The aggregation of amyloid-$\beta 42$ ($\mathrm{A}\beta 42$) peptide, a key pathological event in Alzheimer's disease, is strongly influenced by its solvent environment. While cosolvents are often used in experimental studies, their specific role in modulating the conformational stability and aggregation propensity of $\mathrm{A}\beta 42$ remains poorly understood. We perform molecular dynamics simulations to investigate the effects of three organic solvents—ethanol (EtOH), dimethyl sulfoxide (DMSO), and acetonitrile (ACN)—on the structural dynamics of $\mathrm{A}\beta 42$. Our results reveal a distinct dichotomy: EtOH and DMSO exert a stabilizing effect by promoting the $\alpha$-helical content, reducing Coil formation, and extending the lifetime of intramolecular hydrogen bonds. In contrast, ACN destabilizes the native state and accelerates the formation of aggregation-prone $\beta$-sheet structures. We attribute these opposing effects to the solvents' differential disruption of the peptide's hydrophobic core and their specific interactions with the protein backbone. This work elucidates the microscopic mechanisms by which solvent environment directs $\mathrm{A}\beta 42$ conformational sampling, with implications for understanding aggregation pathways and designing modulating agents.

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• Conformation changes
• Protein dynamics, structure & function
• Protein folding
• Protein structure