Milieu-Initiated Inversion of the Aqueous Polyproline II/β Propensity in the Alanine Tripeptide: Aggregation Origin...

Extending our earlier analogous study of the alanine dipeptide (ADP), we have now analyzed the effect of the external environment on the polyproline II (P) and β relative energies, the P/β propensity, of the alanine tripeptide (ATP). Ab initio calculations of ATP(H₂O)₁₉ and ATP(H₂O)₁₉(HCl) exhibit the same propensity inversion as in ADP: in the pure water case the PP conformation is favored while the addition of the HCl molecule results in the ββ conformation being of lower energy. A comparison, following an intermediate insertion and departure of an HCl molecule, shows that the energy of a hydrogen-bonded (H₂O)₁₉βATP::βATP(H₂O)₁₉ structure is lower than that of the sum of two separate PP systems, i.e., that the aggregated state of the peptide is favored. This arises from the basic physical response to their total environmental influences. Questions about quantitative results from molecular dynamics simulations, obviously needed to analyze longer chains and other side chains, are addressed via rigid water calculations. The desirability of basing studies of amyloid formation on our proposed alternative milieu-folding paradigm is discussed.