Molecular dynamics simulations are performed to investigate
the molecular picture of water sorption in gecko keratin and the influence of
relative humidity (RH) on the local structure and dynamics in water-swollen
keratin. At low RHs, water sorption occurs through hydrogen bonding of
water with the hydrophilic groups of keratin. At high RHs (>80%), additional
water molecules connect to the first “layer” of amide-connected water
molecules (multimolecular sorption) through hydrogen bonds, giving rise to
a sigmoidal shape of the sorption isotherm. This causes the formation of large
chain-like clusters surrounding the hydrophilic groups of keratin, which upon a further increase of the RH form a percolating water
network. An examination of the dynamics of water molecules sorbed in keratin demonstrates that there are two states, bound and
free, for water. The dynamics of water in these states depends on the RH. At low RHs, large-scale translational motions of tightly
bound water molecules to keratin are needed to remake the entire hydration shell of the keratin. At high RHs (>80%), the water
molecules more quickly exchange between the two states. The center-of-mass mean-square displacement of water molecules
indicates a hopping motion of water molecules in the keratin solvation shell. The hopping mechanism is more pronounced at RHs <
80%. At higher RHs, water translation through water clusters (water network) dominates. We have observed two regimes for the
dependence of dynamical properties on the RH: a regime of gradual increase of the dynamics over 10% < RH < 80% and a regime of
drastic dynamic acceleration at RH > 80%. The latter regime begins exactly where the water uptake and the volume swelling also
increase much more and where a drastic change in the elastic properties of gecko keratin has been observed. A nearly linear relation
between the relaxation times for all dynamical processes and the water content of gecko keratin is observed.