Jan Hendrik Bredehöft

Urea, Glycolic Acid, and Glycerol in an Organic Residue Produced by Ultraviolet Irradiation of Interstellar/Pre-Cometary Ice Analogs

More than 50 stable organic molecules have been detected in the interstellar medium (ISM), from ground-based and onboard-satellite astronomical observations, in the gas and solid phases. Some of these organics may be prebiotic compounds that were delivered to early Earth by comets and meteorites and may have triggered the first chemical reactions involved in the origin of life. Ultraviolet irradiation of ices simulating photoprocesses of cold solid matter in astrophysical environments have shown that photochemistry can lead to the formation of amino acids and related compounds. In this work, we experimentally searched for other organic molecules of prebiotic interest, namely, oxidized acid labile compounds. In a setup that simulates conditions relevant to the ISM and Solar System icy bodies such as comets, a condensed CH(3)OH:NH(3) = 1:1 ice mixture was UV irradiated at approximately 80 K. The molecular constituents of the nonvolatile organic residue that remained at room temperature were separated by capillary gas chromatography and identified by mass spectrometry. Urea, glycolic acid, and glycerol were detected in this residue, as well as hydroxyacetamide, glycerolic acid, and glycerol amide. These organics are interesting target molecules to be searched for in space. Finally, tentative mechanisms of formation for these compounds under interstellar/pre-cometary conditions are proposed.

Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry

Comets harbor the most pristine material in our solar system in the form of ice, dust, silicates, and refractory organic material with some interstellar heritage. The evolved gas analyzer Cometary Sampling and Composition (COSAC) experiment aboard Rosetta’s Philae lander was designed for in situ analysis of organic molecules on comet 67P/Churyumov-Gerasimenko. Twenty-five minutes after Philae’s initial comet touchdown, the COSAC mass spectrometer took a spectrum in sniffing mode, which displayed a suite of 16 organic compounds, including many nitrogen-bearing species but no sulfur-bearing species, and four compounds—methyl isocyanate, acetone, propionaldehyde, and acetamide—that had not previously been reported in comets.

Photochirogenesis: photochemical models on the absolute asymmetric formation of amino acids in interstellar space.

Proteins of all living organisms including plants, animals, and humans are made up of amino acid monomers that show identical stereochemical L-configuration. Hypotheses for the origin of this symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which interstellar ultraviolet (UV) circularly polarized light (CPL) induces an enantiomeric excess in chiral organic molecules in the interstellar/circumstellar media. This scenario is supported by a) the detection of amino acids in the organic residues of UV-photo-processed interstellar ice analogues, b) the occurrence of L-enantiomer-enriched amino acids in carbonaceous meteorites, and c) the observation of CPL of the same helicity over large distance scales in the massive star-forming region of Orion. These topics are of high importance in topical biophysical research and will be discussed in this review. Further evidence that amino acids and other molecules of prebiotic interest are asymmetrically formed in space comes from studies on the enantioselective photolysis of amino acids by UV-CPL. Also, experiments have been performed on the absolute asymmetric photochemical synthesis of enantiomer-enriched amino acids from mixtures of astrophysically relevant achiral precursor molecules using UV-circularly polarized photons. Both approaches are based on circular dichroic transitions of amino acids that will be highlighted here as well. These results have strong implications on our current understanding of how lifes precursor molecules were possibly built and how life selected the left-handed form of proteinogenic amino acids.

