SURFACE-ENHANCED RAMAN SCATTERING AS MOLECULAR PROBE OF THE KINETICS OF TRYPTOPHAN SELF-ASSEMBLING

The formation of amyloid fibrils by different proteins and peptides has been thoroughly investigated in the last decades. In more recent studies, it was documented that some amino acids such as tryptophan and phenylalanine also self-assemble into fibrils with amyloid-like characteristics. Similarly to its protein counterpart, this type of metabolite aggregates were also associated with diseases such as Tyrosinemia type II, hypertryptophanemia and Hartnup disease. While an effort was made to ascertain the molecular interactions involved in the formation of amyloid-like metabolite fibrils, only a few were able to probe these interactions directly and in a label-free manner. In this work, surface enhanced Raman scattering (SERS) was utilized to monitor the molecular interactions involved in the self-assembly of tryptophan into amyloid-like structures in water and in PBS buffer, particularly on its initial stages. The information that SERS provides about the vibrational modes of tryptophan allows the characterization of the self-assembling process at a molecular level. The changes observed in the SERS spectra during the aggregation of the amino acid in water correlate well with the structures suggested in previous works. Particularly, some changes in the spectra can be attributed to hydrogen bonding, π-stacking interactions, and hydrophobicity changes during the self-assembling of tryptophan. To better interpret this information, other techniques such as fluorescence essay and computational methods were also used. The self-assembly process of tryptophan was also successfully monitored in PBS buffers, but the results obtained were different. This may indicate differences in the aggregation process of tryptophan in PBS buffer and could be another demonstration of how this methodology can be used to describe in more detail the mechanisms involved in this type of fibril formation.