Handbook of Solvents - George Wypych - ChemTech - Ventech!

24 Estanislao Silla, Arturo Arnau and Iñaki Tuñón
Figure 2.1.8. Neutral and zwitterionic form of glycine.
Figure 2.1.9. Quantum Mechanics predicts the existence of two conformers of glycine in the gas phase whose main
difference is the rotation of the acid group around the axis which joins the two carbons. The structure I, the furthest
from the zwiterionic geometry, is revealed to be the most stable in gas phase.
highlights the importance of the solvent for the autoionized form of the aminoacids to prevail.
An analysis of the specific effects of the solvent in the formation and the stability of
the zwitterion can be addressed carrying out the calculations of the reaction path with and
without a molecule of water (Figure 2.1.10), this being within the philosophy of the
supermolecule calculations. This discrete molecule of solvent has been located such that it
actively participates in the migration of the proton from the oxygen to the nitrogen. 55
That molecule of water forms two hydrogen bonds so much with the neutral glycine as
zwitterionic, and when the glycine is transformed from the neutral configuration to the
zwitterion the interchange of two atoms of hydrogen is produced between the aminoacid
and the molecule of solvent. Thus, we reproduce from a theoretical point of view the process
of protonic Table transfer of an aminoacid with the participation of the solvent
(intermolecular mechanism). Table 2.1.2 shows the relative energies of the three solute-solvent
structures analysed, as well as that of the system formed by the amino acid and
the molecule of solvent individually.
The table shows that the neutral glycine - molecule of water complex is the most stable.
Moreover, the energy barrier which drives the state of transition is much greater than
that which is obtained when Quantum Mechanics is used to reproduce the autoionization of
the glycine with the presence of the solvent by means of an intramolecular process. 55 This
data suggests that even in the case where a larger number of water molecules are included in