The effects of circularly polarized light on amino acid enantiomers produced by the UV irradiation of interstellar ice analogs

Two irradiation experiments on interstellar ice analogs at 80 K under interstellar-like conditions were performed with the LURE SU5 synchrotron beamline to assess, for the first time, the photochemical effect of circularly polarized ultraviolet light (UV CPL) at 167 nm (7.45 eV) with right and left polarizations on such ice mixtures. Methods. This effect was measured by determining the enantiomeric excesses (e.e.s) for two amino acids formed in the solid organic residues produced during the subsequent warm-up of the irradiated samples to room temperature: alanine, the most abundant chiral proteinaceous amino acid produced (both polarizations) and 2,3-diaminopropanoic acid (DAP), a non-proteinaceous amino acid (rightpolarization experiment). These excesses were compared to those measured for the same amino acids produced after unpolarized UV irradiation of the same ice mixtures (expected to be zero), in order to determine the contribution of CPL only. A careful estimate of all the associated uncertainties (statistical and systematic errors) was also developed. Results. It appears that the enantiomeric photochemical effect at this wavelength is weak, since both alanine and DAP e.e.s were found to be small, at most of the order of 1% in absolute values, and tends to be inconclusive since the effects obtained for both amino acids and both polarizations are not those expected. In light of these results, the hypothesis that CPL may be one source responsible for the e.e.s measured for such amino acids in some meteorites and, more generally, that CPL may be directly related to the origin of biomolecular homochirality on Earth is discussed.

Circular Dichroism of Amino Acids in the Vacuum-Ultraviolet Region†

Biopolymers such as nucleic acids and proteins are composed of chiral monomers that show identical stereochemical configuration. Naturally occurring proteins are made up of l-amino acids. Hypotheses for the origin of symmetry breaking in biomolecules include the absolute asymmetric photochemistry model by which circularly polarized (CP) light induces an enantiomeric excess (ee) in chiral organic molecules. This model is supported by both the observation of CP light in the star-forming region of Orion and the occurrence of l-enantiomer-enriched amino acids in carbonaceous meteorites. However, the differential absorption of CP light by amino acid enantiomers, which determines the speed and intensity of enantioselective photolysis, is unknown over a large spectral range. Here we show that significant circular dichroic transitions in amino acids can be observed by extending circular dichroism (CD) spectroscopy to the vacuum-ultraviolet (UV) spectral range. a-H amino acids show the same CD magnitude and sign over a large wavelength range. In a given spectral window CP light is therefore capable of inducing enantiomeric excesses of the same handedness into the proteinogenic amino acids we have studied. Absolute asymmetric photochemistry might thus well have triggered the appearance of l-amino acid based life on Earth. Our results demonstrate that enantiomers of “meteoritic” a-methyl amino acids show dichroic absorption with equal magnitude, yet opposite sign to a-H amino acids. Therefore CP light cannot induce l enantiomeric excesses into a-methyl and a-H amino acids as found in meteorites. To explain the cause of symmetry breaking in biomolecules a well-known theory proposes that CP interstellar UV radiation—similar to that identified in the starforming region of Orion in the infrared—induced enantiomeric excesses into interstellar and circumstellar organic compounds by asymmetric photochemical reactions prior to their deposition on the early Earth. In support of this theory chiral amino acid structures were identified in interstellar ice analogues and a large number of l-enantiomer-enriched amino acids have been identified in the interior of the Murchison and Murray carbonaceous meteorites. To verify the absolute asymmetric photochemistry model the differential CP-light absorption of proteinogenic andmeteoritic amino acid enantiomers requires systematic examination. Until now, the popular and extensively used technique of CD spectroscopy has been used to record electronic CD for chiral molecules in aqueous solution above 190 nm. Water absorbs photons of l< 190 nm, making the vacuum-UV region inaccessible for CD spectroscopy in aqueous solution. By using a synchrotron radiation source for CP light and preparing isotropic amorphous solid-state samples immobilized on MgF2 windows, we have extended electronic CD measurements to the vacuum-UV spectral range. We observed intense CD-active transitions of amino acids between 140 and 190 nm (Figure 1), which are much more intense than the previously known CD bands between 190 and 330 nm. Figure 1a shows the CD spectra for dand l-alanine. As expected, the enantiomers of alanine show dichroic absorption of equal magnitude but opposite sign; the nice mirroring effect shows the high quality of the data. The CD spectra of l-alanine, l-valine, and l-leucine are characterized by maxima between 180 and 190 nm (Figure 1b), l-valine and l-leucine show minima between 160 and 170 nm, and l-serine and l-2-aminobutyric acid show maxima at 165– [*] Prof. Dr. U. J. Meierhenrich Laboratoire des Mol cules Bioactives et des Ar mes UMR 6001 CNRS-UNSA, Universit de Nice-Sophia Antipolis Facult des Sciences, Parc Valrose, 06108 Nice (France) Fax: (+33)4-9207-6151 E-mail: uwe.meierhenrich@unice.fr Homepage: http://www.unice.fr/lcmba/meierhenrich/

