  Chitin and protein are two main building blocks for many natural
biomaterials. The interaction between chitin and protein critically determines
the properties of the composite biological materials. As living organisms
usually encounter complex ambient conditions like water, pH and ions are
critical factors towards the structural integrity of biomaterials. It is
therefore essential to study the chitin-protein interface under different
environmental conditions. Here, an atomistic model consisting of a chitin
substrate and a protein filament is constructed, which is regarded as a
representative of the chitin-protein interface existing in many chitin-based
biomaterials. Based on this model, the mechanical properties of chitin-protein
interface under different moisture and pH values are investigated through
molecular dynamics simulations. The results reveal a weakening effect of water
towards the chitin-protein interface, as well as acidity, i.e. the protonated
protein forms a stronger adhesion to chitin than that in the alkaline
environment. In addition, the effect from side-chain of protein is studied and
it is found that certain kinds of amino acid can form hydrophobic connections
to chitin surface, which means that these peptides partly dodge the weakening
effect of water. Our observation indicates that terminuses and side-chains in
protein are of importance in forming interfacial hydrogen bonds. From our full
atomistic models, we can observe some molecular mechanisms about how protein
interacts with chitin in different conditions, which may spotlight the
engineering on biomaterials with similar interfaces.
