Nykola C. Jones

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.

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.

An undulator-based spherical grating monochromator beamline for low energy electron-molecule scattering experiments

A new synchrotron radiation undulator beamline, SGM2, with an energy range of 12–20 eV, has been commissioned on the ASTRID storage ring at the University of Aarhus. Using a spherical grating monochromator, the beamline is presently optimized for an energy of 15.76 eV (78.65 nm), for the 3p5(2P1/2)11s resonance in argon, for use in electron-molecule scattering experiments. Using this beamline in conjunction with an electron-molecule scattering apparatus, a beam of electrons down to kinetic energies of a few meV with a resolution of ∼1 meV full width at half maximum is routinely produced.

Determination of Degree of Ionization of Poly(allylamine hydrochloride) (PAH) and Poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) in Layer-by-Layer Films using Vacuum Photoabsorption Spectroscopy

Electrostatic and hydrophobic interactions govern most of the properties of supramolecular systems, which is the reason determining the degree of ionization of macromolecules has become crucial for many applications. In this paper, we show that high-resolution ultraviolet spectroscopy (VUV) can be used to determine the degree of ionization and its effect on the electronic excitation energies of layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). A full assignment of the VUV peaks of these polyelectrolytes in solution and in cast or LbL films could be made, with their pH dependence allowing us to determine the pK(a) using the Henderson-Hasselbach equation. The pK(a) for PAZO increased from ca. 6 in solution to ca. 7.3 in LbL films owing to the charge transfer from PAH. Significantly, even using solutions at a fixed pH for PAH, the amount adsorbed on the LbL films still varied with the pH of the PAZO solutions due to these molecular-level interactions. Therefore, the procedure based on a comparison of VUV spectra from solutions and films obtained under distinct conditions is useful to determine the degree of dissociation of macromolecules, in addition to permitting interrogation of interface effects in multilayer films.

Band gap structure modification of amorphous anodic Al oxide film by Ti-alloying

The band structure of pure and Ti-alloyed anodic aluminum oxide has been examined as a function of Ti concentration varying from 2 to 20 at. %. The band gap energy of Ti-alloyed anodic Al oxide decreases with increasing Ti concentration. X-ray absorption spectroscopy reveals that Ti atoms are not located in a TiO2 unit in the oxide layer, but rather in a mixed Ti-Al oxide layer. The optical band gap energy of the anodic oxide layers was determined by vacuum ultraviolet spectroscopy in the energy range from 4.1 to 9.2 eV (300–135 nm). The results indicate that amorphous anodic Al2O3 has a direct band gap of 7.3 eV, which is about ∼1.4 eV lower than its crystalline counterpart (single-crystal Al2O3). Upon Ti-alloying, extra bands appear within the band gap of amorphous Al2O3, mainly caused by Ti 3d orbitals localized at the Ti site.

The ionic states of iodobenzene studied by photoionization and ab initio configuration interaction and DFT computations

New valence electron photoelectron spectra of iodobenzene obtained using synchrotron radiation have been recorded. Ionization energies (IEs) determined using multi-configuration SCF calculation (MCSCF) procedures confirmed the adiabatic IE order as: X(2)B1<A(2)A2<B(2)B2<C(2)B1. Although it is convenient to retain C2v labelling, there is an evidence that minor distortion to CS symmetry occurs at the MCSCF level for the C state. The fifth ionization process shown to be D(2)A1 exhibits dissociation to C6H5 (+) + I both in the experimental and theoretical studies. The calculated Franck-Condon vibrational spectral envelopes, including hot band contributions, for the first four ionic states reproduce the observed peak positions and intensities with reasonable accuracy. In order to simulate the observed spectra, different bandwidths are required for different states. The increase in the required bandwidths for the A(2)A2 and B(2)B2 states is attributed to internal conversion to lower-lying states. The presence of relatively high intensity sequence bands leads to asymmetry of each of the X(2)B1 state bands.

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.

Electronic excitation of furfural as probed by high-resolution vacuum ultraviolet spectroscopy, electron energy loss spectroscopy, and ab initio calculations

The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range.

Spontaneous electric fields in solid films: spontelectrics☆

When dipolar gases are condensed at sufficiently low temperature onto a solid surface, they form films that may spontaneously exhibit electric fields in excess of 108 V/m. This effect, called the ‘spontelectric effect’, was recently revealed using an instrument designed to measure scattering and capture of low energy electrons by molecular films. In this review it is described how this discovery was made and the properties of materials that display the spontelectric effect, so-called ‘spontelectrics’, are set out. A discussion is included of properties that differentiate spontelectrics from ferroelectrics and other species in which spontaneous polarisation may be found. Spontelectric films may be composed of a number of quite mundane dipolar molecules that involve such diverse dipolar species as propane, nitrous oxide or methyl formate. Experimental results are presented for spontelectrics illustrating that the spontelectric field generally decreases monotonically with increasing deposition temperature, with the exception of methyl formate that shows an increase beyond a critical range of deposition temperature. Films of spontelectric material show a Curie temperature above which the spontelectric effect disappears. Heterolayers may also be laid down creating potential wells on the nanoscale. A model is put forward based upon competition between dipole alignment and thermal disorder, which is successful in reproducing the variation of the degree of dipole alignment and the spontelectric field with deposition temperature, including the behaviour of methyl formate. This model and associated data lead to the conclusion that the spontelectric effect is new in solid-state physics and that spontelectrics represent a new class of materials.