Nanodiscs are self-assembled discoidal nanoparticles composed of amphiphilic α-helical scaffold proteins or peptides that wrap themselves around the circumference of a lipid bilayer in a beltlike manner. In this study, an amphiphilic helical peptide that mimics helix 10 of human apoA-I was newly synthesized by solid phase peptide synthesis using Fmoc chemistry, and its physicochemical properties, including surface tension, self-association, and solubilization abilities, were evaluated and related directly to nanodisc formation. The synthesized peptide having hydrophobic and hydrophilic faces behaves like a general surfactant, affording a critical association concentration (CAC) of 2.7 × 10(-5) M and a γCAC of 51.2 mN m(-1) in aqueous solution. Interestingly, only a peptide solution above its CAC was able to microsolubilize L-α-dimyristoylphosphatidylcholine (DMPC) vesicles, and lipid nanodiscs with an average diameter of 9.5 ± 2.7 nm were observed by dynamic light scattering and negative stain transmission electron microscopy. Moreover, the ζ potentials of the lipid nanodiscs were measured for the first time as a function of pH, and the values changed from positive (20 mV) to negative (-30 mV). In particular, nanodisc solutions at acidic pH 4 (20 mV) or basic pH 9 (-20 mV) were found to be stable for more than 6 months as a result of the electrostatic repulsion between the particles.
Pseudorotaxanes composed of an alkylpyridinium and alpha-cyclodextrin (alpha-CD) form supramolecular hydrogels which show sol-gel transitions at 7-67 degrees C depending on the type and amount of the guest compound.
Mixtures of N-alkyl pyridinium compounds [py-N-(CH(2))(n)OC(6)H(3)-3,5-(OMe)(2)](+)(X(-)) (1bCl: n = 10, X = Cl; 1cBr: n = 12, X = Br) and alpha-cyclodextrin (alpha-CD) form supramolecular hydrogels in aqueous media. The concentrations of the two components influences the sol-gel transition temperature, which ranges from 7 to 67 degrees C. Washing the hydrogel with acetone or evaporation of water left the xerogel, and (13)C CP/MAS NMR measurements, powder X-ray diffraction (XRD), and scanning electron microscopy (SEM) revealed that the xerogel of 1bCl (or 1cBr) and alpha-CD was composed of pseudorotaxanes with high crystallinity. (13)C{(1)H} and (1)H NMR spectra of the gel revealed the detailed composition of the components. The gel from 1bCl and alpha-CD contains the corresponding [2]- and [3]pseudorotaxanes, [1b x (alpha-CD)]Br and [1b x (alpha-CD)(2)]Br, while that from 1cBr and alpha-CD consists mainly of [3]pseudorotaxane [1c x (alpha-CD)(2)]Br. 2D ROESY (1)H NMR measurements suggested intermolecular contact of 3,5-dimethoxyphenyl and pyridyl end groups of the axle component. The presence of the [3]pseudorotaxane is indispensable for gel formation. Thus, intermolecular interaction between the end groups of the axle component and that between alpha-CDs of the [3]pseudorotaxane contribute to formation of the network. The supramolecular gels were transformed into sols by adding denaturing agents such as urea, C(6)H(3)-1,3,5-(OH)(3), and [py-N-nBu](+)(Cl(-)).
[reaction: see text] Micelles of an amphiphile that encapsulate the added dye in water can be released, upon addition of alpha-CD, to form pseudorotaxane. The equilibrium between the micelles and the pseudorotaxanes and the absorption spectra of solution are controlled by temperature reversibly.
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