UV femtosecond time-resolved circular dichroism (TRCD) spectroscopy has been used to study the ultrafast changes of chirality in a small molecular biological paradigm sample, 7-dehydrocholesterol (7-DHC). Upon UV-photoexcitation, 7-DHC undergoes a ring opening to produce previtamin D 3 , and two of the chiral centers of 7-DHC are removed, which impacts the overall chirality of the molecule. Here, measurements of this chirality change connected to the ring opening of 7-DHC with a time resolution of 280 fs in the UV are reported. With this method, a previously described discrepancy concerning the photophysics of 7-DHC was clarified. With our setup, the relaxation time of the chirality change was measured to be 1−2 ps, which corresponds to the shortest time constant in the transient absorption (TA) measurements, allowing us to assign that time constant to the ring opening.
Observing chirality changes as they occur is an important topic of research. It provides information that deepens the understanding of biomolecular configuration and conformation under environmental changes. Also, knowing the specific steps in chiral synthesis would simplify the production of specific chiral enantiomers that have a specific function. To gain better insight to the initial steps of conformational and configurational changes, the time‐resolution of chiral spectroscopy is continually pushed toward a shorter time‐scale. Recent advances have produced measurements of chirality changes with a femtosecond time‐resolution. These measurements are hindered by the inherently weak chirality signal, which can be overshadowed by different optical artefacts. This minireview will look at the so far successful techniques which measure chirality changes with femtosecond time‐resolution and discuss the advantages and disadvantages of these techniques. A short outlook will also look at new techniques that could improve the ability to measure chirality changes on an ultrafast time‐scale.
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