Images
The Origin of Biomolecular Asymmetry –
Photochemistry and Enantioselective Analysis of Organic Molecules in
Interstellar Environments with Special Emphasis on Amino Acids.
Meierhenrich U.J.
Bremen University, Dept. Phys. Chemistry, FB 02, Leobener Straße, D-28359 Bremen, Germany
Summary
Biopolymers like DNA and proteins are strongly selective towards the chirality of their monomer units. The use of homochiral monomers is regarded as essential for the construction of biopolymers; the emergence of the molecular asymmetry is therefore considered a fundamental step in Chemical Evolution. This Habilitationsschrift focuses on physico-chemical mechanisms of the origin of biomolecular asymmetry, a phenomenon that has received increasing attention in science literature.
Several controversial theories have been developed to explain an abiogenic origin of the biomolecular chiral purity in terms of the physico-chemical processes involved. Principally, the theories had been classified into random mechanisms and determinate mechanisms. Random mechanisms are undirected processes that lead to an appearance of the biomolecular asymmetry by mere chance. The spontaneous chiral symmetry breaking in autocatalytic systems gives an example. Determinate mechanisms on the other hand involve the interaction of racemic substances with chiral physical driving forces, which cause the prevalence of one enantiomer. Examples of specific driving forces in deterministic scenarios include the adsorption of organic molecules on enantiomorph surfaces of crystals, the directed vortex, the weak force, spin-polarized electrons, and circularly polarized light (CPL). In particular, innovative concepts and results on absolute asymmetric synthesis induced by photochemical reactions are encouraging. Thus, searching for the origins of biomolecular homochirality leads to a strong interest in the fields of asymmetric photochemistry with special emphasis on absolute asymmetric synthesis. For this Habilitationsschrift I outlined the context in which absolute asymmetric photochemistry is of interest, summarized our recent concepts and advances in the field, and discussed briefly the current understanding of the underlying mechanisms involved. Predominantly, for the photochemical formation of optically active molecules in non-racemic yields one distinguishes between enantioselective photolysis and absolute asymmetric photosynthesis.
Conventional attempts on enantioselective photolysis were performed by studying the interaction of racemic organic molecules with r-CPL and l-CPL in aqueous solution. Solitary (pi*, n)-electronic transitions of the amino acids’ carboxylic groups with values close to 212 nm (5.85 eV) were accessible here, because water absorbs below 200 nm making higher energetic electronic transitions inaccessible. By our innovative and new approach, leucine molecules were exposed in their solid state to synchrotron CPL of variable polarization and energy in the French Synchrotron Facility LURE with a newly developed electromagnetic planar/helical crossed undulator. Using this concept, (pi*, pi1)-, (pi*, pi2)-, and even (s*, s)-transitions of amino acids were excited below 200 nm in order to provide more effective optical anisotropies. After photodecomposition of 70 % of the target the analysis of the remaining amino acids resulted in ± 2.6 % e.e. The sign of the induced e.e. was dependent on the direction of circular polarization. Using this concept the variation of the photon’s energy enabled the determination of the anisotropy factor (g) as a function of the wavelength, and from this values the conditions required to obtain significant enantioenrichments were in turn determined.
The pure synthesis of optically active molecules in nonracemic yields induced only by CPL has remained a task difficult to achieve. 30 years ago, photocyclization of alkenes in solution, performed in the presence of iodine, led to the formation of polyaromatic hydrocarbon molecules. By irradiation with CPL chiral hexahelicene was synthesized with optical yields below 2 %. Photoproduction of amino acids has been reported to be possible with the help of initial photon acceptors. Very recently two groups, one from Prof. L. Allamandola at NASA Ames, plus our team, demonstrated at the same time the spontaneous photoformation of a variety of chiral amino acid structures under simulated interstellar conditions. The theoretic background on interstellar ice photochemistry, the detailed experimental conditions, the results, and its implications are reported in this Habilitationsschrift. Since both groups used unpolarized light for the photoreaction the obtained amino acids were racemic as expected. In the very near future related experiments will be performed with CPL for the direct generation of amino acids showing a considerable enantiomeric enrichment.
The obtained
experimental data support the assumption that tiny ice grains can play host
to important reactions when irradiated by ultraviolet light. It is possible
that such ice grains could have become incorporated into the early cloud
that formed our Solar System and ended up on Earth, assisting life to start.
Several lines of evidence suggest that some of the building blocks of life
were delivered to the primitive Earth via (micro-) meteoroids and/or comets.
