A comparison of RNA with DNA in template-directed synthesis

Abstract

The unified `electroweak' theory of the electromagnetic and weak nuclear interactions in physics predicts a small energy difference (ΔEpv) between the left- and right-handed enantiomers of chiral molecules. Thus, electroweak theory provides one of several possible explanations for the origin of biomolecular homochirality (nature's preference for L-amino acids and D-monosaccharides). Recent systematic electroweak quantum-chemical studies find ΔEpv to be an order of magnitude larger than previously anticipated, which has sparked renewed interest in the subject. The present paper addresses, for the first time, the question of the relative stability of certain possible prebiotic precursor molecules suggested in the work of A. Eschenmoser and co-workers: aziridine-2-carbonitrile (CH2NHCHCN) and oxiranecarbonitrile (CH2OCHCN), also commonly referred to as 2-cyanoaziridine and cyanooxirane, respectively. The cis/trans-isomerization pathway of aziridine-2-carbonitrile is initially characterized by standard quantum-chemical techniques. At the highest level of theory employed, the trans-isomer is found to lie by 4.0 kJ mol−1 above its cis-counterpart. The transition state connecting the two is another 74 kJ mol−1 higher in energy. After including unscaled, harmonic zero-point energy corrections, these values change to 3.7 and 69 kJ mol−1, respectively. Using the multi-configuration linear response (MCLR) approach to electroweak quantum chemistry (R. Berger, M. Quack, J. Chem. Phys. 2000, 112, 3148), the energy difference between the enantiomers of the various compounds and transition structures has been computed within the random phase approximation, a special case of the general MCLR technique. (S)-Oxiranecarbonitrile and the (R)-configurations of aziridine-2-carbonitrile are found to be energetically more stable than their mirror images at their equilibrium nuclear configurations. Use of a solvation model to approximate an aqueous environment has a modest, but consistent, effect on the computed parity-violating energies. In the present examples, solvation increases the magnitude of the parity-violating energy by ca. 10%.