Asymmetric Photolysis Research Articles

Uncovering the chiral bias of meteoritic isovaline through asymmetric photochemistry

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.

Magnetic Circular Dichroism in Archean Stratospheric Oxygen: Enantiomeric Excess of Amino Acids Produced in Volcanic Plumes.

While there is consensus that Archean atmosphere was anoxic with O2 pressure, p(O2) <10-6 PAL (present atmospheric level) at sea-level, evidence suggests that p(O2) at stratospheric altitudes of 10-50 km was orders of magnitude higher, a result of photodissociation of CO2 by UVC sunlight and incomplete mixing of O2 with other gases. Molecular O2 is paramagnetic due to triplet ground state. Magnetic circular dichroism (MCD) by stratospheric O2 is examined in earth's magnetic field and shows maximum circular polarization │(I+ - I-)│ at altitude of 15-30 km (I+/I- is intensity of left/right circularly polarized light). While (I+ - I-)/(I+ + I-) is small (~10-10), it is an unexplored source of enantiomeric excess (EE) by asymmetric photolysis of amino acid precursors produced in volcanic eruptions. The precursors reside in stratosphere for periods of over a year due to relative absence of vertical transport. Due to negligible thermal gradient across equator, they are trapped in the hemisphere where they are produced, with interhemispheric exchange time of over a year. The precursors diffuse through altitudes of maximum circular polarization before getting hydrolyzed on ground to amino acids. Enantiomeric excess of ~10-12 is calculated for precursors and amino acids. While small, this EE is orders of magnitude higher than predicted (~10-18) by parity violating energy differences (PVED) and could be the seed for growth of biological homochirality. Preferential crystallization (PC) is described as a plausible mechanism for amplification of solution EE of some amino acids from 10-12 to 10-2, for period of several days.

Enantiomeric excess of amino acids in interstellar ice analogues-asymmetric photolysis of precursors by circularly polarized UV light

ABSTRACT Based on the results from UV-irradiated interstellar ice analogues, we describe a two-step phenomenological theory for enantiomer-selective, asymmetric formation of molecules like amino acid precursors: (1) UV photolysis of interstellar ices to molecular fragments/radicals and formation of racemic population of chiral molecules and (2) asymmetric photolysis of precursors by UV-circularly polarized light (UV-CPL), resulting in preferred growth of one enantiomer. Independent of specifics of step (1), when ice-analogue is irradiated with UV-CPL, enantiomeric excess of amino acids following hydrolysis is limited to |EEice(max)|=|(σ+ − σ−)/(σ+ + σ−)|, 0.5 times the anisotropy factor (g) for amino acid precursors (σ+/− is molecular cross-section for absorption of right/left UV-CPL light in cm2). For amino acids, the calculated EEice(max) ∼ 1 per cent agrees with experiments. Over small duration, EE is given by, |EEice(t)|=|(I+ − I−)(σ+ − σ−)|t/4, (I+/− is photon flux of right/left UV-CPL cm−2 s−1) which agrees with the observed dependence of EE on time-integrated flux of UV-CPL light. For ice irradiated with linearly polarized/unpolarized light followed by irradiation of residue with UV-CPL, |EEres(t)|=|tanh[(I+ − I−)(σ+ − σ−)t/2]| and can be much larger than EEice(max) for ice alone. When both ice and residue are irradiated with UV-CPL, EE = EEice + EEres and depends on the durations of irradiation for ice and residue. Similar analysis by assuming asymmetric photosynthesis of precursors as the underlying mechanism does not agree with experimental observations.

Fluorescence-Detected Circular Dichroism of a Chiral Molecular Monolayer with Dielectric Metasurfaces.

Strong enhancement of molecular circular dichroism (CD) has the potential to enable efficient asymmetric photolysis, a method of chiral separation that has conventionally been impeded by insufficient yield and low enantiomeric excess. Here, we study experimentally how predicted enhancements in optical chirality density near resonant silicon nanodisks boost CD. We use fluorescence-detected circular dichroism (FDCD) spectroscopy to measure indirectly the differential absorption of circularly polarized light by a monolayer of optically active molecules functionalized to silicon nanodisk arrays. Importantly, the molecules and nanodisk antennas have spectrally coincident resonances, and our fluorescence technique allows us to deconvolute absorption in the nanodisks from the molecules. We find that enhanced FDCD signals depend on nanophotonic resonances, in good agreement with simulated differential absorption and optical chirality density, while no signal is detected from molecules adsorbed on featureless silicon surfaces. These results verify the potential of nanophotonic platforms to be used for asymmetric photolysis with lower energy requirements.

Magnetic circular dichroism in Archean atmosphere and asymmetric photolysis of biomolecules: enantiomeric excess of prebiotic sugar.

