Unveiling the SERS Spectrum of 5-Hydroxy-L-Tryptophan: Structure and Binding Mechanisms at Silver Nanoparticle Interface

5-Hydroxy-L-tryptophan (5-HTP) is a biologically relevant metabolite and a promising optical probe in protein studies, with potential applications in neurological diseases. However, its vibrational properties have remained poorly characterized. In this work, we present a comprehensive Raman and surface-enhanced Raman scattering (SERS) study of 5-HTP by integrating experimental spectroscopy with molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The normal Raman spectrum of solid 5-HTP was recorded and accurately reproduced using DFT calculations. We report here the SERS spectra of 5-HTP using silver nanoparticles. The SERS spectrum revealed significant differences in relative band intensities compared to the normal Raman spectrum which is related to the adsorption and orientation of the molecule at the metal surface. MD simulations on neutral and charged silver surfaces demonstrated that 5-HTP does not adopt a unique configuration, but interacts primarily through the carboxylate group and often with the indole ring tilted or parallel to the surface. DFT calculations of 5-HTP adsorbed on silver clusters of ten atoms further support these findings and the role of both chemical and electromagnetic effects in the intensity enhancement. This work establishes the Raman fingerprint of 5-HTP and its SERS enhancement mechanisms offering new possibilities for its application as a label-free Raman probe in biochemical sensing and protein structure investigations.