A diphenylpropynone derivative, DPP2, has been recently demonstrated to target metal-associated amyloid-β (metal-Aβ) species implicated in Alzheimer's disease (AD). DPP2 was shown to interact with metal-Aβ species and subsequently control Aβ aggregation (reactivity) in vitro; however, its cytotoxicity has limited further biological applications. In order to improve reactivity toward Aβ species and lower cytotoxicity, along with gaining an understanding of a structure-reactivity-cytotoxicity relationship, we designed, prepared, and characterized a series of small molecules (C1/C2, P1/P2, and PA1/PA2) as structurally modified DPP2 analogues. A similar metal binding site to that of DPP2 was contained in these compounds while their structures were varied to afford different interactions and reactivities with metal ions, Aβ species, and metal-Aβ species. Distinct reactivities of our chemical family toward in vitro Aβ aggregation in the absence and presence of metal ions were observed. Among our chemical series, the compound (C2) with a relatively rigid backbone and a dimethylamino group was observed to noticeably regulate both metal-free and metal-mediated Aβ aggregation to different extents. Using our compounds, cell viability was significantly improved, compared to that with DPP2. Lastly, modifications on the DPP framework maintained the structural properties for potential blood-brain barrier (BBB) permeability. Overall, our studies demonstrated that structural variations adjacent to the metal binding site of DPP2 could govern different metal binding properties, interactions with Aβ and metal-Aβ species, reactivity toward metal-free and metal-induced Aβ aggregation, and cytotoxicity of the compounds, establishing a structure-reactivity-cytotoxicity relationship. This information could help gain insight into structural optimization for developing nontoxic chemical reagents toward targeting metal-Aβ species and modulating their reactivity in biological systems.

Metal ion homeostasis in conjunction with amyloid-β (Aβ) aggregation in the brain has been implicated in Alzheimer’s disease (AD) pathogenesis. To uncover the interplay between metal ions and Aβ peptides, synthetic, multifunctional small molecules have been employed to modulate Aβ aggregation in vitro. Naturally occurring flavonoids have emerged as a valuable class of compounds for this purpose due to their ability to modulate both metal-free and metal-induced Aβ aggregation. Although, flavonoids have shown anti-amyloidogenic effects, the structural moieties of flavonoids responsible for such reactivity have not been fully identified. In order to understand the structure-interaction-reactivity relationship within the flavonoid family for metal-free and metal-associated Aβ, we designed, synthesized, and characterized a set of isoflavone derivatives, aminoisoflavones (1-4), that displayed reactivity (i.e., modulation of Aβ aggregation) in vitro. NMR studies revealed a potential binding site for aminoisoflavones between the N-terminal loop and central helix on prefibrillar Aβ different from the non-specific binding observed for other flavonoids. The absence or presence of the catechol group differentiated the binding affinities and enthalpy/entropy balance between aminoisoflavones and Aβ. Furthermore, having a catechol group influenced the binding mode with fibrillar Aβ. Inclusion of additional substituents moderately tuned the impact of aminoisoflavones on Aβ aggregation. Overall, through these studies, we obtained valuable insights on the requirements for parity among metal chelation, intermolecular interactions, and substituent variation for Aβ interaction.

There is a significant need for developing compounds that kill Cryptococcus neoformans, the fungal pathogen that causes meningoencephalitis in immunocompromised individuals. Here, we report the mode of action of a designed antifungal peptide, VG16KRKP (VARGWKRKCPLFGKGG) against C. neoformans. It is shown that VG16KRKP kills fungal cells mainly through membrane compromise leading to efflux of ions and cell metabolites. Intracellular localization, inhibition of in vitro transcription, and DNA binding suggest a secondary mode of action for the peptide, hinting at possible intracellular targets. Atomistic structure of the peptide determined by NMR experiments on live C. neoformans cells reveals an amphipathic arrangement stabilized by hydrophobic interactions among A2, W5, and F12, a conventional folding pattern also known to play a major role in peptide-mediated Gram-negative bacterial killing, revealing the importance of this motif. These structural details in the context of live cell provide valuable insights into the design of potent peptides for effective treatment of human and plant fungal infections.

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