Structural characterization and detection of 5-Hydroxy-L-tryptophan with surface-enhanced Raman scattering

5-Hydroxy-L-tryptophan(5-HTP) is an amino acid intermediate for the conversion of tryptophan(Trp) into serotonin(5-HT) in its metabolic pathway. 5-HTP play important roles in neurologic and metabolic processes [1] and is used as optical probe of protein structure and dynamics. The present study describes the vibrational and structural properties of 5-HTP as well as its adsorption on the surface silver and gold nanoparticles using normal Raman and Surface-enhanced Raman Scattering (SERS) [1-2]. The SERS spectra of 5-HTP were recorded at different concentrations and pH values. To interpret the experimental data, we have carried out a combination of density functional theory (DFT) calculations with molecular dynamics (MD) simulations. For the DFT calculations we used the M06-2X functional with 6-311++G(d,p), LANL2DZ mixed basis and the Polarizable Continuum Model (PCM). Computational results were obtained for 5-HTP, Trp, 5-HT and considering different conformations of 5-HTP with Ag and Au atoms. Additionally, in order to observe the conformation of 5-HTP in the surface of the metal, MD simulations were made using GROMACS [4] software with CHARMM [5] topology parameters. The MD simulations showed different possible configurations of 5-HTP that could potentially explain the apparent differences found between the experimental data and the calculated spectra.