International research team examines photoelectric effect with the aid of a COLTRIMS reaction microscope
When light hits a material, electrons can be released from this material – the photoelectric effect. Although this effect played a major role in the development of the quantum theory, it still holds a number of secrets: To date it has not been clear how quickly the electron is released after the photon is absorbed. Jonas Rist, a Ph.D. student working within an international team of researchers at the Institute for Nuclear Physics at Goethe University Frankfurt, has now been able to find an answer to this mystery with the aid of a COLTRIMS reaction microscope which had been developed in Frankfurt: The emission takes place lightning fast, namely within just a few attoseconds – within a billionths of billionths of a second.
FRANKFURT. It is now exactly one hundred years ago that Albert Einstein was awarded the Nobel Prize in Physics for his work on the photoelectric effect. The jury had not yet really understood his revolutionary theory of relativity – but Einstein had also conducted ground-breaking work on the photoelectric effect. With his analysis he was able to demonstrate that light comprises individual packets of energy – so-called photons. This was the decisive confirmation of Max Planck's hypothesis that light is made up of quanta, and paved the way for the modern quantum theory.
Although the photoelectric effect in molecules has been studied extensively in the meantime, it has not yet been possible to determine its evolution over time in an experimental measurement. How long does it take after a light quantum has hit a molecule for an electron to be dislodged in a specific direction? “The length of time between photon absorption and electron emission is very difficult to measure because it is only a matter of attoseconds," explains Till Jahnke, the PhD-supervisor of Jonas Rist. This corresponds to just a few light oscillations. “It has so far been impossible to measure this duration directly, which is why we have now determined it indirectly." To this end the scientists used a COLTRIMS reaction microscope – a measuring device with which individual atoms and molecules can be studied in incredible detail.
The researchers fired extremely intense X-ray light – generated by the synchrotron radiation source BESSY II of Helmholtz-Zentrum Berlin – at a sample of carbon monoxide in the centre of the reaction microscope. The carbon monoxide molecule consists of one oxygen atom and one carbon atom. The X-ray beam now had exactly the right amount of energy to dislodge one of the electrons from the innermost electron shell of the carbon atom. As a result, the molecule fragments. The oxygen and carbon atoms as well as the released electron were then measured.
“And this is where quantum physics comes into play," explains Rist. “The emission of the electrons does not take place symmetrically in all directions." As carbon monoxide molecules have an outstanding axis, the emitted electrons, as long as they are still in the immediate vicinity of the molecule, are still affected by its electrostatic fields. This delays the release slightly – and to differing extents depending upon the direction in which the electrons are ejected.
As, in accordance with the laws of quantum physics, electrons not only have a particle character but also a wave character, which in the end manifests in form of an interference pattern on the detector. “On the basis of these interference effects, which we were able to measure with the reaction microscope, the duration of the delay could be determined indirectly with very high accuracy, even if the time interval is incredibly short," says Rist. “To do this, however, we had to avail of several of the possible tricks offered by quantum physics."
On the one hand the measurements showed that it does indeed only take a few dozen attoseconds to emit the electron. On the other hand, they revealed that this time interval is very heavily dependent on the direction in which the electron leaves the molecule, and that this emission time is likewise greatly dependent on the velocity of the electron.
These measurements are not only
interesting for fundamental research in the field of physics. The models which
are used to describe this type of electron dynamics are also relevant for many chemical
processes in which electrons are not released entirely, but are transferred to
neighbouring molecules, for instance, and trigger further reactions there. “In
the future such experiments could also help to better understand chemical
reaction dynamics therefore," says Jahnke.
Publication: Jonas Rist, Kim Klyssek, Nikolay M. Novikovskiy, Max Kircher, Isabel Vela-Pérez, Daniel Trabert, Sven Grundmann, Dimitrios Tsitsonis, Juliane Siebert, Angelina Geyer, Niklas Melzer, Christian Schwarz, Nils Anders, Leon Kaiser, Kilian Fehre, Alexander Hartung, Sebastian Eckart, Lothar Ph. H. Schmidt,1 Markus S. Schöffler, Vernon T. Davis, Joshua B. Williams, Florian Trinter, Reinhard Dörner,1 Philipp V. Demekhin, Till Jahnke: Measuring the photoelectron emission delay in the molecular frame. Nat Commun 12, 6657 (2021). https://doi.org/10.1038/s41467-021-26994-2
High-tech: COLTRIMS reaction microscope at electron storage ring BESSY II, Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). Photo: Miriam Weller, Goethe University Frankfurt
Ph.D. student Jonas Rist, Goethe University Frankfurt. Photo: Alexander Hartung, Goethe University Frankfurt
Prof. Dr. Till Jahnke
European XFEL and
Institute for Nuclear Physics, Goethe University Frankfurt, Germany
Tel.: + 49 (0)69-798 47023 (Office)
Shedding new light on the role of tumour suppressor protein pVHL
Transforming Growth Factor beta (TGF-β) is a signalling protein whose dysregulation can cause developmental disorders and cancer. Dr Xinlai Cheng and his colleagues at the Goethe University Frankfurt have discovered how a tumour suppressor known as pVHL influences signal transmission involving TGF-β. Their findings suggest possible starting points for developing new drugs.
