Handbook of Solvents - George Wypych - ChemTech - Ventech!

26 Estanislao Silla, Arturo Arnau and Iñaki Tuñón
Figure 2.1.11. The microscopic environment in which the formation of zwitterion takes place is exhibited in these
four “snapshots”. It is possible to see the re-ordering of the structure of the molecule of glycine, along with the
re-ordering of the shell of molecules of water which surround it, at 200, 270, 405 and 440 femtoseconds from the
beginning of the simulation of the process by Molecular Dynamics.
to the zwitterion (Figure 2.1.7). This transition structure corresponds, then, to an
intramolecular protonic transfer from the initial form to the zwiterionic form of the
aminoacid. The calculations carried out reveal an activation energy of 2.39 Kcal/mol, and a
reaction energy of -1.15 Kcal/mol at the MP2/6-31+G** level.
A more visual check of the process submitted to study is achieved by means of hybrid
QM/MM Molecular Dynamics, 58 which permits “snapshots” to be obtained which reproduce
the geometry of the aminoacid surrounded by the solvent. In Figure 2.1.11 are shown
four of these snapshots , corresponding to times of 200, 270, 405 and 440 femtoseconds after
commencement of the process.
In the first of these, the glycine has still not been autoionized. Two molecules of water
(identified as A and B) form hydrogen bonds with the nitrogen of the amine group, whilst