Self-Assembly of Oleyl Bis(2-hydroxyethyl)methyl Ammonium Bromide with Sodium Dodecyl Sulfate and Their Interactions with Zein

Surface tension and aggregation behavior in an aqueous solution of the mixture of cationic surfactant oleyl bis­(2-hydroxyethyl)­methylammonium bromide (OHAB) and anionic surfactant sodium dodecyl sulfate (SDS) have been studied by surface tension, conductivity, turbidity, zeta potential, isothermal titration microcalorimetry (ITC), cryogenic transmission electron microscopy (Cryo-TEM), and dynamic light scattering. The mixture shows pretty low critical micellar concentration and surface tension, and successively forms globular micelles, unilamellar vesicles, multilamellar vesicles, rod-like micelles, and globular micelles again by increasing the molar fraction of OHAB from 0 to 1.00. The cooperation of hydrophobic interaction between the alkyl chains, electrostatic attraction between the headgroups as well as hydrogen bonds between the hydroxyethyl groups leads to the abundant aggregation behaviors. Furthermore, the solubilization of zein by the OHAB/SDS aggregates and their interactions were studied by ITC, total organic carbon analysis (TOC), and Cryo-TEM. Compared with pure OHAB or pure SDS solution, the amount of zein solubilized by the OHAB/SDS mixture is significantly reduced. It means that the mixtures have much stronger abilities in solubilizing zein. This result has also been proved by the observed enthalpy changes for the interaction of OHAB/SDS mixture with zein. Mixing oppositely charged OHAB and SDS reduces the net charge of mixed aggregates, and thus, the electrostatic attraction between the aggregates and zein is weakened. Meanwhile, the large size of the aggregates may increase the steric repulsion to the zein backbone. This work reveals that surfactant mixtures with larger aggregates and smaller CMCs solubilize less zein, suggesting how to construct a highly efficient and nonirritant surfactant system for practical use

Complex Formation and Aggregate Transitions of Sodium Dodecyl Sulfate with an Oligomeric Connecting Molecule in Aqueous Solution

Anionic single-tail surfactant sodium dodecyl sulfate (SDS) and a molecule with multiple amido and amine groups (Lys-12-Lys) were used as building blocks to fabricate oligomeric surfactants through intermolecular interactions. Their interactions and the resultant complex and aggregate structures were investigated by turbidity titration, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, freeze-fracture transmission electron microscopy, ¹H NMR, and 1D NOE techniques. At pH 11.0, the interaction between SDS and Lys-12-Lys is exothermic and mainly resulted from hydrogen bonding among the amido and amine groups of Lys-12-Lys and the sulfate group of SDS and hydrophobic interaction between the hydrocarbon chains of SDS and Lys-12-Lys. At pH 3.0, each Lys-12-Lys carries four positive charges and two hydrogen bonding sites. Then SDS and Lys-12-Lys form complexes Lys-12-Lys­(SDS)₆ and Lys-12-Lys­(SDS)₄ through the head groups by electrostatic attraction and hydrogen bonds assisted by hydrophobic interaction. Moreover, the complexes pack more tightly in their aggregates with the increase of the molar ratio. Especially the Lys-12-Lys­(SDS)₄ and Lys-12-Lys­(SDS)₆ complexes behave like oligomeric surfactants taking Lys-12-Lys as a spacer group, exhibiting a series of aggregates transitions with the increase of concentration, i.e., larger vesicles, smaller spherical micelles, and long threadlike micelles. Therefore, oligomeric surfactants Lys-12-Lys­(SDS)₄ and Lys-12-Lys­(SDS)₆ have been successfully fabricated by using a single chain surfactant and an oligomeric connecting molecule through noncovalent association

Aggregation Behavior of Sodium Lauryl Ether Sulfate with a Positively Bicharged Organic Salt and Effects of the Mixture on Fluorescent Properties of Conjugated Polyelectrolytes

The aggregation behavior of anionic single-chain surfactant sodium lauryl ether sulfate containing three ether groups (SLE3S) with positively bicharged organic salt 1,2-bis­(2-benzylammoniumethoxy)­ethane dichloride (BEO) has been investigated in aqueous solution, and the effects of the BEO/SLE3S aggregate transitions on the fluorescent properties of anionic conjugated polyelectrolyte MPS-PPV with a larger molecular weight and cationic conjugated oligoelectrolyte DAB have been evaluated. Without BEO, SLE3S does not affect the fluorescent properties of MPS-PPV and only affects the fluorescent properties of DAB at a higher SLE3S concentration. With the addition of BEO, SLE3S and BEO form gemini-like surfactant (SLE3S)₂-BEO. When the BEO/SLE3S molar ratio is fixed at 0.25, with increasing the BEO/SLE3S concentration, the BEO/SLE3S mixture forms large, loosely arranged aggregates and then transforms to closely packed spherical aggregates and finally to long thread-like micelles. The photoluminescence (PL) intensity of MPS-PPV varies with the morphologies of the BEO/SLE3S aggregates, while the PL intensity of DAB is almost independent of the aggregate morphologies. The results demonstrate that gemini-like surfactants formed through intermolecular interactions can effectively adjust the fluorescent properties of conjugated polyelectrolytes

