VUV photodynamics and chiral asymmetry in the photoionization of gas phase alanine enantiomers

Conformer-Selective Photoelectron Circular Dichroism

Conformational flexibility and chirality both play a key role in molecular recognition. It is therefore very useful to develop spectroscopic methods that simultaneously probe both properties. It has been theoretically predicted that photoelectron circular dichroism (PECD) should be very sensitive to conformational isomerism. However, experimental proof has been less forthcoming and only exists for a very few favorable cases. Here, we present a new PECD scheme based on resonance-enhanced two-photon ionization (RE2PI) using UV/Vis nanosecond laser excitations. The spectral resolution obtained thereby guarantees conformer-selectivity by inducing resonant conformer-specific ππ* S1←S0 transitions. We apply this experimental scheme to the study of chiral 1-indanol, which exists in two conformers linked by a ring inversion and defined by the position of the hydroxyl group, namely axial and equatorial. We show that the PECD of the equatorial and axial forms considerably differ in sign, magnitude and shape. We also discuss the influence of the total ionization energy, vibronic excitation of intermediate and final states, and relative polarization of the excitation and ionization lasers. Conformer-specificity adds a new dimension to the applications of PECD in analytical chemistry addressing now the general case of floppy systems.

Theoretical Investigation into a Possibility of Formation of Propylene Oxide Homochirality in Space

The preferential synthesis or destruction of a single enantiomer by ultraviolet circularly polarized light (UV-CPL) has been proposed as a possible triggering mechanism for the extraterrestrial origin of homochirality. Herein, we investigate the photoabsorption property of propylene oxide (c-C₃H₆O) for UV-CPL in the Lyman-α region. Our calculations show that c-C₃H₆O was produced by CH₃⁺ and CH₃CH(OH)CH₃ or C₃H₇^• and O (triplet). The computed electronic circular dichroism spectra show that c-C₃H₆O and the intermediate (CH₃CH(OH)CH₂⁺) could absorb the UV-CPL originating from the Lyman-α emitter spectrum, suggesting that the photolysis of c-C₃H₆O or CH₃CH(OH)CH₂⁺ upon irradiation could induce chiral symmetry breakage.

Photoelectron-photoion(s) coincidence studies of molecules of biological interest

Photoelectron-photoion(s) coincidence, PEPICO, experiments with synchrotron radiation have become one of the most powerful tools to investigate dissociative photoionization thanks to their selectivity. In this paper their application to the study of molecular species of biological interest in the gas phase is reviewed. Some applications of PEPICO to the study of potential radiosensitizers, amino acids and small peptides and opportunities offered by the advent of novel methods for the production of beams of these molecules are discussed.

Possible chemical and physical scenarios towards biological homochirality

The single chirality of biological molecules in terrestrial biology raises more questions than certitudes about its origin. The emergence of biological homochirality (BH) and its connection with the appearance of life have elicited a large number of theories related to the generation, amplification and preservation of a chiral bias in molecules of life under prebiotically relevant conditions. However, a global scenario is still lacking. Here, the possibility of inducing a significant chiral bias "from scratch", i.e. in the absence of pre-existing enantiomerically-enriched chemical species, will be considered first. It includes phenomena that are inherent to the nature of matter itself, such as the infinitesimal energy difference between enantiomers as a result of violation of parity in certain fundamental interactions, and physicochemical processes related to interactions between chiral organic molecules and physical fields, polarized particles, polarized spins and chiral surfaces. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently undergone significant advances thanks to the deracemization of conglomerates through Viedma ripening and asymmetric auto-catalysis with the Soai reaction. All these phenomena are commonly discussed as plausible sources of asymmetry under prebiotic conditions and are potentially accountable for the primeval chiral bias in molecules of life. Then, several scenarios will be discussed that are aimed to reflect the different debates about the emergence of BH: extra-terrestrial or terrestrial origin (where?), nature of the mechanisms leading to the propagation and enhancement of the primeval chiral bias (how?) and temporal sequence between chemical homochirality, BH and life emergence (when?). Intense and ongoing theories regarding the emergence of optically pure molecules at different moments of the evolution process towards life, i.e. at the levels of building blocks of Life, of the instructed or functional polymers, or even later at the stage of more elaborated chemical systems, will be critically discussed. The underlying principles and the experimental evidence will be commented for each scenario with particular attention on those leading to the induction and enhancement of enantiomeric excesses in proteinogenic amino acids, natural sugars, and their intermediates or derivatives. The aim of this review is to propose an updated and timely synopsis in order to stimulate new efforts in this interdisciplinary field.

