Uncovering the chiral bias of meteoritic isovaline through asymmetric photochemistry

Abstract

Systematic enrichments of L-amino acids in meteorites is a strong indication that biological homochirality originated beyond Earth. Although still unresolved, stellar UV circularly polarized light (CPL) is the leading hypothesis to have caused the symmetry breaking in space. This involves the differential absorption of left- and right-CPL, a phenomenon called circular dichroism, which enables chiral discrimination. Here we unveil coherent chiroptical spectra of thin films of isovaline enantiomers, the first step towards asymmetric photolysis experiments using a tunable laser set-up. As analogues to amino acids adsorbed on interstellar dust grains, CPL-helicity dependent enantiomeric excesses of up to 2% were generated in isotropic racemic films of isovaline. The low efficiency of chirality transfer from broadband CPL to isovaline could explain why its enantiomeric excess is not detected in the most pristine chondrites. Notwithstanding, small, yet consistent L-biases induced by stellar CPL would have been crucial for its amplification during aqueous alteration of meteorite parent bodies.

Fig. 1. Experimental observation of polarization controllable symmetry breaking in analogs mimicking isovaline adsorbed on interstellar dust grains.

a Schematic structure of the l- and d-enantiomers of isovaline (carbon in gray, hydrogen in white, oxygen in red, and nitrogen in blue). b Scheme of the AU-CD beam line apparatus at the ASTRID2 synchrotron storage ring facility, ISA, Aarhus University, Denmark, used for recording UV circular dichroism (CD) and anisotropy spectra of l- and d-isovaline thin films deposited on UV grade CaF₂ windows. A CaF₂ photoelastic modulator converts horizontally polarized synchrotron radiation into 50 kHz alternating left- and right-circularly polarized light (l- and r-CPL). After passing through enantiopure isovaline films, the transmitted light is recorded using a vacuum UV enhanced photomultiplier. c Scheme of the sublimation-deposition chamber for preparation of racemic thin films of isovaline for asymmetric photolysis experiments. The films are produced by condensation of sublimated dl-isovaline powder on a CaF₂ window. Their thickness is monitored by a quartz crystal microbalance (QCM). d Scheme of the tunable laser set-up employed for asymmetric photolysis experiments on racemic thin films of isovaline. Linearly polarized monochromatic laser radiation (LPL) is reflected by a UV enhanced mirror and is subsequently circularly polarized by a Soleil-Babinet compensator. Monochromatic UV CPL then interacts with a racemic dl-isovaline film condensed on a CaF₂ window. The window constantly moves in the xy direction to allow for homogeneous sample irradiation.

Fig. 2. Chiroptical solid-phase spectra of isovaline.

The anisotropy spectra (thick lines) of the l- and d-enantiomers of isovaline allow for the prediction of enantiomeric excess (ee) inducible by asymmetric photolysis by circularly polarized light at the extent of reaction ξ (thin lines of decreasing intensity at 0.9999, 0.9995, and 0.9990) via numerical solution of the equation reported by Kagan et al.. Source data are provided as a Source Data file.

Fig. 3. Comparing enantiomeric excesses in irradiated versus non-irradiated isovaline films.

a Enantioselective multidimensional gas-chromatography coupled to time-of-flight mass-spectrometry (GC×GC–TOF-MS) ion chromatogram at m/z 168 corresponding to the l- and d-enantiomers of isovaline in the r-CPL irradiated film. b GC×GC–TOF-MS ion chromatogram at m/z 168 of the non-irradiated diluted (1:600) reference sample (sample set IV). Note that ∼1% ee difference of the irradiated film compared to the non-irradiated counterpart cannot be readily visible in the 3D plots.

Fig. 4. Comparative analysis of isovaline’s l-enantiomeric excess in carbonaceous chondrites and inducible by circularly polarized starlight.

a l-enantiomeric excess ee_L of isovaline previously detected in Ivuna-type (CI)^,, Mighei-type (CM)^,,,,,–, CI/CM, and Renazzo-type (CR)^,, carbonaceous chondrites as a function of aqueous alteration (H in OH/H₂O metrics). Symbols represent the mean and error bars indicate ± standard deviations of ee_l in enantioselective analyses. Only the analyses where the standard deviation of %ee_L is reported and is equal to or less than 10% are considered (a complete list along with the number of injections is given in Supplementary Note 5, Supplementary Table 3). The asterisk indicates an anomalous thermally altered CR2.4 chondrite^,. b Comparison of the %ee inducible by ultraviolet circularly polarized light (UV CPL) in solid-, liquid-³³, and gas-phase isovaline calculated from the anisotropy spectra of l-isovaline at the extent of reaction ξ = 0.9999 based on the equation reported by Kagan et al.. The upper estimate of the ee inducible solely by interaction with UV CPL is indicated by the purple bar in (a), corresponding to the purple area in (b). Source data are provided as a Source Data file.