Conformation and Aggregation of LKα14 Peptide in Bulk Water and at the Air/Water Interface

Historically, the protein folding problem has mainly been associated with understanding the relationship between amino acid sequence and structure. However, it is known that both the conformation of individual molecules and their aggregation strongly depend on the environmental conditions. Here, we study the aggregation behavior of the model peptide LKα14 (with amino acid sequence LKKLLKLLKKLLKL) in bulk water and at the air/water interface. We start by a quantitative analysis of the conformational space of a single LKα14 in bulk water. Next, in order to analyze the aggregation tendency of LKα14, by using the umbrella sampling technique we calculate the potential of mean force for pulling a single peptide from an n-molecule aggregate. In agreement with the experimental results, our calculations yield the optimal aggregate size as four. This equilibrium state is achieved by two opposing forces: Coulomb repulsion between the lysine side chains and the reduction of solvent accessible hydrophobic surface area upon aggregation. At the vacuum/water interface, however, even dimers of LKα14 become marginally stable, and any larger aggregate falls apart instantaneously. Our results indicate that even though the interface is highly influential in stabilizing the α-helix conformation for a single molecule, it significantly reduces the attraction between two LKα14 peptides, along with their aggregation tendency.