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C'mon electrons, let's do the twist! Twisting electrons can tell right-handed and left-handed molecules apart.

19 February 2018

Identifying right-handed and left-handed molecules is a crucial step for many applications in chemistry and pharmaceutics. An international research team (CELIA-CNRS/INRS/Berlin Max Born Institute/SOLEIL) has now presented a new original and very sensitive method. The researchers use laser pulses of extremely short duration to excite electrons in molecules into twisting motion, the direction of which reveals the molecules' handedness. The research results appear in Nature Physics.

Are you right handed or left handed? No, we aren't asking you, dear reader; we are asking your molecules. It goes without saying that, depending on which hand you use, your fingers will wrap either one way or the other around an object when you grip it. It so happens that this handedness, or "chirality", is also very important in the world of molecules. In fact, we can argue that a molecule's handedness is far more important than yours: some substances will be either toxic or beneficial depending on which "mirror-twin" is present. Certain medicines must therefore contain exclusively the right-handed or the left-handed twin.

The problem lies in identifying and separating right-handed from left-handed molecules, which behave exactly the same unless they interact with another chiral object. An international research team has now presented a new method that is extremely sensitive at determining the chirality of molecules.

We have known that molecules can be chiral since the 19th century. Perhaps the most famous example is DNA, whose structure resembles a right-handed corkscrew. Conventionally, chirality is determined using so-called circularly polarised light, whose electromagnetic fields rotate either clockwise or anticlockwise, forming a right or left "corkscrew", with the axis along the direction of the light ray. This chiral light is absorbed differently by molecules of opposite handedness. This effect, however, is small because the wavelength of light is much longer than the size of a molecule: the light's corkscrew is too big to sense the molecule's chiral structure efficiently.

The new method, however, greatly amplifies the chiral signal. "The trick is to fire a very short, circularly polarized laser pulse at the molecules," says Olga Smirnova from the Max Born Institute. This pulse is only some tenths of a trillionth of a second long and transfers energy to the electrons in the molecule, exciting them into helical motion. The electrons' motion naturally follows a right or left helix in time depending on the handedness of the molecular structure they reside in.

Their motion can now be probed by a second laser pulse. This pulse also has to be short to catch the direction of electron motion and have enough photon energy to knock the excited electrons out of the molecule. Depending on whether they were moving clockwise or anticlockwise, the electrons will fly out of the molecule along or opposite to the direction of the laser ray.

This lets the experimentalists of CELIA to determine the chirality of the molecules very efficiently, with a signal 1000 times stronger than with the most commonly used method. What's more, it could allow one to initiate chiral chemical reactions and follow them in time. It comes down to applying very short laser pulses with just the right carrier frequency. The technology is a culmination of basic research in physics and has only been available since recently. It could prove extremely useful in other fields where chirality plays an important role, such as chemical and pharmaceutical research.

Having succeeded in identifying the chirality of molecules with their new method, the researchers are now thinking already of developing a method for laser separation of right- and left-handed molecules. Text: Dirk Eidemueller/Forschungsverbund Berlin e.V. - Translation: Peter Gregg


Fig. (click to enlarge)

Fig: Following excitation by an ultra-short circularly polarised laser pulse, electrons follow a right or left helix depending on the handedness of the molecular structure they reside in. Source: Samuel Beaulieu

Original Publication: Nature Physics
S. Beaulieu, A. Comby, D. Descamps, B. Fabre, G. A. Garcia, R. Géneaux, A. G. Harvey, F. Légaré, Z. Mašin, L. Nahon, A. F. Ordonez, S. Petit, B. Pons, Y. Mairesse, O. Smirnova and V. Blanchet: Photoexcitation Circular Dichroism in Chiral Molecules
Nature Physics (2018) online, doi:10.1038/s41567-017-0038-z.


Prof. Dr. Olga Smirnova, Tel.: 030 6392 1340




Flexibility and arrangement - the interaction of ribonucleic acid and water

16 January 2018

Ribonucleic acid (RNA) plays a key role in biochemical processes which occur at the cellular level in a water environment. Mechanisms and dynamics of the interaction between RNA and water were now revealed by vibrational spectroscopy on ultrashort time scales and analyzed by in-depth theory

Ribonucleic acid (RNA) represents an elementary constituent of biological cells. While deoxyribonucleic acid (DNA) serves as the carrier of genetic information, RNA displays a much more complex biochemical functionality. This includes the transmission of information in the form of mRNA, RNA-mediated catalytic function in ribosomes, and the encoding of genetic information in viruses. RNA consists of a sequence of organic nucleobase molecules which are held together by a so-called backbone consisting of phosphate and sugar groups. Such a sequence can exist as a single strand or in a paired double-helix geometry. Both forms are embedded in a water shell and their phosphate and sugar groups are distinct docking points for water molecules. The structure of the water shell fluctuates on a time scale of a few tenth of a picosecond (1 ps = 10-12 s = 1 millionth of a millionth of a second). The interactions of RNA and water and their role for the formation of three-dimensional RNA structures are only understood insufficiently and difficult to access by experiment.

