Effects of Hydration on the Zwitterion Trialanine Conformation by Electronic Structure Theory

Exploration of interfacial hydration networks of zwitterion and nonionized trialanine has been performed using DFT-M062X quantum chemical computations explicitly considering up to 41 water molecules. The step-by-step water molecules peptide surrounding, carried out for unfolded extended (β), polyproline II (PPII) conformations reveals the crucial importance of explicit solvent effects in stabilizing the zwitterion form and the left-handed PPII-helix ubiquitously found at room temperature for short polyalanines. Hydration effects are much greater for the ionized form of the peptide; thus, the zwitterion is about 10 kcal mol^-1 more stable than the nonionized form. For the β → PPII transformation, the two components of free Gibbs energy act in the opposite direction; thus, it is favored by enthalpy but not by entropy. These findings agree with experimental data that report an equilibrium between these conformers modulated by temperature. Thermodynamic functions of the four conformers (β-β, β-PPII, PPII-β, and PPII-PPII) for zwitterion trialanine are similar to those derived for the protonated one (Ala₃H⁺); therefore, the peptidic conformation is independent of the pH of the solution. Rather, it reflects the high propensity of alanine toward PPII helix. The enthalpic preference of the PPII has electrostatic origin and it is owing to a more favorable interaction of dipole of each peptidic residue with water dipole of H-bonded molecules.