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

Abstract

The tip-enhanced Raman scattering (TERS) spectra of bradykinin (BK) and its potent B₂ BK receptor antagonists, [D-Arg⁰,Hyp³,Thi^5,8,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,Thi^5,8,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 Phe^5/8 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,Thi^5,8,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.