Photoelectron Circular Dichroism as a Signature of Subtle Conformational Changes: The Case of Ring Inversion in 1-Indanol

Chirality plays a fundamental role in the molecular recognition processes. Molecular flexibility is also crucial in molecular recognition, allowing the interacting molecules to adjust their structures and hence optimize the interaction. Methods probing simultaneously chirality and molecular conformation are therefore crucially needed. Taking advantage of a possible control in the gas phase of the conformational distribution between the equatorial and axial conformers resulting from a ring inversion in jet-cooled 1-indanol, we demonstrate here the sensitivity of valence-shell photoelectron circular dichroism (PECD) to both chirality and subtle conformational changes, in a case where the photoelectron spectra of the two conformers are identical. For the highest occupied orbital, we observe a dramatic inversion of the PECD-induced photoelectron asymmetries, while the photoionization cross-section and usual anisotropy (β) parameter are completely insensitive to conformational isomerism. Such a sensitivity is a major asset for the ongoing developments of PECD-based techniques as a sensitive chiral (bio)chemical analytical tool in the gas phase.

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

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.

PECD study of a single conformer molecule: a critical comparison of experiment and theory

In this work we address a specific experimental and theoretical quest on the influence of a conformational population in the modeling of PhotoElectron Circular Dichroism spectroscopy. In the last two...

Hydrocarboxyl Radical as a Product of α-Alanine Ultraviolet Photolysis

UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S₂ excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO₂ + H or CO + OH. On the other hand, no photoproducts were detected by S₁ excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

Valence-shell photoelectron circular dichroism of ruthenium(III)-tris-(acetylacetonato) gas-phase enantiomers

Chiral transition-metal complexes are of interest in many fields ranging from asymmetric catalysis and molecular materials science to optoelectronic applications or fundamental physics including parity violation effects. We present here a combined theoretical and experimental investigation of gas-phase valence-shell photoelectron circular dichroism (PECD) on the challenging open-shell ruthenium(III)-tris-(acetylacetonato) complex, Ru(acac)₃. Enantiomerically pure Δ- or Λ-Ru(acac)₃, characterized by electronic circular dichroism (ECD), were vaporized and adiabatically expanded to produce a supersonic beam and photoionized by circularly-polarized VUV light from the DESIRS beamline at Synchrotron SOLEIL. Photoelectron spectroscopy (PES) and PECD experiments were conducted using a double imaging electron/ion coincidence spectrometer, and compared to density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. The open-shell character of Ru(acac)₃, which is not taken into account in our DFT approach, is expected to give rise to a wide multiplet structure, which is not resolved in our PES signals but whose presence might be inferred from the additional striking features observed in the PECD curves. Nevertheless, the DFT-based assignment of the electronic bands leads to the characterisation of the ionized orbitals. In line with other recent works, the results confirm that PECD persists independently on the localization and/or on the achiral or chiral nature of the initial orbital, but is rather a probe of the molecular potential as a whole. Overall, the measured PECD signals on Ru(acac)₃, a system exhibiting D₃ propeller-type chirality, are of similar magnitude compared to those on asymmetric-carbon-based chiral organic molecules which constitute the vast majority of species investigated so far, thus suggesting that PECD is a universal mechanism, inherent to any type of chirality.

Photoelectron Circular Dichroism in the Photodetachment of Amino Acid Anions

The chirality of chemical compounds is of undisputed importance in science and technology. In particular with respect to pharmacological application most molecules of interest cannot be accessed by the powerful techniques developed in recent years for gas phase analytes. Here, we demonstrate that the combination of electrospray ionization (ESI) with the detection of photoelectron circular dichroism (PECD) provides access to chirality information applicable to molecular materials with negligible vapor pressure, for example, amino acids. To this end, glutamic acid and 3,4‐dihydroxyphenylalanine (DOPA) have been electrosprayed into the source of a chirality spectrometer, where photodetachment is enforced and the PECD is detected. The technique can be expected to be conceptually applicable to all chemical systems with chirality based on molecular properties.

