Isotopic chirality and molecular parity violation

Large vibrationally induced parity violation effects in CHDBrI. Chem Commun (Camb) 2023;59:14579-14582. [PMID: 37990542 DOI: 10.1039/d3cc03787h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The isotopically chiral molecular ion CHDBrI+ is identified as an exceptionally promising candidate for the detection of parity violation in vibrational transitions. The largest predicted parity-violating frequency shift reaches 1.8 Hz for the hydrogen wagging mode which has a sub-Hz natural line width and its vibrational frequency auspiciously lies in the available laser range. In stark contrast to this result, the parent neutral molecule is two orders of magnitude less sensitive to parity violation. The origin of this effect is analyzed and explained. Precision vibrational spectroscopy of CHDBrI+ is feasible as it is amenable to preparation at internally low temperatures and resistant to predissociation, promoting long interrogation times (Landau et al., J. Chem. Phys., 2023, 159, 114307). The intersection of these properties in this molecular ion places the first observation of parity violation in chiral molecules within reach.

Enantioselective Synthesis of Enantioisotopomers with Quantitative Chiral Analysis by Chiral Tag Rotational Spectroscopy

Fundamental to the synthesis of enantioenriched chiral molecules is the ability to assign absolute configuration at each stereogenic center, and to determine the enantiomeric excess for each compound. While determination of enantiomeric excess and absolute configuration is often considered routine in many facets of asymmetric synthesis, the same determinations for enantioisotopomers remains a formidable challenge. Here, we report the first highly enantioselective metal‐catalyzed synthesis of enantioisotopomers that are chiral by virtue of deuterium substitution along with the first general spectroscopic technique for assignment of the absolute configuration and quantitative determination of the enantiomeric excess of isotopically chiral molecules. Chiral tag rotational spectroscopy uses noncovalent chiral derivatization, which eliminates the possibility of racemization during derivatization, to perform the chiral analysis without the need of reference samples of the enantioisotopomer.

Perspectives on parity violation in chiral molecules: theory, spectroscopic experiment and biomolecular homochirality

The reflection (or ‘mirror’) symmetry of space is among the fundamental symmetries of physics. It is connected to the conservation law for the quantum number parity and a fundamental ‘non-observable’ property of space (as defined by an absolute ‘left-handed’ or ‘right-handed’ coordinate system). The discovery of the violation of this symmetry – the non-conservation of parity or ‘parity violation’ – in 1956/1957 had an important influence on the further development of physics. In chemistry the mirror symmetry of space is connected to the existence of enantiomers as isomers of chiral (‘handed’) molecules. These isomers would relate to each other as idealized left or right hand or as image and mirror image and would be energetically exactly equivalent with perfect space inversion symmetry. Parity violation results in an extremely small ‘parity violating’ energy difference between the ground states of the enantiomers which can be theoretically calculated to be about 100 aeV to 1 feV (equivalent to 10⁻¹¹ to 10⁻¹⁰ J mol⁻¹), depending on the molecule, but which has not yet been detected experimentally. Its detection remains one of the great challenges of current physical–chemical stereochemistry, with implications also for fundamental problems in physics. In biochemistry and molecular biology one finds a related fundamental question unanswered for more than 100 years: the evolution of ‘homochirality’, which is the practically exclusive preference of one chiral, enantiomeric form as building blocks in the biopolymers of all known forms of life (the l-amino acids in proteins and d-sugars in DNA, not the reverse d-amino acids or l-sugars). In astrobiology the spectroscopic detection of homochirality could be used as strong evidence for the existence of extraterrestrial life, if any. After a brief conceptual and historical introduction we review the development, current status, and progress along these three lines of research: theory, spectroscopic experiment and the outlook towards an understanding of the evolution of biomolecular homochirality.

The reflection (or ‘mirror’) symmetry of space is among the fundamental symmetries of physics. It is connected to the conservation law for the quantum number purity and its violation and has a fundamental relation to stereochemistry and molecular chirality.