In the terrestrial dipolar magnetic field, magnetic circular dichroism (MCD) of UV sunlight by paramagnetic O2 in an Archean atmosphere (mostly CO2 and N2) results in circular polarization anisotropy (~ 10-10). This is used to calculate enantiomeric excess (EE~10-13) of glyceraldehyde (3-carbon sugar) with a model that includes racemic production and asymmetric photolysis of its enantiomers. The sign and magnitude of enantiomeric excess (EE) vary with the Earth's latitude. Unlike random noise fluctuation in spontaneous mirror symmetry breaking (SMSB) models, the sign of EE is deterministic and constant over large areas of prebiotic Earth. The magnitude is several orders greater than the mean amplitude of stochastically fluctuating EE. MCD could provide the initial EE for growth of homochirality by asymmetric autocatalysis.

Interstellar condensed (icy) amino acids and precursors: theoretical absorption and circular dichroism under UV and soft X-ray irradiation

The photophysics of interstellar ices and condensed molecules adsorbed on grains is of primary importance for studies on the origin of the specific handedness of complex organic molecules delivered to the early Earth and of the homochirality of the building blocks of life. Here, we present quantum mechanical calculations based on time-dependent density functional theory for the absorption and circular dichroism (CD) of isovaline and its chiral precursor 5-ethyl-5-methylhydantoin, both observed in meteoritic findings. The systems are considered in their geometrical conformation as extracted from a full solid (icy)matrix, as a shortcut to understand the behaviour of molecules with fixed orientation, and/or taking into account the full solid matrix. In the context of a possible 'condensation-warming plus hydrolysis-recondensation' process, we obtain that: (i) for low-energy excitations, the 'condensed' precursor has a stronger CD with respect to the amino acid, suggesting that the handedness of the latter could be biased by asymmetric photolysis of the precursor in cold environments; (ii) enantiomeric excess could in principle be induced more efficiently in both systems for excitation at higher energies (VUV).X-ray absorption near-edge spectroscopy and related CDresults could serve as support for future experiments on ionization channels. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Homochirality Origin in Nature: Possible Versions

Homochirality and related problems have been reviewed. Relative probability of different versions of homochirality arising were estimated on the example of processes, synthesis of organic compounds in space, asymmetric photolysis under circularly polarized light, synthesis of optically active compounds as a result of parity violation, magnetochiral dichroism, chiral amplification, optical active amino acids synthesis and theirs isotope ratios in meteorites, and some other determinate and indeterminate processes. It has been estimated handedness of moving objects as source of chirality. Possible contributions of these processes in homochirality origin in nature have been evaluated. The view is justified that homochirality (and life) origin may be not an accidental event. Keywords: Asymmetric in vitro and in vivo reactions, enantioselectivity, homochirality.

The origin of biomolecular asymmetry – Insights from cometary and meteoritic matter

The chemistry of life is purely stereospecific. The fundamental biopolymers – proteins and nucleic acids – are intrinsically chiral due to their molecular building blocks, namely L-amino acids and D-sugars. Hypotheses for the evolutionary origin of that strict stereochemical selection include the asymmetric photochemistry model by which circularly polarized (CP) photons induced an enantiomeric excess in chiral organic molecules via asymmetric photolysis. The transfer of a distinct chirality from chiral photons to organic molecules is assumed to have occurred in environments of interstellar molecular clouds. This model is strengthened by the observation of CP light in the star-forming region of Orion. Due to our laboratory experiments mimicking cometary ice evolution, we show that molecules of prebiotic interest such as amino and diamino acids are formed in inter/protostellar environments and might have been subjected to CP vacuum-UV radiation before and during their space journey to the early Earth.

ChemInform Abstract: Synthesis and Chirality of Amino Acids under Interstellar Conditions

Amino acids are the fundamental building blocks of proteins, the biomolecules that provide cellular structure and function in all living organisms. A majority of amino acids utilized within living systems possess pre-specified orientation geometry (chirality); however the original source for this specific orientation remains uncertain. In order to trace the chemical evolution of life, an appreciation of the synthetic and evolutional origins of the first chiral amino acids must first be gained. Given that the amino acids in our universe are likely to have been synthesized in molecular clouds in interstellar space, it is necessary to understand where and how the first synthesis might have occurred. The asymmetry of the original amino acid synthesis was probably the result of exposure to chiral photons in the form of circularly polarized light (CPL), which has been detected in interstellar molecular clouds. This chirality transfer event, from photons to amino acids, has been successfully recreated experimentally and is likely a combination of both asymmetric synthesis and enantioselective photolysis. A series of innovative studies have reported successful simulation of these environments and afforded production of chiral amino acids under realistic circumstellar and interstellar conditions: irradiation of interstellar ice analogues (CO, CO2, NH3, CH3OH, and H2O) with circularly polarized ultraviolet photons at low temperatures does result in enantiomer enriched amino acid structures (up to 1.3% ee). This topical review summarizes current knowledge and recent discoveries about the simulated interstellar environments within which amino acids were probably formed. A synopsis of the COSAC experiment onboard the ESA cometary mission ROSETTA concludes this review: the ROSETTA mission will soft-land on the nucleus of the comet 67P/Churyumov-Gerasimenko in November 2014, anticipating the first in situ detection of asymmetric organic molecules in cometary ices.