Conformer-dependent vacuum ultraviolet photodynamics and chiral asymmetries in pure enantiomers of gas phase proline

Proline is a unique amino-acid, with a secondary amine fixed within a pyrrolidine ring providing specific structural properties to proline-rich biopolymers. Gas-phase proline possesses four main H-bond stabilized conformers differing by the ring puckering and carboxylic acid orientation. The latter defines two classes of conformation, whose large ionization energy difference allows a unique conformer-class tagging via electron spectroscopy. Photoelectron circular dichroism (PECD) is an intense chiroptical effect sensitive to molecular structures, hence theorized to be highly conformation-dependent. Here, we present experimental evidence of an intense and striking conformer-specific PECD, measured in the vacuum ultraviolet (VUV) photoionization of proline, as well as a conformer-dependent cation fragmentation behavior. This finding, combined with theoretical modeling, allows a refinement of the conformational landscape and energetic ordering, that proves inaccessible to current molecular electronic structure calculations. Additionally, astrochemical implications regarding a possible link of PECD to the origin of life's homochirality are considered in terms of plausible temperature constraints.

Condensation Effects on Electron Chiral Asymmetries in the Photoionization of Serine: From Free Molecules to Nanoparticles

Structural changes at the molecular level, occurring at the onset of condensation, can be probed by angle-resolved valence photoelectron spectroscopy, which is inherently sensitive to the electronic structure. For larger condensed systems like aerosol particles, the observation of intrinsic angular anisotropies in photoemission (β parameters) is challenging due to the strong reduction of their magnitude by electron transport effects. Here, we use a less common, more sensitive observable in the form of the chiral asymmetry parameter to perform a comparative study of the VUV photoelectron spectroscopy and photoelectron circular dichroism (PECD) between pure gas phase enantiomers of the amino acid serine and their corresponding homochiral nanoparticles. We observe a relatively large (1%) and strongly kinetic energy-dependent asymmetry, discussed in terms of the emergence of local order and conformational changes potentially counterbalancing the loss of angular information due to electron transport scattering. This demonstrates the potential of PECD as a sensitive probe of the condensation effects from the gas phase to bulk-like chiral aerosol particles surpassing the potential of conventional photoemission observables such as β parameters.

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

Combined IR and UV laser spectroscopic techniques in molecular beams merged with theoretical approaches have proven to be an ideal tool to elucidate intrinsic structural properties on a molecular level. It offers the possibility to analyze structural changes, in a controlled molecular environment, when successively adding aggregation partners. By this, it further makes these techniques a valuable starting point for a bottom-up approach in understanding the forces shaping larger molecular systems. This bottom-up approach was successfully applied to neutral amino acids starting around the 1990s. Ever since, experimental and theoretical methods developed further, and investigations could be extended to larger peptide systems. Against this background, the review gives an introduction to secondary structures and experimental methods as well as a summary on theoretical approaches. Vibrational frequencies being characteristic probes of molecular structure and interactions are especially addressed. Archetypal biologically relevant secondary structures investigated by molecular beam spectroscopy are described, and the influences of specific peptide residues on conformational preferences as well as the competition between secondary structures are discussed. Important influences like microsolvation or aggregation behavior are presented. Beyond the linear α-peptides, the main results of structural analysis on cyclic systems as well as on β- and γ-peptides are summarized. Overall, this contribution addresses current aspects of molecular beam spectroscopy on peptides and related species and provides molecular level insights into manifold issues of chemical and biochemical relevance.

Using photoelectron elliptical dichroism (PEELD) to determine real-time variation of enantiomeric excess

In this work, the photoionization of chiral molecules by an elliptically polarized, high repetition rate, femtosecond laser is probed. The resulting 3D photoelectron angular distribution shows a strong forward-backward asymmetry, which is highly dependent not only on the molecular structure but also on the ellipticity of the laser pulse. By continuously varying the laser ellipticity, we can observe molecular and enantiomer changes in real time at a previously unseen speed and precision. The technique allows enantiomeric excess of a pure compound to be measured with a 5% precision within 3 s, and a 10-min acquisition yields a precision of 0.4%. The isomers camphor and fenchone can be easily distinguished, unlike with conventional mass spectrometry. Preliminary results for the pharmaceutically interesting ibuprofen are also given, showing the capability of photoionization as a means of distinguishing larger molecular systems.

