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.