Computational Comparison of Chemical and Isotopic Approaches to Control the Photoisomerization Dynamics of Light-Driven Molecular Motors

Synthetic molecular motors driven by E/Z photoisomerization reactions are able to produce unidirectional rotary motion because of a structural asymmetry that makes one direction of rotation more probable than the other. In most such motors, this asymmetry is realized through the incorporation of a chemically asymmetric carbon atom. Here, we present molecular dynamics simulations based on multiconfigurational quantum chemistry to investigate whether the merits of this approach can be equaled by an alternative approach that instead exploits isotopic chirality. By first considering an N-methylpyrrolidine–cyclopentadiene motor design, it is shown that isotopically chiral variants of this design undergo faster photoisomerizations than a chemically chiral counterpart, while maintaining rotary photoisomerization quantum yields of similarly high magnitude. However, by subsequently considering a pyrrolinium–cyclopentene design, it is also found that the introduction of isotopic chirality does not provide any control of the directionality of the photoinduced rotations within this framework. Taken together, the results highlight both the potential usefulness of isotopic rather than chemical chirality for the design of light-driven molecular motors, and the need for further studies to establish the exact structural circumstances under which this asymmetry is best exploited.

Unidirectional Rotary Motion in Isotopically Chiral Molecular Motors: A Computational Analysis

Molecular dynamics simulations are performed to explore if isotopic chirality can induce unidirectional rotary motion in molecular motors operated through double-bond photoisomerizations. Using a high-quantum yield motor featuring a chemically asymmetric carbon atom as reference, it is found that isotopically chiral counterparts of this motor sustain such motion almost equally well. Overall, the study reveals a previously unexplored role for isotopic chirality in the design of rotary molecular motors.

A molecular quantum switch based on tunneling in meta-d-phenol C6H4DOH

The concept of a molecular quantum switch is introduced from realistic, quantitative wavepacket analyses of tunneling switching in m-d-phenol.

Polyatomic Candidates for Cooling of Molecules with Lasers from Simple Theoretical Concepts

A rational approach to identify polyatomic molecules that appear to be promising candidates for direct Doppler cooling with lasers is outlined. First-principles calculations for equilibrium structures and Franck-Condon factors of selected representatives with different point-group symmetries (including the chiral nonsymmetric C1) have been performed and a high potential for laser cooling of these molecules is indicated.

Optical discrimination of racemic from achiral solutions

We demonstrate purely optical discrimination between achiral and racemic solutions by selectively triggering an asymmetric photoreaction with femtosecond laser pulses.

Conformational analysis of chiral supramolecular aggregates: modeling the subtle difference between hydrogen and deuterium

A detailed analysis of the conformational states of self-assembled, stereoselectively deuterated benzene-1,3,5-tricarboxamides ((S,S,S)-D-BTAs) reveals four different conformers for the supramolecular polymers. The relative amount of the conformers depends on the solvent structure and the temperature. With the help of a model, the thermodynamic parameters that characterize the different conformational states were quantified as well as the amount of the species that occur at different stages of the polymerization process. The results show that small changes in the stability between different types of conformers formed by (S,S,S)-D-BTAs—in the order of a few J mol(–1)—arise from the combination of interactions between the solvent/supramolecular aggregate, temperature, and solvent structure. While the introduction of a deuterium label allows to sensitively probe the solvophobic effects in the supramolecular aggregation, a rationalization of the observed effects on a molecular level is not yet straightforward but is proposed to result from subtle effects in the vibrational enthalpy and entropy terms of the isotope effect.

Jahn-Teller effects in molecular cations studied by photoelectron spectroscopy and group theory

The traditional "ball-and-stick" concept of molecular structure fails when the motion of the electrons is coupled to that of the nuclei. Such a situation arises in the Jahn-Teller (JT) effect which is very common in open-shell molecular systems, such as radicals or ions. The JT effect is well known to chemists as a mechanism that causes the distortion of an otherwise symmetric system. Its implications on the dynamics of molecules still represent unsolved problems in many cases. Herein we review recent progress in understanding the dynamic structure of molecular cations that have a high permutational symmetry by using rotationally resolved photoelectron spectroscopy and group theory. Specifically, we show how the pseudo-Jahn-Teller effect in the cyclopentadienyl cation causes electronic localization and nuclear delocalization. The fundamental physical mechanisms underlying the vaguely defined concept of "antiaromaticity" are thereby elucidated. Our investigation of the methane cation represents the first experimental characterization of the JT effect in a threefold degenerate electronic state. A special kind of isomerism resulting from the JT effect has been discovered and is predicted to exist in all JT systems in which the minima on the potential-energy surface are separated by substantial barriers.

Isotope chirality and asymmetric autocatalysis: a possible entry to biological chirality

Natural-abundance isotopic substitution in isotopically prochiral groups of otherwise achiral molecules can provide stochastically formed enantiomeric excesses which exceed the sensitivity threshold of sensitive asymmetric autocatalytic (Soai-type) reactions. This kind of induction of chirality should be taken into consideration in in vitro model experiments and offer a new kind of entry into primary prebiotic or early biotic enantioselection in the earliest stages of molecular evolution.

