Project

femtosecond vibrational spectroscopy; vibrational circular dichroism; condensed phase dynamics; chirality; Vibrational Optical Activity; Ultrafast Chemical Reactions; Molecular Structure

Lead
Lay summary
Probing vibrational optical activity with ultrashort infrared laser pulses is a new spectroscopic method, promising detailed structural information on fast chemical and biochemical processes. It is currently being developed in Zürich and only a few other specialized laboratories around the world. Proteins, DNA and many other biologically important molecules are chiral, which means that like our hands they cannot be superimposed with their mirror image. We can detect the "handedness" of chiral molecules because they interact differently with left and right circular polarized light, a phenomenon known as optical activity. The difference in absorption of left and right circular polarized light is called circular dichroism, which is today routinely used to analyze secondary structure content of proteins (such as alpha helices and beta-sheets) and to characterize chiral drugs. More detailed information can be obtained when we study the interaction of chiral molecules with infrared light that can excite vibrations, since these are often localized in specific regions of a molecule. Measured vibrational circular dichroism (VCD) spectra can be compared to reliable quantum chemistry calculations, leading to precise knowledge of the configurations of atoms. The goal of this research project is to advance current technology in order to measure not only the vibrational circular dichroism of molecules in equilibrium, but to detect extremely small VCD changes during fast chemical reactions or during the rearrangement of biomolecules. In order to observe molecular motion on the timescale of picoseconds (this is a thousand times faster than a single operation in a 1GHz computer) we use very short laser pulses both to initiate a reaction or conformational change and to record spectra after a certain time delay (pump-probe technique). This method is already well established for measuring ordinary absorption changes in very fast chemical, physical and biological processes. Our unique capability to probe vibrational optical activity with short infrared laser pulses will also enable us to detect variations in chirality. From this development, new insights into the mechanism of a number of chemical and biochemical reactions can be expected. In addition to answering questions of fundamental research, we hope to stimulate the improvement of conventional vibrational optical activity measurements, which are of increasing commercial importance.

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Last update: 21.02.2013

Active participation

Communication	Title	Media	Place	Year

Number	Title	Start	Funding scheme

We have shown in first proof of principle experiments that ultrafast transient vibrational circular dichroism spectroscopy (VCD) is experimentally possible, but it has not yet become a routine measurement. Crossed polarizer arrangements, which have already been used with some success in electronic transient chiral spectroscopy, promise to provide the necessary gain in signal-to-noise necessary for a wider application of the technique. They also make it possible to record vibrational optical rotary dispersion (VORD) spectra. With this proposal wee seek financial support for a new PhD student as well as for further technical developments, in order to continue our pioneering efforts of the past three years. An already developed crossed-polarizer VORD/VCD setup will be upgraded with better polarizers and an automated rotation stage for more precise alignment. Elimination of these uncertainties should help us to overcome current difficulties to implement broad band detection. At the same time, an independent set-up will be built, which combines a new concept, recently demonstrated by a Korean research group, with noise-reduction techniques known from non-linear vibrational spectroscopy. We also seek a ‘calibration standard’ for chiral transient vibrational spectroscopy, i.e. molecules with two equally accessible enantiomeric forms that exhibit large VCD and VORD changes of opposite sign upon photoexcitation and are photostable. Like terpenes in static VCD or helical transition metal complexes in transient electronic CD spectroscopy, they are needed to test the performance of the experimental setup prior to measurements on more precious samples.The final goal of the project is to be able to resolve, in real time, inherently chiral processes, such as the unidirectional motion of molecular motors as well as conformational changes in biomolecules.