FRANKFURT/HEIDELBERG. Signal transmission inside cells is a complex process. TGF-β, for example, regulates many cell functions during the early development of both humans and animals, but also in adult organisms. The mechanisms involved are not yet fully understood. It is, however, clear that activated TGF-β initially binds to receptors located on the cell surface. Inside the cell, the TGF-β receptors in their turn activate a protein called SMAD3, which then forms complexes with SMAD4 that translocate to the cell nucleus. There the SMAD proteins mediate the extent to which genes are activated and translated into proteins and other gene products.
Researchers at the Goethe University Frankfurt, Heidelberg University, the German Cancer Research Center (DKFZ), Heidelberg University Hospital and the University Hospital in Jena have now discovered how the von Hippel-Lindau tumour suppressor protein (pVHL) intervenes in this signalling pathway. Tumour suppressors are proteins whose defects or reduced presence in multicellular organisms are associated with a high risk that cells will degenerate into tumour cells. In the Journal of Cell Biology the scientists report the first evidence that pVHL degrades the SMAD3 protein. This occurs before SMAD3 and SMAD4 associate. pVHL thus inhibits the signalling chain that starts with activated TGF-β. “We obtained evidence of this both in cultures of human cells and in Drosophila," says the last author, Dr Xinlai Cheng. “This suggests that at a very early stage in evolution pVHL assumed the regulatory function that we have now brought to light."
Xinlai Cheng has been leading a junior research group at the Buchmann Institute for Molecular Life Sciences at the Goethe University Frankfurt since 2019. He began the investigations at the Institute of Pharmacy and Molecular Biotechnology at Heidelberg University. His mentor, Professor Stefan Wölfl, explained an important finding that emerged from the new-found connection between pVHL and the TGF-β signalling pathway: “pVHL is known to be involved in how cells 'feel' oxygen and react to varying oxygen availability. As a result, a cell's oxygen supply also mediates TGF-β signal transmission."
The researchers' discovery opens up new opportunities for
developing drugs to combat cancer. “If we could, for example, use a substance
to specifically regulate pVHL activity, we would also influence the TGF-β
signalling pathway, which in turn plays a major role in the formation of
tumours, and metastases in particular," says Xinlai Cheng. Tumour cells are
good at adapting to their environment inside the organism and to variations in oxygen
availability. Their very flexible cellular activity helps them to do so. This
activity is regulated by factors including the TGF-β signalling pathway.
Publication: Jun Zhou, Yasamin Dabiri, Rodrigo A. Gama-Brambila, Shahrouz Ghafoory, Mukaddes Altinbay, Arianeb Mehrabi, Mohammad Golriz, Biljana Blagojevic, Stefanie Reuter, Kang Han, Anna Seidel, Ivan Đikić, Stefan Wölfl, Xinlai Cheng: pVHL-mediated SMAD3 degradation suppresses TGF-β signaling. Journal of Cell Biology (2022) 221 (1): e202012097 https://doi.org/10.1083/jcb.202012097
Caption: Stained liver tissue shows the complementary occurrence of pVHL and SMAD proteins: Where pVHL (green) is abundant, SMAD2/3 (red) is scarce, and vice versa. Cell nuclei are stained blue. The lower right picture shows all three colours combined. Photos: Xinglai Cheng/Goethe University
Dr. rer. nat. habil. Xinlai Cheng
Buchmann Institute for Molecular Life Sciences Chemical Biology
Goethe University Frankfurt
Phone +49 69 798-42718
Institut of Pharmacy and Molecular Biotechnology –
Pharmaceutical Biology, Pharmaceutical Bioanalytics and Molecular Cell Biology
Phone +49 6221-544880