5. Conclusions
Using Next generation QTAIM we discovered Sσcharacter chirality Cσ for the dominant torsional C1-N7BCP in HH (ordinary) i.e. formally achiral glycine. The associated bond-flexing Fσ and bond-axiality Aσ possessed Rσ andSσ character respectively. The introduction of the D and T isotopes caused a reversal of theRσ and Sσ character torsional C1-N7 BCP of the bond-flexing Fσ and bond-axiality Aσ. The presence of theSσ character chirality Cσ found for ordinary HH glycine was preserved after the addition of the D isotope but after replacement of the D isotope with the T isotope the chirality Cσ reversed to Rσcharacter. This reversal of the chirality Cσ, depending on the presence of a D or T isotope on the alpha carbon adds to the debate as to the nature of the extraterrestrial origins of chirality in simple amino acids. This is since either D or T isotopes of hydrogen may be present, albeit at rather low concentrations, in these hostile extraterrestrial environments.
We also considered the effects of the addition of the D and T isotopes on the bond-twist Tσ, bond-flexing Fσand the bond-axiality Aσ of the non-torsional C-H/D/TBCP s.
This theoretical analysis represents a new challenge for the eventual observation of glycine chiral properties. Interestingly, for vibrational spectroscopies the “classic” nuclear isotopic effects can be also exploited to identify any new weak signals not corresponding to the energy ranges of the main HH species.
As an example, the effect of the addition of the T isotope to the X3 and X10 sites on lowering the Fσ and Aσvalues for the O5-H6 BCP and C2-O5 BCP are expected to also affect the C2-O5-H6 group undergoing a bending vibration, which is strongly coupled to the methylene group wagging for HH species, but is expected to be uncoupled for HT. Our analysis therefore, can be used for the interpretation of infrared (IR) spectra to provide explanations of classical mass-dependent isotopic shifts as well as modifications in mode coupling and intensity changes.
The feasibility however, of such studies is still very limited, not only due to expected very weak chiroptical signals but even creation of alpha carbon monosubstitued D/T glycine in suitable amounts. As a result, theoretical analysis with NG-QTAIM at present is a unique highly sensitive method for the detection of isotopic chirality in glycine, an amino acid that is present in meteoritic organic compounds.
Furthermore this analysis demonstrates the possibility of directly relating the Rσ /Sσ chirality to the specific D/T isotopic substitution for a molecule in natural conditions, without the need to introduce external electric fields or perform challenging and difficult to interpret spectroscopic experiments. In future, experiments such as those undertaken by Beaulieuet al. on neutral molecules28 could be undertaken to detect coherent helical motion of bound electrons of formally achiral glycine and therefore assignRσ /Sσ chirality, possibly including the D isotope of glycine.
Funding: This research was funded by the National Natural Science Foundation of China grant number: 21673071. The One Hundred Talents Foundation of Hunan Province is also gratefully acknowledged for their support of S.J. and S.R.K.