High-Resolution Spectroscopic Studies and Theory of Parity Violation in Chiral Molecules

We review the high-resolution spectroscopic approach toward the study of intramolecular dynamics, emphasizing molecular parity violation. Theoretical work in the past decade has shown that parity-violating potentials in chiral molecules are much larger (typically one to two orders of magnitude) than anticipated on the basis of older theories. This makes experimental approaches toward small molecular parity-violating effects promising. The concepts and results of intramolecular dynamics derived from spectroscopy are analyzed as a sequence of symmetry breakings. We summarize the concepts of symmetry breakings (de facto and de lege) in view of parity violation in chiral molecules. The experimental schemes and the current status of spectroscopic experiments on molecular parity violation are established. We discuss the promises of detecting and accurately measuring parity-violating energy differences Δpv E on the order of 10−11 J mol−1 (approximately 100 aeV) in enantiomers of chiral molecules with regard to their contribution to fundamental physics in the framework of the standard model of particle physics and more speculative future fundamental symmetry tests such as for the combined charge conjugation, parity, and time-reversal (CPT) symmetry violation.

Inhibition of monometalated methionine aminopeptidase: inhibitor discovery and crystallographic analysis

Two divalent metal ions are commonly seen in the active-site cavity of methionine aminopeptidase, and at least one of the metal ions is directly involved in catalysis. Although ample structural and functional information is available for dimetalated enzyme, methionine aminopeptidase likely functions as a monometalated enzyme under physiological conditions. Information on structure, as well as catalysis and inhibition, of the monometalated enzyme is lacking. By improving conditions of high-throughput screening, we identified a unique inhibitor with specificity toward the monometalated enzyme. Kinetic characterization indicates a mutual exclusivity in binding between the inhibitor and the second metal ion at the active site. This is confirmed by X-ray structure, and this inhibitor coordinates with the first metal ion and occupies the space normally occupied by the second metal ion. Kinetic and structural analyses of the inhibition by this and other inhibitors provide insight in designing effective inhibitors of methionine aminopeptidase.

High Resolution Rovibrational Spectroscopy of Chiral and Aromatic Compounds

The analysis of selected rovibrationally resolved infrared spectra of some relatively heavy and large polyatomic molecules is reviewed. A short historical summary of the development of high resolution interferometric Fourier transform infrared (FTIR) spectrometers is given and the possibilities of the currently most highly resolving FTIR spectrometer, which is commercially available in the Bruker IFS 125 series, are discussed. The computational tools necessary to analyse FTIR spectra are described briefly. As examples of rovibrational analysis the spectra of three selected molecules CHCl(2)F, CDBrClF, and pyridine (C(5)H(5)N) are discussed. The spectrum of CHCl(2)F, a fluorochlorohydrocarbon, is of interest for a better understanding of the chemistry of the Earth's atmosphere. CDBrClF is a chiral molecule and therefore the analysis of its rovibrational spectra provides the basis for carrying out further experiments towards the detection of molecular parity violation. The analysis of the pyridine FTIR spectra illustrates the potential of the new generation of FTIR spectrometers in the study of spectra and rovibrational dynamics of aromatic systems and molecules of potential biological interest.

Synthesis, characterization and development of a high-throughput methodology for the discovery of botulinum neurotoxin a inhibitors

Botulinum neurotoxins (BoNTs), etiological agents of the deadly food poisoning disease botulism, are the most toxic proteins currently known. Although only a few hundred cases of botulism are reported in the United States annually, there is growing interest in BoNTs attributable to their potential use as biological warfare agents. Neurotoxicity results from cleavage of the soluble NSF-attachment protein receptor complex proteins of the presynaptic vesicles by the BoNT light chain subunit, a Zn endopeptidase. Few effective inhibitors of BoNT/A LC (light chain) activity are known, and the discovery process is hampered by the lack of an efficient high-throughput assay for screening compound libraries. To alleviate this bottleneck, we have synthesized the peptide SNAPtide and have developed a robust assay for the high-throughput evaluation of BoNT/A LC inhibitors. Key aspects for the development of this optimized assay include the addition of a series of detergents, cosolvents, and salts, including 0.01% w/v Tween 20 to increase BoNT/A LC catalysis, stability, and ease of small molecule screening. To evaluate the effectiveness of the assay, a series of hydroxamate-based small molecules were synthesized and examined with BoNT/A LC. The methodology described is superior to other assays reported to date for the high-throughput identification of BoNT/A inhibitors.

