Amino Acid Anions in Organic Ionic Compounds. An ab Initio Study of Selected Ion Pairs

The combination of amino acids in
their deprotonated and thus anionic
form with a choline cation gives origin to a new and potentially important
class of organic ionic compounds. A series of such neutral ion pairs
has been investigated by first principle methods. The results reveal
intriguing structural motives as well as regular patterns in the charge
distribution and predict a number of vibrational and optical properties
that could guide the experimental investigation of these compounds.
The replacement of choline with its phosphocholine analogue causes
the spontaneous reciprocal neutralization of cations and anions, taking
place through the transfer of a proton between the two ions. Systems
of this kind, therefore, provide a wide and easily accessible playground
to probe the ionic/polar transition in organic systems, while the
easy transfer of H+ among neutral and ionic species points
to their potential application as proton conductors. The analysis
of the ab initio data highlights similarities as well as discrepancies
from the rigid-ions force-field picture and suggests directions for
the improvement of empirical models.