Transition-State-Dependent Spontaneous Generation of Reactive Oxygen Species by Aβ Assemblies Encodes a Self-Regulated Positive Feedback Loop for Aggregate Formation.

Amyloid-β (Aβ) peptides exhibit distinct biological activities across multiple physical length scales, including monomers, oligomers, and fibrils. The transition from Aβ monomers to pathological aggregates correlates with the emergence of chemical toxicity, which plays a critical role in the progression of neurodegenerative disorders. However, the relationship between the physical state of Aβ assemblies and their chemical toxicity remains poorly understood. Here, we show that Aβ assemblies can spontaneously generate reactive oxygen species (ROS) through transition-state-specific inherent nonenzymatic redox activity. During the transition from initial monomers to intermediate oligomers or condensates to final fibrils, interfacial electrochemical environments emerge and vary at the liquid-liquid and liquid-solid interfaces. Determined by the vibrational Stark effect using electronic pre-resonance stimulated Raman scattering microscopy, the interfacial field of such assemblies is on the order of 10 MV/cm. Interfacial activity, which depends on the Aβ transition state, can modulate the spontaneous oxidation of hydroxide anions, which leads to the formation of hydroxyl radicals. Interestingly, this redox activity modifies the chemical composition of Aβ and establishes a self-regulated positive feedback loop that accelerates aggregation and promotes fibril formation, which represents a new functioning mechanism of Aβ aggregation beyond physical cross-linking. Leveraging this mechanistic insight, we identified small molecules capable of disrupting the feedback loop by scavenging hydroxyl radicals or perturbing the interface, thereby inhibiting fibril formation. Our findings provide a nonenzymatic model of neurotoxicity and reveal the critical role of physical interfaces in modulating the chemical dynamics of biomolecular assemblies. These results offer a novel framework for therapeutic intervention in Alzheimer's disease and related neurodegenerative disorders.