title: Water adsorption at two unsolvated peptides with a protonated lysine residue: from self-solvation to solvation.
abstract: We study the initial steps of the interaction of water molecules with two unsolvated peptides: Ac-Ala(5)-LysH(+) and Ac-Ala(8)-LysH(+). Each peptide has two primary candidate sites for water adsorption near the C-terminus: a protonated carboxyl group and the protonated ammonium group of LysH(+), which is fully hydrogen-bonded (self-solvated) in the absence of water. Earlier experimental studies have shown that H(2)O adsorbs readily at Ac-Ala(5)-LysH(+) (a non-helical peptide) but with a much lower propensity at Ac-Ala(8)-LysH(+) (a helix) under the same conditions. The helical conformation of Ac-Ala(8)-LysH(+) has been suggested as the origin of the different behavior. We here use first-principles conformational searches (all-electron density functional theory based on a van der Waals corrected version of the PBE functional, PBE+vdW) to study the microsolvation of Ac-Ala(5)-LysH(+) with one to five water molecules and the monohydration of Ac-Ala(8)-LysH(+). In both cases, the most favorable water adsorption sites break intramolecular hydrogen bonds associated with the ammonium group, in contrast to earlier suggestions in the literature. A simple thermodynamic model yields Gibbs free energies ΔG(0)(T) and equilibrium constants in agreement with experiments. A qualitative change of the first adsorption site does not occur. For few water molecules, we do not consider carboxyl deprotonation or finite-temperature dynamics, but in a liquid solvent, both effects would be important. Exploratory ab initio molecular dynamics simulations illustrate the short-time effects of a droplet of 152 water molecules on the initial unsolvated conformation, including the deprotonation of the carboxyl group. The self-solvation of the ammonium group by intramolecular hydrogen bonds is lifted in favor of a solvation by water.
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Theory Department, Berlin, Germany.
authors.forename: Sucismita
authors.initials: S
authors.lastname: Chutia
authors.forename: Mariana
authors.initials: M
authors.lastname: Rossi
authors.forename: Volker
authors.initials: V
authors.lastname: Blum
mesh_terms: D000327:Adsorption; D006860:Hydrogen Bonding; D008239:Lysine / Q000737:chemistry*; D008744:Methylamines / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D010455:Peptides / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D008744:Methylamines; D010455:Peptides; D012997:Solvents; D014867:Water; C027451:methylamine; D008239:Lysine