Photodissociation dynamics of tryptophan and the implication of asymmetric photolysis

Photodissociation of amino acid tryptophan in a molecular beam at wavelengths of 212.8 and 193 nm, corresponding to excitation to the second and third absorption bands, was investigated using multimass ion imaging techniques. The respective wavelengths also represent excitation to the edge of a positive circular dichroism band and the center of a negative circular dichroism band of L-tryptophan. Only one dissociation channel was observed at both photolysis wavelengths: C(8)NH(6)CH(2)CHNH(2)COOH-->C(8)NH(6)CH(2)+CHNH(2)COOH. Dissociation rates were found to be 1.3x10(6) and 5x10(6) s(-1) at the respective wavelengths. Comparison to theoretical calculation indicates that dissociation occurs on the ground state after internal conversion. Implication of asymmetric photolysis is discussed.

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.

Chiroptical Properties of Amino Acids: A Density Functional Theory Study

Amino acids are involved in many scientific theories elucidating possible origins of life on Earth. One of the challenges when discussing the evolutionary origin of biopolymers such as proteins and oligonucleotides in living organisms is the phenomenon that these polymers implement monomers of exclusively one handedness, a feature called biomolecular homochirality. Many attempts have been made to understand this process of racemic symmetry breaking. Assuming an extraterrestrial origin of the molecular building blocks of living organisms, their susceptibility to asymmetric photolysis by the absorption of circularly polarized electromagnetic radiation in interstellar space was proposed. In order to predict whether the interaction of circularly polarized light with various racemic amino acids can induce an enantiomeric excess, we investigated the electronic and chiroptical properties of the amino acids valine and isovaline by a molecular modelling approach based on quantum chemistry (Density Functional Theory). The average spectra of both L-valine and L-isovaline have been produced on the basis of Boltzmann population analysis using computed spectra for the various conformations of each amino acid.

Heterocyclic chemistry

The year 2005 was very busy and productive for the heterocyclic community. Particular highlights include a new and facile procedure for the synthesis of N-sulfonyloxaziridines, the first catalytic asymmetric photolysis reaction, which yields pyrrolizidines in up to 70% ee, a new regioselective diamination of terminal dienes, which yields cyclic ureas in excellent yields, and the first catalytic asymmetric hetero-Diels–Alder reaction of nitroso compounds with acyclic dienes.

Enantioselective Synthesis of Near Enantiopure Compound by Asymmetric Autocatalysis Triggered by Asymmetric Photolysis with Circularly Polarized Light

Right- and left-handed circularly polarized light (CPL) has been proposed as one of the origins of homochirality of biomolecules. However, the enantiomeric excess induced by CPL has been only very low (<2% ee). We found the unprecedented example of asymmetric autocatalysis triggered directly by a chiral physical factor, that is, right- and left-handed CPL, leading to a near enantiopure compound. Asymmetric photolysis of racemic pyrimidyl alkanol by r-CPL irradiation followed by asymmetric autocatalysis affords (R)-pyrimidyl alkanol with >99.5% ee. On the other hand, irradiation with l-CPL affords (S)-pyrimidyl alkanol with >99.5% ee. Thus, chiral physical power, such as CPL, in conjunction with asymmetric autocatalysis, provides a highly enantioenriched compound.

UV circular polarisation in star formation regions: the origin of homochirality?

Ultraviolet circularly polarised light has been suggested as the initial cause of the homochirality of organic molecules in terrestrial organisms, via enantiomeric selection of prebiotic molecules by asymmetric photolysis. We present a theoretical investigation of mechanisms by which ultraviolet circular polarisation may be produced in star formation regions. In the scenarios considered here, light scattering produces only a small percentage of net circular polarisation at any point in space, due to the forward throwing nature of the phase function in the ultraviolet. By contrast, dichroic extinction can produce a fairly high percentage of net circular polarisation ( approximately 10%) and may therefore play a key role in producing an enantiomeric excess.

Highly Enantioselective Asymmetric Synthesis by the Design of Asymmetric Catalysts and Reaction Media

AbstractFor Abstract see ChemInform Abstract in Full Text.

Circular Polarisation in Star Forming Regions: The Origin of Homochirality?