Decoupling vibration and electron energy dependencies in the photoelectron circular dichroism of a terpene, 3-carene

A fresh perspective on the interaction of electron and nuclear motions in photon induced dynamical processes can be provided by the coupling of photoelectron angular distributions and cation vibrational states in the photoionization of chiral molecules using circularly polarized radiation. The chiral contributions, manifesting as a forward-backward asymmetry in the photoemission, can be assessed using Photoelectron Circular Dichroism (PECD), which has revealed an enhanced vibrational influence exerted on the outgoing photoelectron. In this paper, we investigate the PECD of a rigid chiral monoterpene, 3-carene, using single-photon vacuum ultraviolet ionization by polarized synchrotron radiation and selecting energies from the ionization threshold up to 19.0 eV. By judicious choice of these photon energies, two factors that influence PECD asymmetry values, electron kinetic energy and ion vibrational level, can be effectively isolated, allowing a clear demonstration of the very marked vibrational effects. A slow photoelectron spectrum is used to examine the vibrational structure of the isolated outermost valence (HOMO) photoelectron band, and peak assignments are made with the aid of a Franck-Condon simulation. Together, these provide an estimate of the adiabatic ionization energy as 8.385 eV. The reported chiral asymmetry from the randomly oriented 3-carene enantiomers reaches a maximum of over 21%. Theoretical PECD calculations, made both for the fixed equilibrium molecular geometry and also modeling selected normal mode vibration effects, are presented to provide further insight.

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

The biomolecular homochirality in living organisms has been investigated for decades, but its origin remains poorly understood. It has been shown that circular polarized light (CPL) and other energy sources are capable of inducing small enantiomeric excesses (ees) in some primary biomolecules, such as amino acids or sugars. Since the first findings of amino acids in carbonaceous meteorites, a scenario in which essential chiral biomolecules originate in space and are delivered by celestial bodies has arisen. Numerous studies have thus focused on their detection, identification, and enantiomeric excess calculations in extraterrestrial matrices. In this review we summarize the discoveries in amino acids, sugars, and organophosphorus compounds in meteorites, comets, and laboratory-simulated interstellar ices. Based on available analytical data, we also discuss their interactions with CPL in the ultraviolet (UV) and vacuum ultraviolet (VUV) regions, their abiotic chiral or achiral synthesis, and their enantiomeric distribution. Without doubt, further laboratory investigations and upcoming space missions are required to shed more light on our potential extraterrestrial molecular origins.

Synchrotron-radiation vacuum-ultraviolet circular dichroism spectroscopy in structural biology: an overview

Circular dichroism spectroscopy is widely used for analyzing the structures of chiral molecules, including biomolecules. Vacuum-ultraviolet circular dichroism (VUVCD) spectroscopy using synchrotron radiation can extend the short-wavelength limit into the vacuum-ultraviolet region (down to ~160 nm) to provide detailed and new information about the structures of biomolecules in combination with theoretical analysis and bioinformatics. The VUVCD spectra of saccharides can detect the high-energy transitions of chromophores such as hydroxy and acetal groups, disclosing the contributions of inter- or intramolecular hydrogen bonds to the equilibrium configuration of monosaccharides in aqueous solution. The roles of hydration in the fluctuation of the dihedral angles of carboxyl and amino groups of amino acids can be clarified by comparing the observed VUVCD spectra with those calculated theoretically. The VUVCD spectra of proteins markedly improves the accuracy of predicting the contents and number of segments of the secondary structures, and their amino acid sequences when combined with bioinformatics, for not only native but also nonnative and membrane-bound proteins. The VUVCD spectra of nucleic acids confirm the contributions of the base composition and sequence to the conformation in comparative analyses of synthetic poly-nucleotides composed of selected bases. This review surveys these recent applications of synchrotron-radiation VUVCD spectroscopy in structural biology, covering saccharides, amino acids, proteins, and nucleic acids.

Probing ultrafast dynamics of chiral molecules using time-resolved photoelectron circular dichroism

Measuring the ultrafast dynamics of chiral molecules in the gas phase has been a long standing and challenging quest of molecular physics. The main limitation to reach that goal has been the lack of highly sensitive chiroptical measurement. By enabling chiral discrimination with up to several 10% of sensitivity, photoelectron circular dichroism (PECD) offers a solution to this issue. However, tracking ultrafast processes requires measuring PECD with ultrashort light pulses. Here we compare the PECD obtained with different light sources, from the extreme ultraviolet to the mid-infrared range, leading to different ionization regimes: single-photon, resonance-enhanced multiphoton, above-threshold and tunnel ionization. We use single and multiphoton ionization to probe the ultrafast relaxation of fenchone molecules photoexcited in their first Rydberg states. We show that time-resolved PECD enables revealing dynamics much faster than the population decay of the Rydberg states, demonstrating the high sensitivity of this technique to vibronic relaxation.

