Fibrillation/defibrillation of myoglobin decorated with gold nanoparticles probed through nanometal surface energy transfer mechanism

Amyloid fibrillation and protein aggregation are closely associated with several pathological and structural transformations, making it important to understand their conformational heterogeneity and microenvironmental properties. In the present study, structural transitions of equine skeletal myoglobin (EMb) conjugated with gold nanoparticles (AuNPs) from native to amorphous aggregates, cross-β amyloid fibrils, and partially refolded states induced by sodium dodecyl sulfate (SDS) were investigated through nanometal surface energy transfer (NSET)-based photophysical approaches. Intrinsic aromatic amino acid residues, along with Coumarin-153 (C153) and Rhodamine-6G (Rh6G) as interior- and surface-sensitive fluorescent probes, respectively, were employed to probe the microenvironmental heterogeneity and energy-transfer behavior of different conformational states. Intrinsic fluorescence studies demonstrated state-dependent quenching predominantly governed by non-radiative decay pathways, where amorphous aggregates exhibited the highest quenching efficiency due to enhanced structural disorder and greater fluorophore accessibility to AuNPs. Fluorescence lifetime measurements of C153 revealed an open interior matrix in amorphous aggregates that promoted maximum energy-transfer efficiency, whereas the ordered cross-β-sheet architecture of amyloid fibrils shielded the probe within the fibrillar core, resulting in reduced energy transfer and enhanced excited-state lifetime. Time-resolved anisotropy studies further indicated that, despite their open structure, amorphous aggregates possessed a comparatively rigid interior microenvironment, while amyloid fibrils exhibited weaker immobilization within the fibrillar core but a relatively rigid exterior environment. Both amorphous aggregates and amyloid fibrils were found to possess less flexible interior and exterior environments than native bioconjugates. Furthermore, the photophysical behavior of intrinsic and extrinsic fluorophores supported the defibrillation of protein structures at SDS concentrations above the critical micelle concentration. Overall, this study demonstrates the effectiveness of NSET-based photophysical techniques for monitoring protein aggregation, amyloid fibrillation, microenvironmental heterogeneity, and defibrillation processes in bioconjugates.