Elucidating the Adverse Interactions of L -Tryptophan and 5-Hydroxytryptophan Self-Assemblies with Prebiotic Membranes and Neuronal Cells: An Investigation Using Fluorescence Spectroscopy and Lifetime Imaging Microscopy

L -tryptophan ( l -Trp) and 5-Hydroxytryptophan (5-HTP) are essential precursors in serotonin biosynthesis and widely used as over-the-counter dietary supplements. We hypothesize that, due to prolonged uptake and accumulation in the human body, these amino acids can spontaneously form fibrillar self-assemblies at higher concentrations, capable of interacting with model membranes, possibly constituting the underlying mechanistic basis for their cytotoxicity and associated neurodegenerative risks. The self-assemblies of l -Trp and 5-HTP were characterized using field emission scanning electron microscopy (FESEM) and fluorescence lifetime imaging microscopy (FLIM). We investigated their interfacial interactions with fatty acid model membranes, evaluating structural perturbations using fluorescence spectroscopy, molecular dynamics (MD) simulations, dynamic light scattering (DLS), and FLIM, and complementing these with in vitro cellular viability assays using SH-SY5Y cell lines. Self-assembly of l -Trp and 5-HTP formed distinct flake-like and rod-like structures, respectively, with different rigidity. Fluorescence spectroscopy and FLIM analyses demonstrated that these assemblies altered local microenvironmental polarity and significantly compromised bilayer integrity─a disruptive integration further corroborated by atomistic MD simulations. Crucially, in vitro assays linked these interfacial interactions to a marked decrease in SH-SY5Y cell viability, establishing membrane disruption as a plausible mechanism driving the neurotoxicity of these assemblies at biointerfaces.