Scientists from the Max Born Institute have now observed the interaction of RNA with its water shell in real time. In their new experimental method, vibrations of the RNA backbone serve as sensitive noninvasive probes of the influence of neighboring water molecules on the structure and dynamics of RNA. The so-called two-dimensional infrared spectroscopy allows for mapping the time evolution of vibrational excitations and for determining molecular interactions within RNA and between RNA and water. The results show that water molecules at the RNA surface perform tipping motions, so-called librations, within a fraction of a picosecond whereas their local spatial arrangement is preserved for a time range longer than 10 ps. This behavior deviates strongly from that of neat water and is governed by the steric boundary conditions set by the RNA surface. Individual water molecules connect neighboring phosphate groups and form a partly ordered structure which is mediated by their coupling to the sugar units.

The librating water molecules generate an electrical force by which the water fluctuations are transferred to the vibrations of RNA. The different backbone vibrations display a diverse dynamical behavior which is determined by their local water environment and reflects its heterogeneity. RNA vibrations also couple mutually and exchange energy among themselves and with the water shell. The resulting ultrafast redistribution of excess energy is essential for avoiding a local overheating of the sensitive macromolecular structure. This complex scenario was analyzed by detailed theoretical calculations and simulations which, among other results, allowed for the first complete and quantitative identification of the different vibrations of the RNA backbone. Comparative experiments with DNA reveal similarities and characteristic differences between these two elementary biomolecules, showing a more structured water arrangement around RNA. The study highlights the strong potential of non-invasive time-resolved vibrational spectroscopy for unraveling the interplay of structure and dynamics in complex biomolecular systems on molecular length and time scales.


Fig. 1 (click to enlarge)

Fig. 1: Left: Structure of a RNA double helix. The blue spheres represent sodium counterions. Right: Enlarged segment of the sugar-phosphate backbone of RNA, including bridging water molecules. Vibrations of the RNA backbone serve as sensitive real time probes for mapping the influence of the neighboring water molecules on RNA's structure and dynamics.

Fig. 2 (click to enlarge)
Fig. 2: Two-dimensional vibrational spectra of RNA (upper panel) and DNA (lower panel) in the frequency range of the sugar-phosphate vibrations of the backbone. The RNA spectrum displays additional bands (contours) along the frequency diagonal ν13 and a more complex distribution of off-diagonal peaks. In addition to the frequency positions the line shapes of the individual bands (contours) give insight in details of the interactions with neighboring water molecules.

Original Publication:
E. M. Bruening, J. Schauss, T. Siebert, B. P. Fingerhut, T. Elsaesser: Vibrational Dynamics and Couplings of the Hydrated RNA Backbone: A Two-Dimensional Infrared Study.
J. Phys. Chem. Lett. 9, 583-587 (2018). DOI: 10.1021/acs.jpclett.7b03314.


Dr. Benjamin Fingerhut, Tel.: 030 6392 1404

Prof. Dr. Thomas Elsaesser, Tel.: 030 6392 1400




Marc Vrakking named Editor-in-Chief of the Journal of Physics B

1 January 2018

Prof. Marc Vrakking, director of Division A of the Max Born Institute, has been named Editor-in-Chief of the Journal of Physics B from January 1st 2018. In this capacity he succeeds Prof. Paul Corkum of the University of Ottawa, who served as Editor-in-Chief since 2011.

The Journal of Physics B is one of the publications of the Institute of Physics, and is a prominent journal serving the Atomic, Molecular and Optical (AMO) Physics community. In particular in the emerging fields of strong field physics, attosecond science and free electron laser science the journal has a prominent role, as is manifest by numerous important papers (including Topical Reviews, Tutorials, Special Issues and Roadmaps) that the journal continuously publishes. As such, the journal is strongly aligned with the research program in Division A of the Max Born Institute.


Prof. Marc Vrakking Tel. (030) 6392 1200


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