Probing Intermolecular Interactions of Amyloidogenic Fragments of SOD1 by Site-Specific Tryptophan and Its Noncanonical Derivative.

Transient amyloid intermediates are likely to be cytotoxic and play an essential role in amyloid-associated neurodegenerative diseases. Characterization of their structural and dynamic evolution is the key to elucidating the molecular mechanism of amyloid formation. Here, combining circular dichroism (CD), exciton couplet theory, and Fourier transform infrared spectroscopy with site-specific tryptophan (Trp) and its noncanonical derivative 5-cyano-tryptochan (Trp5CN), we developed a method to monitor strand-to-strand tertiary and sheet-to-sheet quaternary interactions in the aggregation cascades of an amyloidogenic fragment from protein SOD128-38 (with the sequence KVKVWGSIKGL). We found that the exciton couplet generated from the Bb band of Trp can be used as a probe for side chain interactions. Its sensitivity can be further improved by four times with the incorporation of Trp5CN. We further observed a red-shift of ∼2 cm-1 and a broadening of ∼2 cm-1 in the IR band generated from the CN stretch during the aggregation, which we attributed to the transition from a corkscrew-like structure to a cross-linked intermediate phase. We show here that the integration of optical methods with unique aromatic side chain-related probes is able to elucidate amyloid intermolecular interactions and even capture elusive transient intermediates on and off the amyloid assembling pathway.