Threonine versus isothreonine in synthetic peptides analyzed by high-resolution liquid chromatography/tandem mass spectrometry

Molecular Properties of Hydroxyamino Acids in Water

Aliphatic hydroxyamino acids, namely, α-hydroxyglycine, α-hydroxyalanine, serine, threonine, and homoserine, were studied by quantum chemical calculations using two methods in water as a solvent. A hybrid variant of DFT-B3LYP was applied to optimize the geometry of neutral molecules, molecular cations, and anions for the canonical and zwitterionic form of amino acids. In the energy minimum, vibrational analysis was applied, enabling the evaluation of thermodynamic functions (internal energy, enthalpy, entropy, and Gibbs energy) of individual species and absolute oxidation and reduction potentials for redox couples. In the B3LYP preoptimized geometry, the advanced DLPNO-CCSD(T) method was applied to include the major part of the interelectron correlation energy. Calculated molecular descriptors were compared with previously studied molecules by the same method, and the whole set for 17 amino acids was processed by advanced statistical methods such as cluster analysis and principal component analysis. Calculated oxidation potentials correlate with the adiabatic ionization energies along a straight line, and analogously, the calculated reduction potential correlates with the electrophilicity index. The ionization energy in α-amino acids is systematically influenced (reduced) by the functional groups such as hydroxyl, methyl, ethyl, and iso-propyl; it decreases along a series of α-, β-, γ-, and δ-amino acids.