Amino acid gas phase circular dichroism and implications for the origin of biomolecular asymmetry

Abstract

Life on Earth employs chiral amino acids in stereochemical l-form, but the cause of molecular symmetry breaking remains unknown. Chiroptical properties of amino acids – expressed in circular dichroism (CD) – have been previously investigated in solid and solution phase. However, both environments distort the intrinsic charge distribution associated with CD transitions. Here we report on CD and anisotropy spectra of amino acids recorded in the gas phase, where any asymmetry is solely determined by the genuine electromagnetic transition moments. Using a pressure- and temperature-controlled gas cell coupled to a synchrotron radiation CD spectropolarimeter, we found CD active transitions and anisotropies in the 130–280 nm range, which are rationalized by ab initio calculation. As gas phase glycine was found in a cometary coma, our data may provide insights into gas phase asymmetric photochemical reactions in the life cycle of interstellar gas and dust, at the origin of the enantiomeric selection of life’s l-amino acids.

Chiroptical activity of gas-phase propylene oxide predicting the handedness of interstellar circular polarization in the presolar nebula

“…The anisotropy factor g as a function of wavelength was studied by recording simultaneously the CD ∆ε and absorption ε spectra of each enantiomer of propylene oxide using a gas of analytes in a dedicated sample cell with a length of 500 mm at the AU-CD beam line on ASTRID2 at Aarhus University ( 52 ). Figure 1B shows the CD and associated absorption spectra of S -(−)-propylene oxide along with the most dominant resolved experimental electronic and vibronic transitions highlighting the high resolution of gas-phase synchrotron CD spectroscopy.…”

“…These experimental data are thus inadequate for validation of theoretical approaches. We have recently refreshed the field of gas-phase CD and anisotropy spectroscopy with a new experimental setup that combines an entirely new gas cell with synchrotron radiation from the AU-CD beam line at ASTRID2, Aarhus (Denmark) ( 52 ).…”

Trends in angle-resolved molecular photoelectron spectroscopy

“…246 In this sense it can be considered complementary to established chiral techniques like CD in absorption, very weak and hard to detect with dilute samples, which is therefore most employed with solutions. Gas phase CD measurements can nevertheless be achieved with special techniques, 247,248 and have notably been recently demonstrated in mass selected photodetachment of DNA strands. 249…”

“…246 In this sense it can be considered complementary to established chiral techniques like CD in absorption, very weak and hard to detect with dilute samples, which is therefore most employed with solutions. Gas phase CD measurements can nevertheless be achieved with special techniques, 247,248 and have notably been recently demonstrated in mass selected photodetachment of DNA strands. 249 While PECD may require special techniques to bring delicate samples in the gas phase, it may probe systems, like anions or cations that cannot survive in solution, or in typical ultrahigh vacuum environments, like the study of surfaces and adsorbates.…”

Dual-Beam Circular Dichroism Spectroscopy of Jet-Cooled Chiral Molecules

“…In recent decades, CD spectroscopy of gas-phase molecules has attracted much attention because it can provide mass- and conformation-selective CD spectra. − Since the first CD measurement of jet-cooled 3-methylcyclopentanone, resonance-enhanced multiphoton ionization (REMPI) spectroscopy has been employed to measure the CD of gaseous molecules. ,, Photoelectron CD spectroscopy has also been used to investigate the chiroptical properties of gas-phase molecules and clusters. − Cavity ring-down polarimetry has been developed to measure the CD of chiral molecules in the gas phase . The matrix isolation technique has been employed to obtain vibrational CD spectra of chiral molecules in cryogenic rare gas matrices. , Nevertheless, obtaining CD spectra of gas-phase molecules is still challenging because of the low molecular densities and small CD values.…”