Four branched-chain aliphatic α-amino acidsα-alanine,
valine, leucine, and isoleucine (1–4)were investigated by quantum-chemical calculations in water
as a solvent by two methods. The B3LYP variant of DFT calculations
was used to obtain the electronic structure and molecular descriptors
of these species in their canonical amino acid form as well as the
related zwitterionic form in three oxidation states (cation, neutral
molecule, and anion). A total of 24 species were subjected to full
geometry optimization and complete vibration analysis. Quantities
related to ionization or affinity processes were evaluated under adiabatic
conditions. The calculated standard reaction Gibbs energy facilitates
evaluation of the absolute oxidation and reduction potential. The
absolute reduction potential correlates with the electrophilicity
index, and the absolute oxidation potential correlates with the adiabatic
ionization energy. This finding makes it possible to skip the tedious
vibrational analysis and use electronic properties to estimate the
redox potentials. The molecular descriptors were compared with the
calculated properties of four linear amino acids (glycine, β-alanine,
GABA, and DAVA). Parallel calculations using the DLPNO–CCSD(T)
method gave analogous results for 24 species. The absolute oxidation
potential was related to the antioxidant activity index, which showed
only a moderate antioxidant activity of 1–4.