Peptide Orientation Strongly Affected by the Nanoparticle Size as Revealed by Sum Frequency Scattering Spectroscopy

The orientation of proteins at interfaces has a profound effect on the function of proteins. For nanoparticles (NPs) in a biological environment, protein orientation determines the toxicity, function, and identity of the NP. Thus, understanding how proteins orientate at NP surfaces is a critical parameter in controlling NP biochemistry. While planar surfaces are often used to model NP interfaces for protein orientation studies, it has been shown recently that proteins can orient very differently on NP surfaces. This study uses sum frequency scattering vibrational spectroscopy of the model helical leucine-lysine (LK) peptide on NPs of different sizes to determine the cause for the orientation effects. The data show that, for low dielectric constant materials, the orientation of the helical LK peptide is a function of the coulombic forces between peptides across different particle volumes. This finding strongly suggests that flat model systems are only of limited use for determining protein orientation at NP interfaces and that charge interactions should be considered when designing medical NPs or assessing NP biocompatibility.