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Dipartimento di Fisica - Politecnico di Milano

Chirality-Sensitive Optical Spectroscopy

What do oranges and lemons have in common, yet make them different? It’s not their taste, shape or color. Both orange and lemons contain limonene, a molecule responsible for their distinctive smell. But limonene in lemons are mirror image to those of oranges, much like our left and right hands. This handedness of molecules – containing the same atoms but arranged in mirror-image configurations – is known as chirality.

As it turns out, chirality has a huge impact on the function of (bio)-molecules. Almost all biomolecules (proteins, nucleic acids, sugars etc.) are chiral and our cells respond in the desired way only to one enantiomer (mirror configurations), whereas the other one may have harmful side effects. This means that determining chirality is very important for drug developments and in medicine.

Here is the big Challenge: How to distinguish one chiral molecule from the other? The method commonly used to detect enantiomers is circular dichroism (CD) spectroscopy. It exploits the fact that circular polarized light is absorbed differently by left-handed and right-handed enantiomers. But commercial CD spectrometers works at single wavelength and measurement of a complete CD spectrum requires sequential scanning of a monochromator.

The main goal of the current project is to develop a high-sensitivity broadband CD spectrometer to characterize chirality. Our approach will employ a ground-breaking methodology, which allows one to measure the chiral signal field simultaneously over a very broad wavelength range in the time domain via Fourier-transform (FT) spectroscopy. This approach is based on heterodyne amplification of the chiral field with a local oscillator and employs an innovative birefringent common-path interferometer. By scanning the time delay between chiral signal and local oscillator one can record an interferogram whose FT provides the steady-state broadband CD and circular birefringence (CB) spectra simultaneously.

Towards time-resolved chiroptical spectroscopy: Since chirality arises from the lack of mirror symmetry, CD is a very sensitive probe of molecular conformations and non-symmetric arrangements of chromophores. Following the evolution of molecular structure in the course of a light-triggered chemical reaction or biological process would enable one to gain fundamental insights into the reaction mechanisms and pathways. The project aims to develop an experimental setup for time-resolved CD measurements, using a pump-probe configuration for probing chiroptical dynamics of (bio)-molecules in the excited state.

Once accomplished, this would be a seminal result in ultrafast optics as it would allow to combine high time resolution with the exquisite structural sensitivity of CD, thus coming one step closer to fulfilling the chemist’s dream of making a ‘molecular motion picture’ of a photoinduced process. This will impact all fields of structural molecular biology including protein folding/misfolding related diseases such as Alzheimer’s and Parkinson’s, and DNA photodamage which is the first step in the development of cancer. Since CD spectroscopy is also employed to study chiral-optical properties of materials, the project has the potential to impact many areas of material science research, including chiral nano-photonics and ultrafast spinotronics.