Intense Vibronic Modulation of the Chiral Photoelectron Angular Distribution Generated by Photoionization of Limonene Enantiomers with Circularly Polarized Synchrotron Radiation

Photoionization of the chiral monoterpene limonene has been investigated using polarized synchrotron radiation between the adiabatic ionization threshold, 8.505 and 23.5 eV. A rich vibrational structure is seen in the threshold photoelectron spectrum and is interpreted using a variety of computational methods. The corresponding photoelectron circular dichroism-measured in the photoelectron angular distribution as a forward-backward asymmetry with respect to the photon direction-was found to be strongly dependent on the vibronic structure appearing in the photoelectron spectra, with the observed asymmetry even switching direction in between the major vibrational peaks. This effect can be ultimately attributed to the sensitivity of this dichroism to small phase shifts between adjacent partial waves of the outgoing photoelectron. These observations have implications for potential applications of this chiroptical technique, where the enantioselective analysis of monoterpene components is of particular interest.

d-Amino acids in molecular evolution in space - Absolute asymmetric photolysis and synthesis of amino acids by circularly polarized light

Living organisms on the Earth almost exclusively use l-amino acids for the molecular architecture of proteins. The biological occurrence of d-amino acids is rare, although their functions in various organisms are being gradually understood. A possible explanation for the origin of biomolecular homochirality is the delivery of enantioenriched molecules via extraterrestrial bodies, such as asteroids and comets on early Earth. For the asymmetric formation of amino acids and their precursor molecules in interstellar environments, the interaction with circularly polarized photons is considered to have played a potential role in causing chiral asymmetry. In this review, we summarize recent progress in the investigation of chirality transfer from chiral photons to amino acids involving the two major processes of asymmetric photolysis and asymmetric synthesis. We will discuss analytical data on cometary and meteoritic amino acids and their potential impact delivery to the early Earth. The ongoing and future ambitious space missions, Hayabusa2, OSIRIS-REx, ExoMars 2020, and MMX, are scheduled to provide new insights into the chirality of extraterrestrial organic molecules and their potential relation to the terrestrial homochirality. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

Conformational effects in photoelectron circular dichroism

Photoelectron circular dichroism (PECD) is a novel type of spectroscopy, which presents surprising sensitivity to conformational effects in chiral systems. While classical photoelectron spectroscopy mainly responds to conformational effects in terms of energy level shifts, PECD provides a rich and detailed response to tiny changes in electronic and structural properties by means of the intensity dispersion of the circular dichroism as a function of photoelectron kinetic energy. In this work, the basics of PECD will be outlined, emphasizing the role of interference from the [Formula: see text] outgoing partial wave of the photoelectron in the PECD transition matrix element, which is responsible for the extreme sensitivity to conformational effects. Examples using molecular systems and interfaces will shed light on the powerful application of PECD to classical conformational effects such as group substitution, isomerism, conformer population and clustering. Moreover, the PECD results will be reported in challenging new fields where conformations play a key role, such as vibrational effects, transient chirality and time- resolved experiments. To date, PECD has mostly been based on synchrotron radiation facilities, but it also has a future as a table-top lab experiment by means of multiphoton ionization. An important application of PECD as an analytical tool will be reported. The aim of this review is to illustrate that in PECD, the presence of conformational effects is essential for understanding a wide range of effects from a new perspective, making it different from classical spectroscopy.

The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid

The valence threshold photoionization of oleic acid has been studied using synchrotron VUV radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. An oleic acid aerosol beam was impacted on a copper thermodesorber, heated to 130 °C, to evaporate the particles quantitatively. Upon threshold photoionization, oleic acid produces the intact parent ion first, followed by dehydration at higher energies. Starting at ca. 10 eV, a large number of fragment ions slowly rise suggesting several fragmentation coordinates with quasi-degenerate activation energies. However, water loss is the dominant low-energy dissociation channel, and it is shown to be closely related to the unsaturated carbon chain. In the lowest-barrier process, one of the four allylic hydrogen atoms is transferred to the carboxyl group to form the leaving water molecule and a cyclic ketone fragment ion. A statistical model to analyze the breakdown diagram and measured rate constants yields a 0 K appearance energy of 9.77 eV, which can be compared with the density functional theory result of 9.19 eV. Alternative H-transfer steps yielding a terminal C=O group are ruled out based on energetics and kinetics arguments. Some of the previous photoionization mass spectrometric studies also reported 2 amu and 26 amu loss fragment ions, corresponding to hydrogen and acetylene loss. We could not identify such peaks in the mass spectrum of oleic acid.

