Lysozyme Interaction with Phospholipid Nanodroplets Probed by Sum Frequency Scattering Vibrational Spectroscopy

When a nanoparticle (NP) is introduced into a biological environment, its identity and interactions are immediately attributed to the dense layer of proteins that quickly covers the particle. The formation of this layer, dubbed the protein corona, is in general a combination of proteins interacting with the surface of the NP and a contest between other proteins for binding sites either at the surface of the NP or upon the dense layer. Despite the importance for surface engineering and drug development, the molecular mechanisms and structure behind interfacial biomolecule action have largely remained elusive. We use ultrafast sum frequency scattering (SFS) spectroscopy to determine the structure and the mode of action by which these biomolecules interact with and manipulate interfaces. The majority of work in the field of sum frequency generation has been done on flat model interfaces. This limits some important membrane properties such as membrane fluidity and dimensionality─important factors in biomolecule-membrane interactions. To move toward three-dimensional (3D) nanoscopic interfaces, we utilize SFS spectroscopy to interrogate the surface of 3D lipid monolayers, which can be used as a model lipid-based nanocarrier system. In this study, we have utilized SFS spectroscopy to follow the action of lysozyme. SFS spectra in the amide I region suggest that there is lysozyme at the interface and that the lysozyme induces an increased lipid monolayer order. The binding of lysozyme with the NP is demonstrated by an increase in acyl chain order determined by the ratio of the CH₃ symmetric and CH₂ symmetric peak amplitudes. Furthermore, the lipid headgroup orientation s-PO₂^- change strongly supports lysozyme insertion into the lipid layer causing lipid disruption and reorientation. Altogether, with SFS, we have made a huge stride toward understanding the binding and structure change of proteins within the protein corona.