Anisotropy spectra of amino acids.

Biopolymers such as enzymes and nucleic acids are composed of homochiral monomers; their molecular symmetry is broken. The origin of biomolecular symmetry breaking— a crucial step in the origin of life—remains unknown. Among various random and deterministic hypotheses that have been proposed, one well-known hypothesis is based on a photochemical model by which chiral photons, in the form of circularly polarized (CP) light, induce an enantioenrichment by interacting with racemic organic molecules, a process known as enantioselective photolysis. According to this model, asymmetric photoreactions took place in the extreme vacuum of interstellar space, prior to the delivery of enantioenriched chiral organic molecules to the early Earth. The hypothesis proposes that CP electromagnetic radiation, such as that detected in the Orion molecular cloud, interacts asymmetrically with chiral organic molecules in interstellar ices and with the early precursors of carbonaceous meteorites. Both enantiomers absorb CP photons triggering photolysis, but one enantiomer has a slightly smaller absorption coefficient. This enantiomer is photo-destroyed less rapidly than its optical antipode and it will therefore become enantioenriched. The induced enantiomeric excess (ee) is determined by the extent of reaction x and is function of the anisotropy factor g, defined by De/e, the ratio between the differential extinction coefficient De, and the extinction coefficient e. However, the sign and magnitude of g depend on the wavelength of the CP light. Here we report anisotropy spectra of amino acids yielding g(l) values, which were recorded for solid amorphous films in a wavelength range between 130 and 350 nm. The anisotropy spectra were measured with a new experimental setup at the synchrotron radiation facility ASTRID at Aarhus University (Denmark). The anisotropy spectra obtained for amino acids in the solid phase show well-resolved zero-crossings, extrema, and g values up to 0.024. These data allow: 1) the prediction of the sign of the induced ee, 2) the determination of the kinetics and the ee values of the enantioselective photolysis, and 3) the selection of the wavelength of the CP light best suited for inducing enantioenrichment. The enantioselective photolysis of a racemic mixture by CP light is an asymmetric transformation that can be represented by two competitive pseudo-first-order reactions with unequal rate constants, kR and kS, for the R and S enantiomer, respectively. The rate constants are proportional to the molar absorption coefficients (eR and eS, respectively), and the efficiency of the enantioselective photolysis depends on the difference between kR and kS or, in this case as Kuhn already outlined, on the anisotropy factor g [Eq. (1)]. 6] More recently it has been shown by Nakamura et al. that Equation (1) is valid even for non-firstorder kinetics.

Photolysis of rac-Leucine with Circularly Polarized Synchrotron Radiation

Amino acids that pass the RNA machinery in living organisms occur in L‐configuration. The question on the evolutionary origin of this biomolecular asymmetry remains unanswered to this day. Amino acids were detected in artificially produced interstellar ices, and L‐enantiomer‐enriched amino acids were identified in CM‐type meteorites. This hints at a possible interstellar/circumstellar origin of the amino acids themselves as well as their stereochemical asymmetry. Based upon the current knowledge about the occurrence of circularly‐polarized electromagnetic radiation in interstellar environments, we subjected rac‐leucine to far‐UV circularly‐polarized synchrotron radiation. Asymmetric photolysis was followed by an analysis in an enantioselective GC/MS system. Here, we report on an advanced photolysis rate of more than 99% for leucine. The results indicate that high photolysis rates can occur under the chosen conditions, favoring enantioselective photolysis. In 2014, the obtained results will be reexamined by cometary mission Rosetta.

