nature.

The PSI research team’s discovery sheds light on the fundamental processes of the human body, such as vision. The SwissFEL X-ray free-electron laser allows researchers to record and analyze such rapid biological processes. The SwissFEL camera’s filming speed is a billion times faster than a high-speed camera, making it the only way to study in detail the fundamental processes of the human body such as vision.

The study shows the vital role of SwissFEL in Swiss research. Working with large research facilities involves much more than simply pressing a shutter button. The PhD student Thomas Gruhl has spent years developing a method of producing high-quality rhodopsin crystals capable of delivering ultra-high resolution data. Ultimately, only these data made it possible to perform the necessary measurements at SwissFEL and, before the SwissFEL was built, at the X-ray free-electron laser SACLA in Japan.

In addition to investigating the transformation of retinal, researchers are also developing methods for investigating dynamic processes in proteins that are not normally activated by light. They use artificial means to make such molecules responsive to light by making appropriate changes to the binding partners or mixing proteins with binding partners in the crystal so quickly that they can be examined at the SwissFEL.