Does Adsorption at Hydroxyapatite Surfaces Induce Peptide Folding? Insights from Large-Scale B3LYP Calculations

Large-scale periodic quantum mechanical calculations (509 atoms, 7852 atomic orbitals) based on the hybrid B3LYP functional focused on the peptide folding induced by the adsorption on the (001) and (010) hydroxyapatite (HA) surfaces give interesting insights on the role of specific interactions between surface sites and the peptide, which stabilize the helix conformation over the “native” random coil ones for in silico designed model peptides. The two peptides were derived from the 12-Gly oligomer, with one (P1, C-tGGKGGGGGGEGGN-t) and two (P2, C-tGGKGGKEGGEGGN-t) glutamic acid (E) and lysine (K) residue mutations. The most stable gas-phase “native” conformation for both peptides resulted in a random coil (RC) structure, with the helix (H) conformation being ≈100 kJ mol–1 higher in free energy. The two peptide conformations interact with the HA (001) and (010) surfaces by C═O groups via Ca2+ ions, by hydrogen bond between NH2 groups and the basic PO43– groups and by a relevant fraction due to dispersion forces. Peptide adsorption was studied on the dry (001) surface, the wet one envisaging 2 H2O per surface Ca2+ and, on the latter, also considering the adsorption of microsolvated peptides with 4 H2O molecules located at sites responsible of the interaction with the surface. The P1 mutant does prefer to be adsorbed as a random coil by ≈160 kJ/mol, whereas the reverse is computed for P2, preferring the helix conformation by ≈50 kJ/mol. Adsorption as helix of both P1 and P2 mutants brings about proton transfer toward the HA surfaces with a large charge transfer component to the interaction energy.