  We present a structural data set of the 20 proteinogenic amino acids and
their amino-methylated and acetylated (capped) dipeptides. Different
protonation states of the backbone (uncharged and zwitterionic) were considered
for the amino acids as well as varied side chain protonation states.
Furthermore, we studied amino acids and dipeptides in complex with divalent
cations (Ca2+, Ba2+, Sr2+, Cd2+, Pb2+, and Hg2+). The database covers the
conformational hierarchies of 280 systems in a wide relative energy range of up
to 4 eV (390 kJ/mol), summing up to an overall of 45,892 stationary points on
the respective potential-energy surfaces. All systems were calculated on equal
first-principles footing, applying density-functional theory in the generalized
gradient approximation corrected for long-range van der Waals interactions. We
show good agreement to available experimental data for gas-phase ion
affinities. Our curated data can be utilized, for example, for a wide
comparison across chemical space of the building blocks of life, for the
parametrization of protein force fields, and for the calculation of reference
spectra for biophysical applications.
