We derive structural and binding energy trends for twenty amino acids, their dipeptides,
and their interactions with the divalent cations Ca
2+, Ba
2+, Sr
2+, Cd
2+, Pb
2+, and Hg
2+. The underlying data set consists of more than 45,000 first-principles predicted
conformers with relative energies up to ~4 eV (~400 kJ/mol). We show that only very
few distinct backbone structures of isolated amino acids and their dipeptides emerge
as lowest-energy conformers. The isolated amino acids predominantly adopt structures
that involve an acidic proton shared between the carboxy and amino function. Dipeptides
adopt one of two intramolecular-hydrogen bonded conformations C
5 or
. Upon complexation with a divalent cation, the accessible conformational space shrinks
and intramolecular hydrogen bonding is prevented due to strong electrostatic interaction
of backbone and side chain functional groups with cations. Clear correlations emerge
from the binding energies of the six divalent ions with amino acids and dipeptides.
Cd
2+ and Hg
2+ show the largest binding energies–a potential correlation with their known high acute
toxicities. Ca
2+ and Pb
2+ reveal almost identical binding energies across the entire series of amino acids
and dipeptides. This observation validates past indications that ion-mimicry of calcium
and lead should play an important role in a toxicological context.