Molecular dynamics simulations and elastic neutron scattering experiments have been used to highlight how the structural organization of hydration water is able in some cases to locally constrain atomic movements at biologic interfaces. Using fully hydrated small peptides as models of protein interfaces, we show that the length of the side chains and the hydrophilic backbone have specific signatures. The dynamics of the side chain, which is part of biomolecules, have not only a crucial role in the whole flexibility as compared to the backbone, but also modify the values of transition temperatures. The analysis of the activation energies of methyl group dynamics suggests that the interaction between hydrophobic side chain and surrounding water plays an important role in the whole flexibility as well. We suggest that the progressive water cluster organization, around hydrophobic interfaces increases the activation energy and that a plateau regime is reached only when an extended hydrogen-bond network is established. The cluster size corresponds to a single layer of water molecules.