Probing Ultrafast Chirality Changes in the Infrared


We are interested in spectroscopic methods which are both structure-sensitive and fast enough to gain direct information about the rearrangement of atoms in light-induced chemical reactions. Two related techniques that are both sensitive to molecular geometry are two-dimensional infrared (2D-IR) and vibrational circular dichroism (VCD) spectroscopy.
Both rely on the usually very well defined orientation of vibrational transition dipole moments of localized vibrations. Spectroscopy employing polarized laser pulses can thus give direct information on the relative orientations of different parts of a molecule and their changes during chemical reactions.
The recording of VCD spectra of transient species is an experimental challenge currently addressed in our group. With this new technique we hope to be able to detect short-lived chiral intermediates in photochemical reactions and local conformational changes during the folding peptides.
In a similar way, we have developed highly sensitive ultrafast polarization measurements to test, for example, novel biomimetic potoswitches for new applications in biophysics
A common objective is to make polarised IR spectroscopy more universally useful by pushing the detection limits, improving the signal-to-noise ratio and by exploring new observables in chiral and nonlinear measurements.

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Development of time-resolved chiral infrared spectroscopy

Vibrational circular dichroism (VCD) measures the difference in absorption of left and right-handed circular polarized light for vibrational transitions of molecules. For the C=O stretch vibrations in peptides the VCD signal is closely related and specific to secondary structure. For organic molecules density functional theory (DFT) can routinely and reliably predict VCD spectra, and the absolute configuration of compounds can thus often be determined without crystallization.

The observation of changes in the very small VCD signals on fast timescales promises to become a powerful new tool to investigate, for example, protein folding dynamics or rearrangements of chiral molecules. We could demonstrate that such experiments are indeed possible by measuring, for the first time, transient VCD signals with picosecond time resolution.

In order to increase the sensitivity of the method, we have tested a number of measurement schemes that are capable of enhancing the very tiny VCD signals and are compatible with the use of fast array detectors. This requires the very precise control of polarization and phase difference of infrared femtosecond laser pulses, which has proven to be useful also for other non-linear spectroscopic methods.

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Polarization enhanced Linear Dichroism and 2D-IR spectroscopy

The careful control and fast modulation of the femtosecond laser pulse polarization which we implemented for chiral spectroscopy can be equally useful for unconventional measurement schemes in pump-probe and 2D-IR spectroscopy.

In an adaptation of an earlier development for nanosecond flash photolysis by Kliger and coworkers (Kliger, citation) we use polarizers to separate and gain independent control of “signal” and “local oscillator” fields in pump-probe measurements. This allows us to amplify signals by more than an order of magnitude and to determine angles between transition dipole moments in molecules during ultrafast reactions with increased precision.

When applied to 2D-IR spectroscopy, polarization enhancement can be used in combination with a very simple interferometer setup for highly sensitive measurements, scattering and background suppression and singling out individual Liouville pathways.

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Time-resolved IR-spectroscopy of novel photoswitches

A team around Prof. Massimo Olivucci at the University of Siena, Italy and Bowling Green, Ohio has recently been successful in the computer design and synthesis of a new class of biomimetic photoswitches. They differ from commonly used switches in size, polarity, synthetic flexibility, and remarkably feature a reaction mechanism mimicking the photoisomerization of the chromophore of the visual pigment Rhodopsin. Femtosecond time-resolved IR spectroscopy is an ideal tool for the characterization of these novel nanomashines, providing accurate information on isomerization quantum yields and timeconstants. Additional light-sources are used to prepare the switches in their metastable configuration in order to test their bi-directional functionality.

Funded by the Forschungskredit, Werner Legat and the University of Zurich

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Conformational dynamics of thiopeptides

Thiopeptides are peptides in which one (or more) of the backbone carbonyl oxygen atoms are substituted by a sulfur atom. This one-atom substitution can be used to create a photoswitch inside the peptide backbone. Selective excitation and isomerization of the thiopeptide bond allows us to induce conformational dynamics in an essentially native petpide and study, for example, hydrogen bond breaking by transient 2D-IR spectroscopy.

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