SERS characterization of neuropeptide Y and its C-terminal fragments deposited onto colloidal gold nanoparticle...

It has been suggested that the family of neuropeptide Y (NPY) peptides is a promising target for the neuroprotective therapy; therefore, knowledge of the structure of these biologically active compounds and their behavior at solid/liquid interface is important in order to design new analogues. Because there is still a lack of detailed information on the behavior of NPY and its mutated analogues at the solid/liquid interfaces, in this work surface-enhanced Raman spectroscopy (SERS) analysis was used to investigate NPY and its native NPY^3-36, NPY^13-36, and NPY^22-36 and mutated acetyl-(Leu^28,31)-NPY^24-36C-terminal fragments, acting on Y₂ receptors (Y₂R), in order to determine their possible metal surface/molecule interactions. In these studies, colloidal gold nanoparticle surface served as a solid surface, whereas an aqueous solution was used as a liquid medium. The observed differences in the band intensities, wavenumbers, and widths allowed us to draw conclusions on an adsorption mode of NPY and on changes in this mode upon the shortening of the peptide chain and increase in solution pH (from pH 3 to pH 11). Briefly, three different species of Tyr were identified onto the colloidal gold surface depending upon the length of the peptide chain and solution pH. Tyrosine (TyrOH) is present in a basic medium. Tyrosinate (TyrO^-) is present in an acidic solution, whereas phenoxyl radical (Tyr^*) appears at neutral pH for peptides having relatively short peptide chain (acetyl-(Leu^28,31)-NPY^24-36). The elongation of the peptide chain partially (NPY^13-36 and NPY^22-36) or completely (NPY^3-36 and NPY) protects the Tyr residue against conversion to the radical form.