Understanding the Effect of Secondary Structure on Molecular Interactions of Poly-L-lysine with Different Substrates by SFA

Nonspecific adsorption of proteins on biomaterial surfaces challenges the widespread application of engineered materials, and understanding the impact of secondary structure of proteins and peptides on their adsorption process is of both fundamental and practical importance in bioengineering. In this work, poly-L-lysine (PLL)-based alpha-helices and beta-sheets were chosen as a model system to investigate the effect of secondary structure on peptide interactions with substrates of various surface chemistries. Circular dichroism (CD) was used to confirm the presence of both alpha-helix and beta-sheet structured PLL in aqueous solutions and upon adsorption to quartz, where these secondary structures seemed to be preserved. Atomic force microscopy (AFM) imaging showed different surface patterns for adsorbed alpha-helix and beta-sheet PLL. Interactions between PLL of different secondary structures and various substrates (i.e., PLL, Au, mica, and poly(ethylene glycol) (PEG)) were directly measured using a surface forces apparatus (SFA). It was found that beta-sheet PLL films showed higher adsorbed layer thicknesses in general. Adhesion energies of beta-sheet versus Au and beta-sheet versus beta-sheet were considerably higher than that of alpha-helix versus Au and alpha-helix versus alpha-helix systems, respectively. Au and beta-sheet PLL interactions seemed to be more dependent on the salt concentration than that of alpha-helix, while the presence of a grafted PEG layer greatly diminished any attraction with either PLL structure. The molecular interaction mechanism of peptide in different secondary structures is discussed in terms of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, Alexander-de Gennes (AdG) steric model and hydrogen bonding, which provides important insight into the fundamental understanding of the interaction mechanism between proteins and biomaterials.