Spectroscopic Response of Chiral Proteophenes Binding to Two Chiral Insulin Amyloids

Abstract

We recently reported on 8 different pentameric oligothiophenes, denoted proteophenes, with different amino acid substitution patterns along the conjugated thiophene backbone. The proteophenes were forming chiral both molecular (solvent dissolved) and self-assembled nano-architectonic structures, the latter having approximately 5-6 times greater ICD responses. Herein, two proteophenes, HS-84-E-E and HS-84-K-K, were further investigated in terms of binding to two chiral variants of insulin amyloid fibrils. Binding curves based on fluorescence revealed strong primary binding sites for both proteophenes and more unspecific secondary binding for especially the latter. Interestingly, their induced circular dichroism (ICD) responses were similar to or stronger than for their solvent form and showed a complicated alteration upon binding suggesting that the microstructure at the binding site governs the helical structure of the ligand. Hyperspectral fluorescence microscopy and FLIM revealed more details on the molecular binding in terms of proteophene emission decay-times. Vibrational circular dichroism (VCD) analysis showed that VCD signal of amyloid fibrils was enhanced upon binding of proteophenes, suggesting changes in the amyloid microstructures.