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The Millimeter Wave Circular Dichroism Spectroscopy as a Powerful Tool for the Exploration of Molecules

Final Report Summary - PRAMICIDIS (The Millimeter Wave Circular Dichroism Spectroscopy as a Powerful Tool for the Exploration of Molecules)

The main goal of the project PRAMICIDIS supported by European Framework Programme FP7-MC-ERG is to carry out research in the field of millimetre wave circular dichroism spectroscopy, which can provide absolutely new information about molecular structure. Circular dichroism (CD) is a form of spectroscopy based on the differential absorption of left- and right-handed circularly polarised light by chiral molecule. This method generally enables determining the conformation of these molecules. For small molecular systems, absolute configuration of molecule can be determined by means of combination of measured and ab-initio simulated spectra.

While ultraviolet (UV)-visible CD and infra-red CD spectroscopy are well-established techniques, they still have several drawbacks (low resolved spectra and sometimes insensitivity to final structural variation in case of UV-visible CD, and low signal intensity and complication by the fact that typical solvent - water - often absorbs in the range where structural features exhibit differential absorption of circularly polarised light in case of infra-red CD). To overcome all the drawbacks of both ECD and CD in the infra-red region of spectra, the circular dichroism spectroscopy in the millimetre wave region of spectra seems to be very promising because nearly all biomolecules are charged (ones contain ionic or polar constituents) thus they absorb strongly in this spectral region and should be source of CD effect in millimetre waves due to the rotational motion or collective vibrational mode. Due to increasing of natural resolution of this method with increasing of wavelength of used millimetre wave radiation, the sensitivity to fine structural variation is also increasing. This method should be able to recognise very fine structure changes of chiral biomolecules in comparison with ECD and CD in the infra-red region and could provide 'fingerprinting' spectroscopy of chiral biomolecules which can really strongly help biomedical research.

The research objectives have included a construction of the very first millimetre wave circular dichroism spectrometer, its broad testing and debugging, and the experiments with real chiral molecules. This would start for validate this technique to be an unambiguous and universal approach to the structure determination of chiral macromolecules.

All particular objectives were fulfilled and main result is the very first millimetre wave circular dichroism (MCD) spectrometer, which was constructed. This spectrometer covers a spectral range from 100 to 600 GHz. The heart of the MCD spectrometer - circular polariser - is based on a polarising Michelson interferometer. When a length of one interferometer arm is changed, the phase relationship between two electromagnetic waves originally polarised in two perpendicular planes produces series of polarisation states. When the length of arm is changed by a quarter of a wavelength, left and right circular polarised radiation is alternately produced. The MCD spectrometer uses for detection of millimetre wave radiation, a pyroelectric detector. The researcher also created own control programme for driving MCD spectrometer. Unfortunately, a measurement on real chiral molecule has not shown any detectable CD signal. As real chiral molecule the (R)-(+)-propylene oxide was chosen. Arguments for its selection were simple structure (meaning simpler analysing of spectra) and expected high rotational strengths of rotational transitions at millimetre wave frequencies (meaning high CD signal). The reason for unobserved CD signal can be: firstly, very low intensity of CD signal, which is expected to be 10^-6 to 10^-7 of delta A/A; and/or secondly - potential insufficient sensitivity of the purchased pyroelectric detector. Although MCD signal has not yet been observed by anyone in the world, the researcher of this project will continue in an effort to detect it. Recently built MCD spectrometer and potential cooperation with some research group having the bolometer, which could be more sensitive in compare to pyroelectric detector and enables to use higher modulation frequencies, should help. The detection of this phenomenon will have huge impact on explanation of the theory of rotational circular dichroism and also on the biomedical research.

The expected socio-economic impact was fully satisfied. Due to this reintegration grant, Dr Kania had the possibility to conduct very interesting research and to interact with people both from the similar and outstanding research fields (optics, mechanics, and physics). Thanks to the reintegration grant, he had the possibility to control the whole project for the first time, having all duties and responsibilities which moves him forward in his professional career. Moreover, he was able to get a research position at host institution due to this project and he could present his research quality. This resulted in a longer-term contract at the host institution. He could also transfer the obtained knowledge about polarisation, optical interferometry and behaviour of the millimetre-wave radiation to young students. His teaching task (education of analytical chemistry) brought him into the close contact with a large amount of students. This interaction gives a promising potential for the future because one of his aims is also to attract talented students and to provide transfer of obtained knowledge to them.

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