Functionalization of insulin nanofibrils with fluorophores involved in cascade Förster resonance energy transfer

Abstract

Amyloid protein nanofibrils, self-assembled soft systems, currently attract considerable attention in biomedical and nanotechnological aspects. The present study was undertaken to evaluate the possibility of functionalization of the insulin nanofibrils with three styryl pyridinium dyes (SPD) per se and in combination with the classical amyloid marker thioflavin T (ThT) and squaraine dyes SQ4 and SQ1. The dye–protein complexation was quantified in terms of the association constant, the binding stoichiometry and the molar fluorescence of the bound dye. The binding specificity of SPD was found to result in the amyloid specificity of the cascade Förster resonance energy transfer (FRET) in the chromophore chain ThT → SPD → SQ4 → SQ1, with the efficiencies of energy transfer in the donor–acceptor pairs ThT–SPD and SPD–SQ4 being markedly higher for the fibrillar protein compared to its non-fibrillized counterpart. It was demonstrated that larger spectral overlaps between ThT and styryl pyridinium dyes with alkyl chains renders these SPD more effective relay fluorophores in the three-step FRET from ThT to SQ1 than previously explored benzanthrone and phosphonium dyes. The quantitative analysis of the FRET data with the stretched exponential model showed that the insulin fibril volume is partially accessible to the examined fluorophores with the dimensionality of spatial distribution of the fibril-bound donors and acceptors varying from 1D to 3D. The results obtained suggest that protein nanofibrils represent a promising photoluminescent material ensuring fluorophore positioning and orienting favorable for directional multistep energy transfer.