Temperature dependence of Congo red binding to amyloid β12–28

pH-dependent effect of Congo Red on the growth of Aspergillus nidulans and Aspergillus niger

The azo dye Congo Red (CR) is frequently used as an agent to elicit cell wall integrity stress in fungi. This highly toxic aromatic, heterocyclic compound contains two azo bonds as chromophore, which are responsible for protonation under acidic conditions, leading to changes in the molecular structure of the dye and the color of the solution. The investigation of how CR affects the growth of Aspergillus nidulans and Aspergillus niger on surface cultures provided us with evidence about its pH-dependent toxicity. Reducing the starting pH of the media from 7 to 3 decreased both the toxicity of CR and the dose-dependence of its toxicity substantially. These changes can be explained by the pH-dependent structural changes of CR and its precipitation at low pH. The pH also depended on the fungi; they could induce a decrease or even an increase, which could be important in the loss of dose-dependence. Our experiments led to the conclusion that in studies to evaluate the antifungal effect of CR, properly buffered solutions with pH values adjusted to above 5 are highly recommended to achieve a well-detectable and dose-dependent antifungal effect. However, for decolorization of CR solutions, lower pH is suggested where the decreased toxicity and solubility of CR could help this process.

Design of Gallic Acid-Glutamine Conjugate and Chemical Implications for Its Potency Against Alzheimer's Amyloid-β Fibrillogenesis

Epigallocatechin-3-gallate (EGCG) has been widely recognized as a potent inhibitor of Alzheimer's amyloid-β (Aβ) fibrillogenesis. We found that gallic acid (GA) has superior inhibitory effects over EGCG at the same mass concentrations and assumed the pivotal role of the carboxyl group in GA. Therefore, we designed five GA-derivatives to investigate the significance of carboxyl groups in modulating Aβ fibrillogenesis, including carboxyl-amidated GA (GA-NH₂), GA-glutamic acid conjugate (GA-E), and GA-E derivatives with amidated either of the two carboxyl groups (GA-Q and GA-E-NH₂) or with two amidated-carboxyl groups (GA-Q-NH₂). Intriguingly, only GA-Q shows significantly stronger potency than GA and extends the life span of the AD transgenic nematode by over 30%. Thermodynamic studies reveal that GA-Q has a strong binding affinity for Aβ₄₂ with two binding sites, one stronger (site 1, K_a1 = 3.1 × 10⁶ M^-1) and the other weaker (site 2, K_a2 = 0.8 × 10⁶ M^-1). In site 1, hydrogen bonding, electrostatic interactions, and hydrophobic interactions all have contributions, while in site 2, only hydrogen bonding and electrostatic interactions work. The two sites are confirmed by molecular simulations, and the computations specified the key residues. GA-Q has strong binding to Asp23, Gly33, Gly38, Ala30, Ile31, and Leu34 via hydrogen bonding and electrostatic interactions, while it interacts with Phe19, Ala21 Gly25, and Asn27 via hydrophobic interactions. Consequently, GA-Q destroys Asp23-Lys28 salt bridges and restricts β-sheet/bridge structures. The thermodynamic and molecular insight into the GA-Q functions on inhibiting Aβ fibrillogenesis would pave a new way to the design of potent molecules against Alzheimer's amyloid.

Regulation of Artemisinin and Its Derivatives on the Assembly Behavior and Cytotoxicity of Amyloid Polypeptides hIAPP and Aβ

The misfolding and aggregation of human islet amyloid polypeptide (hIAPP) and amyloid-β (Aβ) protein are closely associated with type 2 diabetes mellitus (T2DM) and Alzheimer's disease, respectively. Inhibitors of amyloid peptides include short peptides, aromatic organic molecules, nanoparticles, and even metal compounds. Sesquiterpenoid artemisinins are widely used in anti-malaria treatments, and they may modulate glucose homeostasis against diabetes. However, the antidiabetic mechanism of these compounds remains unclear. In this work, four compounds, namely, artemisinin (1), dihydroartemisinin (2), artesunate (3), and artemether (4), were exploited to inhibit the assembly behavior of hIAPP and compared with that of Aβ. Although structurally distinct from other aromatic inhibitors of amyloid peptides, these sesquiterpenoids effectively altered the two peptides' fibril morphologies and disaggregated the mature fibrils mostly to the monomers. The interaction of artemisinins with the two peptides demonstrated a spontaneous, exothermic, and entropy-driven binding process predominantly through hydrophobic and hydrogen bonding interactions. Moreover, they reversed cytotoxicity and membrane leakage by reducing peptides' oligomerization. The results suggested that these compounds had better inhibition and disaggregation capability against hIAPP than against Aβ. Furthermore, the effects of these compounds' structural modification on the amyloid fibril formation of the two peptides were observed. The molecular screening offered a new perspective for artemisinins as promising inhibitors against amyloidosis related diseases.

Molecules of Congo red caught hopping between insulin fibrils: a chiroptical probe of the dye–amyloid binding dynamics

Binding dynamics of insulin amyloid–Congo red complex were probed through a new approach based on induced circular dichroism.

Molecular drug targets and therapies for Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is characterized by normal memory loss and cognitive impairment in humans. Many drug targets and disease-modulating therapies are available for treatment of AD, but none of these are effective enough in reducing problems associated with recognition and memory. Potential drug targets so far reported for AD are β-secretase, Γ-secretase, amyloid beta (Aβ) and Aβ fibrils, glycogen synthase kinase-3 (GSK-3), acyl-coenzyme A: cholesterol acyl-transferase (ACAT) and acetylcholinesterase (AChE). Herbal remedies (antioxidants) and natural metal-chelators have shown a very significant role in reducing the risk of AD, as well as lowering the effect of Aβ in AD patients. Researchers are working in the direction of antisense and stem cell-based therapies for a cure for AD, which mainly depends on the clearance of misfolded protein deposits — including Aβ, tau, and alpha-synuclein. Computational approaches for inhibitor designing, interaction analysis, principal descriptors and an absorption, distribution, metabolism, excretion and toxicity (ADMET) study could speed up the process of drug development with higher efficacy and less chance of failure. This paper reviews the known drugs, drug targets, and existing and future therapies for the treatment of AD.