These results suggest that interstellar chemistry may have played a significant
part in supplying Earth with some of the organic materials needed to trigger
life. Furthermore, since new stars and planets are formed within the same
clouds in which new amino acids are being created, this probably increases
the chance that life has evolved elsewhere.
However, back in our field of absolute asymmetric photochemistry it is
hard to obtain evidence by laboratory simulation experiments only. Because
of this, the determination of enantiomeric excesses has become part of space
scientific programs. Today, the measurement of enantioenrichments is included
for the first time in one of the cornerstone missions of the European Space
Agency ESA, Nordwijk, the Netherlands, and the Max-Planck-Institut für
Aeronomie in Katlenburg-Lindau, Germany. This program, the cometary mission
ROSETTA was designed to identify chiral organic molecules in situ on the
surface of a comet and to determine values of enantiomeric excesses therein.
The ROSETTA orbiter will be launched January 12th, 2003 in order to carry
the small subsatellite ROSETTA Lander RoLand to comet 46P/Wirtanen, which
is designed to detach from the orbiter and land on the surface of the comet.
RoLand’s Cometary Sampling and Composition Experiment COSAC is designed
to identify complex organic molecules and to search for enantiomeric enhancements
in cometary matter. In the case of success of this mission we would be
able to specify the extraterrestrial precursors of life based on chiral
carbon chemistry in its prebiotic forms.
The development of the complete COSAC experiment is now ultimately finalized. The basic specifications with respect to the detection of enantiomeric excesses in cometary organic molecules are reported in this work. In order to analyse the cometary sample with respect to its chemical and enantiomeric composition in situ the matter will be heated for obtaining material in gaseous phase. The heating process will be accomplished in small ovens into which the sample has been injected by RoLand’s Sample Drill and Distribution Subsystem. The ovens will be mounted on a ‘carrousel’ and moved by rotation, from a position where the sample will be filled in, to the so-called ‘tapping station’, where the ovens are going to be closed and heated stepwise to levels programmable by ground commands. The pressure developed during each heating step will be measured and recorded, too, because it is indicative of the total amount of gas released. The analytes can be transformed into volatile components by a special development of a gas phase derivatization technique. The analytes in the gaseous phase will be given on capillary columns coated with chiral as well as non-chiral stationary phases. The chromatographically separated analytes are going to be detected, identified, and qualified on miniature thermal-conductivity detectors TCDs, which are coupled with a multi-reflectron TOF mass spectrometer. In this work, the test, the validation, and the selection of chiral stationary Chirasil-Val and cyclodextrin phases for the intended capillary gas-chromatographic identification of enantiomer organic compounds on 46P/Wirtanen are described. Apart from this, the test of the COSAC instrumentation with simulated cometary matter is reported in which chiral amino acids were successfully identified in our laboratory.
Preliminary enantiomer separations for the COSAC experiment showed that – among numerous organic molecules – branched chiral hydrocarbons can easily be separated into their corresponding enantiomers. A geochemical application of such enantiomer separations was performed. Isoprenoid structures are branched structures of hydrocarbons. Stereochemical investigations of isoprenoid structures in fossil samples are considered important in providing new insights into geochemical questions such as the origin, pathways of formation, and geological age of branched hydrocarbons along with their emergence of molecular homochirality. First studies on enantiomer separations of isoprenoid molecules in ancient sediments are described here.
With respect to the origin of biomolecular asymmetry we additionally prepare another experiment for BepiColombo, the ESA cornerstone mission 5 to Mercury. The concept goes back to the fact that CPL can be identified in galaxies, stars, the interstellar medium, and planets by photo-polarimetric measurements with Earth based telescopes. The observed values for Venus, Moon, Mars, and Jupiter were ‘symmetric’ and showed accordance with theory. However, for planet Mercury asymmetric parameters in the fractional CPL were measured that do not fit with calculations. For BepiColombo we proposed to investigate this phenomenon using a concept which includes two instruments. The first instrument is a high-resolution optical polarimeter, capable to determine and map the CPL by remote scanning of Mercury’s surface from the Mercury Planetary Orbiter MPO. The second instrument is an in situ sensor for the detection of the enantiomorphism of surface crystals and minerals, proposed to be included in the Mercury Lander MSE. With the combination of these instruments we hope to supply crucial information about the origin of biomolecular asymmetry. The required instrumentation is presented and discussed here.
Habilitationsschrift, University of Bremen (2002)
Last updated September 2003