Anisotropy-Guided Enantiomeric Enhancement in AlanineUsing Far-UV Circularly Polarized Light

Abstract

All life on Earth is characterized by its asymmetry - both the genetic material and proteins are composed of homochiral monomers. Understanding how this molecular asymmetry initially arose is a key question related to the origins of life. Cometary ice simulations, L-enantiomeric enriched amino acids in meteorites and the detection of circularly polarized electromagnetic radiation in star-forming regions point to a possible interstellar/protostellar generation of stereochemical asymmetry. Based upon our recently recorded anisotropy spectra g(λ) of amino acids in the vacuum-UV range, we subjected amorphous films of racemic (13)C-alanine to far-UV circularly polarized synchrotron radiation to probe the asymmetric photon-molecule interaction under interstellar conditions. Optical purities of up to 4% were reached, which correlate with our theoretical predictions. Importantly, we show that chiral symmetry breaking using circularly polarized light is dependent on both the helicity and the wavelength of incident light. In order to predict such stereocontrol, time-dependent density functional theory was used to calculate anisotropy spectra. The calculated anisotropy spectra show good agreement with the experimental ones. The European Space Agency's Rosetta mission, which successfully landed Philae on comet 67P/Churyumov-Gerasimenko on 12 November 2014, will investigate the configuration of chiral compounds and thereby obtain data that are to be interpreted in the context of the results presented here.

Photochemical Chiral Symmetry Breaking in Alanine

“…However, the enantioselectivity of the photolysis reaction strongly depends on the reaction rate, and the higher ees obtained in experiments rely on photolytic yields of 99.97%, achieved with radiation times of several hours 11 , while we can only account for photolysis of up to 93% due to a much shorter pulse duration. Using the empirically found formula for the lower limit for the inducible ee 11,37 with an extent of reaction of ξ = 93% we obtain a lower bound for the inducible ee L of approximately 1.6%…”

“…where time and the environmental conditions 11,38 . However, the enantioselectivity of the photolysis reaction strongly depends on the reaction rate, and the higher ees obtained in experiments rely on photolytic yields of 99.97%, achieved with radiation times of several hours 11 , while we can only account for photolysis of up to 93% due to a much shorter pulse duration.…”

“…Here we focus on the photodynamics of the amino acid alanine (Ala), which has been subject to numerous experimental studies 11,12,[33][34][35][36][37][38] and of significant interest in theoretical works [39][40][41][42][43] . Our aim is to explore the photochemistry of Ala induced by the interaction with circularly polarized UV light.…”

Light on Chirality: Absolute Asymmetric Formation of Chiral Molecules Relevant in Prebiotic Evolution

“…Later experiments on the photolysis of racemic amorphous films that lacked long‐range ordering and that were of defined film thickness resulted in a greater enantiomeric purity of 5.2 % ee , but were not confirmed by the opposite helicity . Recently, the same group reported CPL‐induced ee values of up to 4 % for both helicities of incident light in alanine accompanied by time‐dependent (TD)‐DFT calculations . Another sophisticated experimental setup to study the asymmetric photolysis of solid state amino acids was introduced by Takahashi et al .…”

The Astrophysical Formation of Asymmetric Molecules and the Emergence of a Chiral Bias

“…Unfortunately, the determination of the ee after l -CPL irradiation was not feasible due to an insufficient quantity of remaining amino acid. However, the expected mirror effect between r - and l -CPL has been observed [28,29]; to ensure no experimental bias due to contamination, isotropic films of 13 C labelled racemic Ala were prepared and combined with improved analyses through the use of enantioselective multidimensional gas chromatography coupled to a time-of-flight mass spectrometer (GC × GC-TOFMS). The sample was irradiated with r - and l -CPL at 184 nm and 200 nm, corresponding to the n→π* transition in the carboxylate anion along with the n(COO − )→σ*(N-H) transition and to the π→π* transition, respectively.…”