Parity non conservation: NMR parameters in chiral molecules

Chiral discrimination in nuclear magnetic resonance spectroscopy

Chirality is a fundamental property of molecules whose spatial symmetry is characterized by the absence of improper rotations, making them not superimposable to their mirror image. Chiral molecules constitute the elementary building blocks of living species and one enantiomer is favoured in general (e.g. L-aminoacids and D-sugars pervade terrestrial homochiral biochemistry) because most chemical reactions producing natural substances are enantioselective. Since the effect of chiral chemicals and drugs on living beings can be markedly different between enantiomers, the quest for practical spectroscopical methods to scrutinize chirality is an issue of great importance and interest. Nuclear magnetic resonance (NMR) is a topmost analytical technique, but spectrometers currently used are 'blind' to chirality, i.e. unable to discriminate the two mirror-image forms of a chiral molecule, because, in the absence of a chiral solvent, the spectral parameters, chemical shifts and spin-spin coupling constants are identical for enantiomers. Therefore, the development of new procedures for routine chiral recognition would offer basic support to scientists. However, in the presence of magnetic fields, a distinction between true and false chirality is mandatory. The former epitomizes natural optical activity, which is rationalized by a time-even pseudoscalar, i.e. the trace of a second-rank tensor, the mixed electric dipole/magnetic dipole polarizability. The Faraday effect, magnetic circular dichroism and magnetic optical activity are instead related to a time-odd axial vector. The present review summarizes recent theoretical and experimental efforts to discriminate enantiomers via NMR spectroscopy, with the focus on the deep connection between chirality and symmetry properties under the combined set of fundamental discrete operations, namely charge conjugation, parity (space inversion) and time (motion) reversal.

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).

NMR and parity violation: low-temperature dependence in 1H CRAMPS and 13C CP/MAS ssNMR spectra of alanine enantiomer

Life is based on L-amino acids and D-sugars rather than the enantiomeric D-amino acids and L-sugars. This broken symmetry is now believed to be a feature of fundamental physics-a result of symmetry-breaking induced by the weak force, which makes one enantiomer slightly more stable than the other. An amplification mechanism based on quantum mechanical tunneling could give rise to a second-order phase transition. In order to understand the transition mechanism, we measured the temperature dependence of 1H CRAMPS solid state NMR and 13C CP/MAS spectra of D- and L-alanine crystals from 295 K through to 220 K. Obvious difference of NMR behaviors between two enantiomers was observed in the phase transition which may be related to one suggested by Salam, caused biochirality among twenty amino acids.

Computational studies of the electroweak origin of biomolecular handedness in natural sugars

The violation of parity by the weak interactions ensures that enantiomeric chiral molecules have inequivalent energies. These parity violating energy differences have been calculated, using ab initio methods, for the biologically important sugars deoxyribose, ribose, arabinose, xylose and lyxose. It is found that in each case the choice of which enantiomer is of lower energy is dependent on the molecular conformation adopted, particularly the type of furanose ring pucker. In general the D-enantiomers are favoured for molecules having a C 2 -endo pucker, whereas the L-series are preferred for C 3 -endo puckers. The significance of these energy differences for the transition from a prebiotic racemic geochemistry to a homochiral biochemistry in terrestrial evolution is discussed.

Electroweak enantioselection and the origin of life

All biomolecules are of one hand--but what determines which hand? Why is life based on L-amino acids and D-sugars rather than D-amino acids and L-sugars? We believe the symmetry-breaker could be the weak force, which causes enantiomers to differ very slightly in energy. In this paper we present our calculations of this parity-violating energy difference (PVED) for a range of important biomolecules, and in nearly all cases it is indeed the 'natural' enantiomer which is the more stable.