Tip-enhanced Raman spectroscopy of bradykinin and its B2 receptor antagonists adsorbed onto colloidal suspended Ag nanowires

The tip-enhanced Raman scattering (TERS) spectra of bradykinin (BK) and its potent B2 BK receptor antagonists, [D-Arg0,Hyp3,Thi5,8,L-Pip7]BK and [D-Arg0,Hyp3,Thi5,D-Phe7,L-Pip8]BK, approximately with a size of about 40 nm, adsorbed onto colloidal suspended Ag nanowires with diameter in the range of 350–500 nm and length of 2–50 μm were recorded. The metal surface plasmon resonance and morphology of the Ag nanowires were studied by ultraviolet-visible (UV-Vis) spectroscopy and scanning electron microscopy (SEM). Briefly, it was shown that two C-terminal amino acids of BK and [D-Arg0,Hyp3,Thi5,8,L-Pip7]BK are involved in the interaction with the colloidal suspended Ag nanowire surface, whereas three last amino acids of the [D-Arg0,Hyp3,Thi5,D-Phe7,L-Pip8]BK sequence attached the Ag surface. Thus, BK adsorbs on the colloidal suspended Ag nanowires mainly through the Phe5/8 ring (tilted orientation) and the one oxygen atom of the carboxylate group and the H2N–C–NH–CH2– fragment of Arg9. In the case of [D-Arg0,Hyp3,Thi5,8,L-Pip7]BK, the Thi8 ring (through the lone electron pair on the sulfur atom) and the both oxygen atoms of the carboxylate group and the amine group of Arg9 mainly participated in the interaction with the Ag nanowire surface. For [D-Arg0,Hyp3,Thi5,D-Phe7,L-Pip8]BK, the D-Phe7 ring, the Pip8 ring, and the Arg9 side-chain assisted in the peptide interaction with the Ag surface. The obtained results emphasize the importance of the C-terminal part of these peptides in the adsorption process onto the colloidal suspended Ag nanowires.