Protein Engineering for Construction Material Performance: Effect of Protein Denaturation on Air-Entraining Function

In this study, we evaluate how denaturing agents, including sodium dodecyl sulfate, urea, and lignin, unfold protein molecular structure and, in turn, influence air-entraining performance in cement paste. Seven proteins were studied at different concentrations. The proteins and denaturants were prepared in solution to induce the binding of denaturants to proteins and the unfolding of proteins, resulting in the formation of protein-denaturant complexes. Surface activity, foaming properties, fresh paste flowability, and setting times were examined. The hardened microstructure of cement paste was characterized by X-ray microcomputed tomography and scanning electron microscopy. Finally, freeze-thaw experiments were conducted to evaluate the durability of the cement paste under freezing conditions. Tensiometry and foaming experiments revealed that although the surface activity is very dependent on each specific protein and denaturant, certain protein-denaturant complexes can result in the highest surface activity. Micro-CT analysis of air voids with fixed concentrations of proteins and denaturants showed that certain protein-denaturant complexes induce multiple times higher air-entrained porosity than the additive air-entrained porosities of their individual components, emphasizing the effects of denaturation and synergistic interactions at the bubble interface. Among the tested protein systems, those modified with Lig and especially SDS exhibited the largest changes in the air-entrained microstructure and the highest improvement in freeze-thaw resistance observed through the reduction in mass loss over 300 cycles. These include complexes such as Lys and Alb modified with Lig and SDS, and CP modified with Lig. However, not all combinations were beneficial, and some protein-denaturant pairs resulted in lower air-entrained porosity than only protein, highlighting that denaturation can also disrupt favorable protein conformations or that the combination could not possess compatibility at the interface. While controlled unfolding of proteins using denaturants can greatly influence air-entraining performance, this influence can vary greatly and requires a better understanding of the contributing factors.