Photochirogenesis: Photochemical Models on the Origin of Biomolecular Homochirality

Current research focuses on a better understanding of the origin of biomolecular asymmetry by the identification and detection of the possibly first chiral molecules that were involved in the appearance and evolution of life on Earth. We have reasons to assume that these molecules were specific chiral amino acids. Chiral amino acids have been identified in both chondritic meteorites and simulated interstellar ices. Present research reasons that circularly polarized electromagnetic radiation was identified in interstellar environments and an asymmetric interstellar photon-molecule interaction might have triggered biomolecular symmetry breaking. We review on the possible prebiotic interaction of ‘chiral photons’ in the form of circularly polarized light, with early chiral organic molecules. We will highlight recent studies on enantioselective photolysis of racemic amino acids by circularly polarized light and experiments on the asymmetric photochemical synthesis of amino acids from only one C and one N containing molecules by simulating interstellar environments. Both approaches are based on circular dichroic transitions of amino acids that will be presented as well.

Anisotropy Spectra for Enantiomeric Differentiation of Biomolecular Building Blocks

All biopolymers are composed of homochiral building blocks, and both D-sugars and L-amino acids uniquely constitute life on Earth. These monomers were originally enantiomerically differentiated under prebiotic conditions. Particular progress has recently been made in support of the photochemical model for this differentiation: the interaction of circularly polarized light with racemic molecules is currently thought to have been the original source for lifes biological homochirality. The differential asymmetric photoreactivity of particular small molecules can be characterized by both circular dichroism and anisotropy spectroscopy. Anisotropy spectroscopy, a novel derivative of circular dichroism spectroscopy, records the anisotropy factor g = Δε/ε as a function of the wavelength. Anisotropy spectroscopy promisingly affords the wavelength-dependent determination of the enantiomeric excess (ee) inducible into chiral organic molecules by photochemical irradiation with circularly polarized light. Anisotropy spectra of small molecules therefore provide unique means for characterizing the different photochemical behaviors between enantiomers upon exposure to various wavelengths of circularly polarized light. This chapter will: (1) present the theory and configuration of anisotropy spectroscopy; (2) explain experimentally recorded anisotropy spectra of selected chiral biomolecules such as amino acids; and (3) discuss the relevance of these spectra for the investigation of the origin of the molecular homochirality observed in living organisms. This review describes a new chiroptical technique that is of significance for advances in asymmetric photochemistry and that is also highly relevant for the European Space Agency Rosetta Mission, which will determine enantiomeric excesses (ees) in chiral organic molecules in cometary ices when it lands on Comet 67P/Churyumov-Gerasimenko in November 2014.

Functionalization of a self-assembled monolayer driven by low-energy electron exposure.

Self-assembled monolayers (SAMs) of 10-undecene-1-thiol on Au were functionalized with nitrogen-containing groups using an approach in which multilayer ammonia (NH(3)) films were deposited at low temperature onto the SAMs and subsequently exposed to 15 eV electrons. The result of this process was investigated after removal of the remaining NH(3) by annealing to room temperature using high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). HREELS shows that the CC double bonds disappear during electron exposure, while XPS gives evidence that about 25% of the terminal double bonds of the SAM were functionalized. Also, XPS shows that a sufficiently thick NH(3) layer protects the underlying SAM from electron-induced damage. The process suggested here thus represents a particularly gentle approach to the functionalization of ultrathin molecular layers. Thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD) experiments on condensed layers of NH(3) reveal production of N(2) but show that significant amounts of the initial NH(3) as well as N(2) produced during electron exposure desorb. Hydrogen released upon formation of N(2) is held responsible for the reduction of double bonds and protection of the SAMs from damage.