Probing the Self-Aggregation of <scp>l</scp>-Tryptophan into Spherical Microstructures and Their Selective Interactions with Bilirubin

The aggregation of proteins, peptides and amino acids has been a keen subject of interest owing to their implications in metabolic disorders. In this work, we investigated the self-aggregation of the unmodified aromatic amino acid l-tryptophan (Trp) into unusual spherical microstructures. Using fluorescence spectroscopy and field emission scanning electron microscopy (FE-SEM), we detail the time-dependent transformation of monomeric tryptophan into spherical aggregates with distinct fluorescence characteristics (λ_ex = 345 nm, λ_em = 430 nm) compared to the monomer. Notably, the fluorescence intensity of these aggregates is selectively quenched in the presence of bilirubin, demonstrating exceptional sensitivity in the picomolar concentration range. The developed assay proved applicable and reliable for real sample analysis. Thermodynamic parameters derived from temperature-dependent fluorescence intensity measurements indicated that the aggregation process is spontaneous and driven by noncovalent interactions. Further evidence of bilirubin's strong association with the aggregates was obtained through competitive interaction studies with human serum albumin (HSA). This work offers insights into the aggregation behavior of single aromatic amino acids and their potential applications in detecting critical analytes.