Small oligomers of the amyloid β (Aβ) peptide, rather than the monomers or the fibrils, are suspected to initiate Alzheimer's disease (AD). However, their low concentration and transient nature under physiological conditions have made structural investigations difficult. A method for addressing such problems has been developed by combining rapid fluorescence techniques with slower two-dimensional solid-state NMR methods. The smallest Aβ40 oligomers that demonstrate a potential sign of toxicity, namely, an enhanced affinity for cell membranes, were thus probed. The two hydrophobic regions (residues 10-21 and 30-40) have already attained the conformation that is observed in the fibrils. However, the turn region (residues 22-29) and the N-terminal tail (residues 1-9) are strikingly different. Notably, ten of eleven known Aβ mutants that are linked to familial AD map to these two regions. Our results provide potential structural cues for AD therapeutics and also suggest a general method for determining transient protein structures.

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Observations like high Zn(2+) concentrations in senile plaques found in the brains of Alzheimer's patients and evidences emphasizing the role of Zn(2+) in amyloid-β (Aβ)-induced toxicity have triggered wide interest in understanding the nature of Zn(2+)-Aβ interaction. In vivo and in vitro studies have shown that aggregation kinetics, toxicity, and morphology of Aβ aggregates are perturbed in the presence of Zn(2+). Structural studies have revealed that Zn(2+) has a binding site in the N-terminal region of monomeric Aβ, but not much is precisely known about the nature of binding of Zn(2+) with aggregated forms of Aβ or its effect on the molecular structure of these aggregates. Here, we explore this aspect of the Zn(2+)-Aβ interaction using one- and two-dimensional (13)C and (15)N solid-state NMR. We find that Zn(2+) causes major structural changes in the N-terminal and the loop region connecting the two β-sheets. It breaks the salt bridge between the side chains of Asp(23) and Lys(28) by driving these residues into nonsalt-bridge-forming conformations. However, the cross-β structure of Aβ(42) aggregates remains unperturbed though the fibrillar morphology changes distinctly. We conclude that the salt bridge is not important for defining the characteristic molecular architecture of Aβ(42) but is significant for determining its fibrillar morphology and toxicity.

Morpholinium-based amide-functionalized ionic liquids (ILs) [C(n)AMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [CnAMorph][Br] ILs at the air-solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γ(cmc)), and much lower critical micelle concentration (cmc) for [C(n)AMorph][Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔG(m)°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (N(agg)) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D (1)H-(1)H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C16AMorph][Br] has shown distinct micellization behavior as compared to [C8AMorph][Br] and [C12AMorph][Br] in corroboration with observations made from other techniques.

We compare the biophysical and structural aspects of the interaction of amphiphilic ionic liquids containing 1-alkyl-3-methylimidazolium cation ([C n MIM] + , n = 8, 12, or 16) with membranes composed of zwitterionic 1-palmitoyl-2-oleoyl-snglycero-3-phosphocholine (POPC) or anionic 1-palmitoyl-2-oleoyl-sn-glycero-3phospho-rac-glycerol (POPG). Liposome affinity and permeabilization were determined using ζ-potential and fluorescence studies, correlated with the cytoxicity of [C n MIM] + Br − toward HeLa cell lines. Membrane affinity is strongest in the case of [C 16 MIM] + Br − followed by [C 12 MIM] + Br − and [C 8 MIM] + Br − for both membranes, and trends remained the same in the case of membrane permeability and cytotoxicity. Solid-state NMR spectroscopy was used to localize [C n MIM] + inside the lipid bilayers and to study their impact on the head group and acyl chain structures and dynamics of the lipid molecules. The charged ring moiety of the [C n MIM] + is localized in the lipid−water interface of the membranes irrespective of the chain length and membrane surface charge. While [C 8 MIM] + binds the membrane most weakly, it induces the largest disorder in the lipid chain region. A lack of fast flip-flop motions of the amphiphiles in the case of long chain [C 16 MIM] + is suggested to render the membrane unstable, which increases its permeability. Between the lipid molecules, the POPC membrane incurs larger disorder in lipid chain packing upon insertion of [C n MIM] + molecules. The study provides structural details of the impact of increasing chain lengths in [C n MIM] + on the structural properties of lipid bilayers.

The interaction of amphiphilic ionic liquids containing an 1-alkyl-3-methylimidazolium cation ([CMIM]), which shows acute cytotoxicity toward marine and bacterial life, with zwitterionic 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho-choline (POPC) and anionic 1-palmitoyl-2-oleoyl- sn-glycero-3-phospho- rac-glycerol (POPG) membranes was investigated. Biophysical parameters of this interaction were quantified by fluorescence spectroscopy, isothermal titration calorimeter, and solution-state NMR measurements. [CMIM] inserts into the membrane and induces vesicle leakage at relatively low concentration (<1 mM). Zwitterionic POPC membranes are more leakage-prone, but the binding of [CMIM] cations is stronger to anionic POPG membranes. A higher rate of exchange of membrane-bound [CMIM] is suspected to play a key role in membrane leakage. Furthermore, solid-state NMR spectroscopy was employed to determine the location of [CMIM] in lipid membranes and its impact on the structure and dynamics of the bilayer. The study provides a molecular understanding of the membrane permeabilizing effect of the [CMIM] mediated by its detergent-like structure.

Background: Curcumin reduces the risk of Alzheimer disease via an unknown mechanism. Results: Curcumin-incubated A␤ 42 aggregates retain the hairpin architecture but have disruptions in the turn region (surprising similarity with Zn 2ϩ incubation). Conclusion: Salt bridge-containing turn region is a major determinant of morphology and toxicity. Significance: Identification of crucial structural changes provides a checkpoint for developing effective AD therapeutics.

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