Peptide Hydration Phenomena through a Combined Quantum Chemical and Bottom-Up Approach

The M06-2X, TPSS, and B3PW91 density functionals and the classical ab initio MP2 method were used to study microsolvation around the protonated trialanine, Ala3H+. All adopted electronic structure approaches show the formation of wires or compact ring clusters of water molecules strongly bonded to peptidic polar groups through hydrogen bonds with hydration energy ranging from − 93 to –66 kcal mol–1. Independently from the adopted electronic structure methods, explicit water molecules favor peptidic chain with the polyproline II (PPII) conformation, thus the electronic energy stability order of the four unfolded conformers follows the sequence: PPII-PPII > β-PPII ∼ PPII-β > β-β, while entropy favors the reversed order. The delicate balance of electronic energy (or enthalpy) and entropy modulated by the temperature accounts for the change in abundance of the PPII and β conformations experimentally observed. The proposed bottom-up approach has been developed following the energetically dominant polar groups of peptide and water dipoles interactions. The intrapeptide dipole decoupling, caused by the β → PPII transformation, and the consequent greater dipole coupling with water molecules provide a rational base to explain the energy gain due to the explicit water coordination to PPII residues.