Molecular description of the formation and structure of plasticized globular protein films

To optimize the properties of plasticized globular proteins films, a clear comprehension of the structure and molecular events occurring during film formation is required. In this work, the structural organization of beta-lactoglobulin (beta-lg) films plasticized with diethyelene glycol are investigated for the first time during the entire film formation process by attenuated total reflectance and transmission infrared spectroscopy. The films are made by a common two-step procedure consisting of a first heat treatment (80 degrees C/30 min) followed by the casting of the film-forming solution for dehydration. Heating at 80 degrees C leads to the self-aggregation of the proteins with a conversion of regular secondary structures into antiparallel beta-sheets. The kinetics of the conformational conversion shows that approximately 10% of the amino acids are involved in beta-sheets after the first step. Dehydration induces a further aggregation, with approximately 46% of the amino acids involved in beta-sheets in the final film. Water evaporation results in the association of the aggregates formed during the heating step. The presence of the plasticizer during water removal is essential as it allows specific conformational rearrangements into extended beta-sheets and ordering of the polypeptide chains. This work underlines that the assembly of building blocks is common in beta-lg networks and it emphasizes the widespread occurrence of beta-structures in synthetic and natural protein networks.