Stereomutation Tunneling Switching Dynamics and Parity Violation in Chlorineperoxide Cl−O−O−Cl

In a search for efficient spectroscopic avenues toward experiments on molecular parity violation, we investigate the stereomutation tunneling processes in the axially chiral chlorine isotopomers of Cl2O2 by the quasi-adiabatic channel reaction path Hamiltonian (RPH) approach and the corresponding parity violating potentials by means of quantum chemical calculations including our recently developed Multiconfiguration linear response (MC-LR) approach to electroweak quantum chemistry. The calculated ground-state torsional tunneling splittings for all isotopomers of Cl2O2 are much smaller than the parity violating energy differences Delta(pv)E between the enantiomers of these molecules and therefore parity violation is predicted to dominate the quantum dynamics of stereomutation at low energies. We also compare these with torsional ground-state tunneling splittings and parity violating energy differences of the whole series of axially chiral HXYH(+) isotopomers (with X, Y= Cl(+), O, S, Se, Te). A comparison with our previous results for the homologous molecule Cl2S2 shows that for Cl2O2 a spectroscopic high-resolution analysis should be easier and the energy region of large tunneling splittings should be more easily accessible by IR excitation. We thus propose a scheme using "tunneling switching" with vibrational excitation in order to carry out the measurement of time-dependent parity violation in superposition states of initially well-defined parity. We discuss the advantages and drawbacks of such an experiment that can be carried out entirely in the IR spectral range (for Cl2O2 or related molecules).

Metal-Mediated Inhibition of Escherichia coli Methionine Aminopeptidase: Structure−Activity Relationships and Development of a Novel Scoring Function for Metal−Ligand Interactions

We report the discovery of thiabendazole as a potent inhibitor (K(i) = 0.4 microM) of Escherichia coli methionine aminopeptidase (ecMetAP) and the synthesis and pharmacological evaluation of thiabendazole congeners with activity in the upper nanomolar range. Elucidation of the X-ray structure of ecMetAP in complex with thiabendazole and an unrelated inhibitor that was independently described by another group showed that that both compounds bind to an additional Co(II) ion at the entrance of the active site. This unexpected finding explains the inactivity of the compounds under in vivo conditions. It also allows us to discuss the structure-activity relationships of this series of compounds in a meaningful way, based upon docking runs with an auxiliary metal ion. We describe a new scoring function for the evaluation of metal-mediated inhibitor binding that, unlike the previously used scoring function implemented in the docking program, allows us to distinguish between active and inactive compounds. Finally, conclusions for the structure-based design of in vivo-active inhibitors of ecMetAP are drawn.

Steps towards molecular parity violation in axially chiral molecules. I. Theory for allene and 1,3-difluoroallene

In view of exploring possibilities for an experimental investigation of molecular parity violation we report quantum-chemical calculations of the parity-conserving and parity-violating potentials in the framework of electroweak quantum chemistry in allene C3H4 and 1,3-difluoroallene C3H2F2, which is nonplanar and axially chiral in the electronic ground state but expected to be nearly planar and achiral in several electronically excited states. The parity-violating potentials Epv for allene and 1,3-difluoroallene calculated with the multiconfiguration linear-response (MC-LR) approach of Berger and Quack [J. Chem. Phys. 112, 3148 (2000)] show qualitatively similar behavior as a function of torsional angle tau with maximum values of about 0.5 pJ mol(-1) for C3H4 and 2 pJ mol(-1) for C3H2F2. However, in the latter case they are asymmetrically shifted around tau=90 degrees , with a nonzero value at the chiral equilibrium geometry resulting in a parity-violating energy difference between enantiomers DeltapvE=Epv(P)-Epv(M)=1.2 pJ mol(-1) (equivalent to about 10(-13) cm(-1)). The calculated barrier heights corresponding to the nonrigid (multiple, and in part chiral) transition states in 1,3-difluoroallene fall in the range of 180-200 kJ mol(-1). These high barriers result in hypothetical tunneling splittings much smaller than DeltapvE and thus parity violation dominates over tunneling for the stereomutation dynamics in 1,3-difluoroallene. Therefore, DeltapvE is predicted to be a spectroscopically measurable energy difference. Two of the lower excited electronic states of C3H2F2 (1A and 3A) are calculated to be planar or quasiplanar, allowing, in principle, for spectroscopic state selection of states of well-defined parity. The results are discussed in relation to possible schemes of measuring parity violation in chiral molecules.