An imaging photoelectron-photoion coincidence investigation of homochiral 2R,3R-butanediol clusters

We report an experimental investigation of homochiral cluster formation in seeded molecular beam expansions of (2R,3R)-butanediol. Synchrotron radiation vacuum ultraviolet photoionization measurements have been performed using a double imaging electron-ion spectrometer in various configurations and modes of operation. These include measurements of the cluster ion mass spectra, wavelength scanned ion yields, and threshold electron spectra. Protonated cluster ions ranging up to n = 7 have been observed and size-selected photoelectron spectra and photoelectron circular dichroism (PECD) have been recorded by velocity map imaging, recorded in coincidence with ions, at a number of fixed photon energies. Translation temperatures of the cluster ions have been further examined by ion imaging measurements. As well as the sequence of protonated clusters with integral numbers of butanediol monomer units, a second series with half-integral monomer masses is observed and deduced to result from a facile cleavage of a butanediol monomer moiety within the nascent cluster. This second sequence of half-integral masses displays quite distinct behaviours. PECD measurements are used to show that the half-integral mass cluster ions do not share a common parentage with whole integer masses. Using an analogy developed with simple theoretical calculations of butanediol dimer structures, it is inferred that the dissociative branching into integral and half-integral ion mass sequences is controlled by the presence of different butanediol monomer conformations within the hydrogen bonded clusters.

Observation of Enhanced Chiral Asymmetries in the Inner-Shell Photoionization of Uniaxially Oriented Methyloxirane Enantiomers

Most large molecules are chiral in their structure: they exist as two enantiomers, which are mirror images of each other. Whereas the rovibronic sublevels of two enantiomers are almost identical (neglecting a minuscular effect of the weak interaction), it turns out that the photoelectric effect is sensitive to the absolute configuration of the ionized enantiomer. Indeed, photoionization of randomly oriented enantiomers by left or right circularly polarized light results in a slightly different electron flux parallel or antiparallel with respect to the photon propagation direction-an effect termed photoelectron circular dichroism (PECD). Our comprehensive study demonstrates that the origin of PECD can be found in the molecular frame electron emission pattern connecting PECD to other fundamental photophysical effects such as the circular dichroism in angular distributions (CDAD). Accordingly, distinct spatial orientations of a chiral molecule enhance the PECD by a factor of about 10.

Photochemical Chiral Symmetry Breaking in Alanine

We introduce a general theoretical approach for the simulation of photochemical dynamics under the influence of circularly polarized light to explore the possibility of generating enantiomeric enrichment through polarized-light-selective photochemistry. The method is applied to the simulation of the photolysis of alanine, a prototype chiral amino acid. We show that a systematic enantiomeric enrichment can be obtained depending on the helicity of the circularly polarized light that induces the excited-state photochemistry of alanine. By analyzing the patterns of the photoinduced fragmentation of alanine we find an inducible enantiomeric enrichment up to 1.7%, which is also in good correspondence to the experimental findings. Our method is generally applicable to complex systems and might serve to systematically explore the photochemical origin of homochirality.

Multilayer based soft-x-ray polarimeter at MAX IV Laboratory

A high precision five rotation-axes polarimeter using transmission multilayers as polarizers and reflection multilayers as analyzers has been designed and manufactured. To cover the extreme ultraviolet regime, Mo/Si, Cr/C, Sc/Cr, and W/B4C multilayers for transmission and reflection have also been designed and produced. The polarimeter mechanics is supported on a hexapod to simplify the alignment relative to photon beam. The instrument is designed so that it can be easily transferred between different beamlines.

Determination of accurate electron chiral asymmetries in fenchone and camphor in the VUV range: sensitivity to isomerism and enantiomeric purity

Photoelectron circular dichroism is a chiroptical effect able to distinguish isomers and to determine accurately the enantiopurity of chiral compounds.

N-Acetylglycine Cation Tautomerization Enabled by the Peptide Bond

We present a combined experimental and theoretical study of the ionization of N-acetylglycine molecules by 48 keV O(6+) ions. We focus on the single ionization channel of this interaction. In addition to the prompt fragmentation of the N-acetylglycine cation, we also observe the formation of metastable parent ions with lifetimes in the microsecond range. On the basis of density functional theory calculations, we assign these metastable ions to the diol tautomer of N-acetylglycine. In comparison with the simple amino acids, the tautomerization rate is higher because of the presence of the peptide bond. The study of a simple biologically relevant molecule containing a peptide bond allows us to demonstrate how increasing the complexity of the structure influences the behavior of the ionized molecule.