Structural Changes in Proteins at Air-Water Interfaces

We study the behavior of five proteins (protein G, egg-white lysozyme, hydrophobin, tryptophan cage and LTP1) at the air–water and oil–water interfaces by all-atom molecular dynamics. The proteins are found to change orientation and get distorted when pinned to the interface. This behavior is consistent with the empirical way of introducing the interfaces in a coarse-grained model through a force that depends on the hydropathy indices of the residues. Proteins couple to the oil–water interface stronger than to the air–water one. They diffuse slower at the oil–water interface but do not depin from it, whereas depinning events are observed at the other interface. The reduction of the disulfide bonds slows the diffusion down. We use the empirical coarse-grained model to study properties of protein layers at the air-water interface. In particular, we demonstrate existence of glassy effects as evidenced by slowing down of diffusion with increasing concentration of proteins. We also show that layers of two barley proteins, LTP1 and its ligand adduct LTP1b, flatten out at the interface and can make a continuous and dense film that should be responsible for formation and stability of foam in beer. The degree of the flattening depends on the protein - the layers of LTP1b should be denser than those of LTP1 – as well as on the presence of glycation and the degree of reduction in the number of disulfide bonds. We also show that the interfacial forces can untie proteins with shallow knots, but they can also make knots in proteins that are natively unknotted. The physics of proteins at the air-water interface can be captured by a simple lattice model which allows for larger statistics of the pinning-depinning processes and an analysis of a Marangoni-like effect induced by a temperature gradient. Collaboration with D. B. Allen, M. Chwastyk, R. L. Leheny, D. H. Reich and Y. Zhao.