The origin of homochirality is one of the longest-standing puzzles in understanding the origins of life. In the laboratory, illumination by circularly polarised UV radiation (asymmetric photolysis) is an effective means of producing an enantiomeric excess in an otherwise racemic mix of chiral molecules. In the natural world, however, it has proven difficult to identify a suitable source of Circularly Polarised Light (CPL). Recent observations of L-excesses of 2–9% for a number of α-methyl amino acids in the Murchison meteorite and our discovery of large degrees of CPL in some star forming regions has added weight to the suggestion that the origin of homochirality is extra-terrestrial. Here we report initial modelling of the production of that CPL.

不斉触媒および反応場の設計に基づく高エナンチオ選択的不斉合成反応の研究

Highly enantioselective asymmetric syntheses, i.e., enantioselective addition of alkylmetal reagents to aldehydes and imines, enantioselective conjugate addition of dialkylzincs to enones, and asymmetric autocatalysis have been developed by designing asymmetric catalysts (or chiral ligands) and chiral environment of the reactions. Automultiplication of chiral compounds by asymmetric autocatalysis was discovered where a chiral product acts as a chiral catalyst for its own production. Asymmetric autocatalysis with>99.5% ee in a yield of > 99% was attained using pyrimidyl alkanol as an asymmetric autocatalyst in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde. The asymmetric autocatalysis exhibits significant positive non-linear effect on ee. Asymmetric autocatalyst with very low (ca. 0.00005%) ee enhances its ee significantly up to > 99.5% ee during the consecutive asymmetric autocatalysis without the assistance of any other chiral auxiliary. Moreover, amino acids and [6] helicene with very low ee which are produced by asymmetric photolysis and photosynthesis using circularly polarized light serve as chiral initiators of asymmetric autocatalysis, and pyrimidyl alkanol with high ee is obtained. Inorganic chiral crystals such as quartz and sodium chlorate, [2.2] paracyclophanes, allenes, chiral cobalt complexes due to the topology of coordination, chiral primary alcohols due to the deuterium substitution also work as chiral initiators. Asymmetric autocatalysis is capable to discriminate efficiently the chirality of various chiral compounds. Moreover, the reactions of pyrimidinecarbaldehyde with diisopropylzinc without adding chiral substance exhibit the formation of enantioenriched S or R pyrimidyl alkanols with approximate stochastic distribution, which indicates the reaction is a spontaneous absolute asymmetric synthesis. These results correlate for the first time the proposed origins of chirality of organic compounds with the chirality of organic compounds with very high ee.

分析化学的手法による多成分系D-,L-アミノ酸の光学分割の精査とその限界

The separation of hydrolyzed chiral amino acids in multiple components, such as high-molecular-weight interstellar type complex organics, was verified to compare with meteoritic amino acids. Reversed-phase high-performance liquid chromatography (RP-HPLC) gave±0.34% (n = 7) regarding the standard deviation(σ) of enantiomeric excesses (%D-%L) of D, L-alanine in a hydrolyzed fraction of complex organics. The chiral separation of D, L-alanine by the RP-HPLC method might be more convenient than gas chromatograph combined with the mass spectrometry (GC/MS) method in asymmetric synthesis verification. Possible pathways for the asymmetric synthesis of amino acid precursors (not asymmetric photolysis of monomer amino acids) were considered in terms of the detection limit of statistically significant enantiomeric excesses.

Asymmetric Photolysis of 2-Phenylcycloalkanones with Circularly Polarized Light: A Kinetic Model for Magnetic Field Effects

Abstract Magnetic field effects (MFE) on asymmetric photolysis involving a biradical intermediate have been investigated on a kinetic model where an intersystem crossing (ISC) of the intermediate is taken into account. Changes in the enantiomeric excesses (ee) of chiral substances with circularly polarized light (CPL) irradiation have been simulated, and the necessary conditions for observing the MFE were obtained. The asymmetric photolysis of racemic 2-phenylcycloalkanones (2-PCAs) with CPL has been carried out in both the presence and absence of a magnetic field. Since the anisotropy g factors of 2-PCAs are considerably large, the CPL-induced ee are achieved to a few percent after 90% decomposition though the MFE are not observed. The photolysis mechanism of 2-PCAs in an air-saturated solution has been also clarified. Triplet acyl–benzyl biradicals, formed via photochemical α-cleavage of 2-PCAs, react with O2 dissolved in the solution and result in the formation of acetophenone and alkenoic acids. The bimolecular reactions are diffusion-controlled and the rates are comparable to those of ISC to the singlet biradicals for all 2-PCAs. The recombination yields of the biradicals are sufficiently large. However, the biradical ISC rate shows little magnetic field dependence, which explains the absence of the MFE in this asymmetric photolysis.