Tryptophan Tight Binding to Gold Nanoparticles Induces Drastic Changes in Indole Ring Raman Markers

Abstract

Experimental conditions favoring the tryptophan tight binding to the surface of citrate-reduced gold nanoparticles (Cit-AuNPs) were explored. For this, the adsorption of three molecular compounds, free amino acid (Trp), tripeptide NH2-Gly-Trp-Gly-CONH2, and lanreotide, a synthetic cyclic somatostatin analogue, on large-size (∼70 nm) Cit-AuNPs was analyzed. UV–visible absorption, transmission electron microscopy, and surface-enhanced Raman scattering (SERS) were jointly used in the present investigation. At low pH (∼3.5), when the repulsive electrostatic interactions between gold particles are sufficiently reduced, both peptides can induce strong NP aggregation, leading particularly to substantial changes in the characteristic tryptophan Raman markers, as well as to the appearance of strong SERS markers at 1228 and 1113 cm–1. In contrast, upon increasing the pH toward a neutral value, the mentioned Trp Raman markers tend to adopt a spectral shape comparable to that observed in solution (bulk). In free amin...