Coacervation of Cationic Gemini Surfactant with <i>N</i>‑Benzoylglutamic Acid in Aqueous Solution

Coacervation of cationic gemini surfactant hexamethylene-1,6-bis­(dodecyldimethylammonium bromide) (12–6–12) with pH-sensitive <i>N</i>-benzoylglutamic acid (H₂Bzglu) has been investigated by potentiometric pH-titration, turbidity titration, dynamic light scattering (DLS), isothermal titration calorimetry (ITC), TEM, ¹H NMR, and light microscopy. Phase boundaries of the 12–6–12/H₂Bzglu mixture were obtained over the pH range from 2 to 9 and in the H₂Bzglu concentration range from 30.0 to 50.0 mM at pH 4.5. When the H₂Bzglu concentration is beyond 30.0 mM, the 12–6–12/H₂Bzglu mixed solution undergoes the phase transitions from soluble aggregate, to precipitate, coacervate, and soluble aggregate again as pH increases. The results indicate that coacervation occurs at extremely low 12–6–12 concentration and lasts over a wide surfactant range, and can be enhanced or suppressed by changing pH, 12–6–12/H₂Bzglu molar ratio and H₂Bzglu concentration. The coacervates present a disorderly connected lay structure. Coacervation only takes place at pH 4–5, where the aggregates are nearly charge neutralized, and a minimum H₂Bzglu concentration of 30.0 mM is required for coacervation. In this pH range, H₂Bzglu mainly exist as HBzglu^–. The investigations on intermolecular interactions indicate that the aggregation of 12–6–12 is greatly promoted by the strong electrostatic and hydrophobic interactions with the HBzglu^– molecules, and the interaction also promotes the formation of dimers, trimers, and tetramers of HBzglu^– through hydrogen bonds. The double chains of 12–6–12 and the HBzglu^– oligomers can play the bridging roles connecting aggregates. These factors endow the mixed system with a very high efficiency in generating coacervation

Interactions of Phospholipid Vesicles with Cationic and Anionic Oligomeric Surfactants

This work studied the interactions of 1,2-dioleoyl-<i>sn</i>-glycero-3-phosphocholine (DOPC) with cationic ammonium surfactants and anionic sulfate or sulfonate surfactants of different oligomeric degrees, including cationic monomeric DTAB, dimeric C₁₂C₃C₁₂Br₂, and trimeric DDAD as well as anionic monomeric SDS, dimeric C₁₂C₃C₁₂(SO₃)₂, and trimeric TED-(C₁₀SO₃Na)₃. The partition coefficient <i>P</i> of these surfactants between the DOPC vesicles and water was determined with isothermal titration microcalorimetry (ITC) by titrating concentrated DOPC solution into the monomer solution of these surfactants. It was found that the <i>P</i> value increases with the increase of the surfactant oligomeric degree. Moreover, the enthalpy change and the Gibbs free energy for the transition of these surfactants from water into the DOPC bilayer become more negative with increasing the oligomeric degree. Meanwhile, the calcein release experiment proves that the surfactant with a higher oligomeric degree shows stronger ability of changing the permeability of the DOPC vesicles. Furthermore, the solubilization of the DOPC vesicles by these oligomeric surfactants was studied by ITC, turbidity, and dynamic light scattering, and thus the phase boundaries for the surfactant/lipid mixtures have been determined. The critical surfactant to lipid ratios for the onset and end of the solubilization for the DOPC vesicles derived from the phase boundaries decrease remarkably with increasing the oligomeric degree. Overall, the surfactant with a larger oligomerization degree shows stronger ability in incorporating into the lipid bilayer, altering the membrane permeability and solubilizing lipid vesicles, which provides comprehensive understanding about the effects of structure and shape of oligomeric surfactant molecules on lipid–surfactant interactions

Disaggregation Ability of Different Chelating Molecules on Copper Ion-Triggered Amyloid Fibers

Dysfunctional interaction of amyloid-β (Aβ) with excess metal ions is proved to be related to the etiology of Alzheimer’s disease (AD). Using metal-binding compounds to reverse metal-triggered Aβ aggregation has become one of the potential therapies for AD. In this study, the ability of a carboxylic acid gemini surfactant (SDUC), a widely used metal chelator (EDTA), and an antifungal drug clioquinol (CQ) in reversing the Cu²⁺-triggered Aβ(1–40) fibers have been systematically studied by using turbidity essay, BCA essay, atomic force microscopy, transmission electron microscopy, and isothermal titration microcalorimetry. The results show that the binding affinity of Cu²⁺ with CQ, SDUC, and EDTA is in the order of CQ > EDTA > SDUC, while the disaggregation ability to Cu²⁺-triggered Aβ(1–40) fibers is in the order of CQ > SDUC > EDTA. Therefore, the disaggregation ability of chelators to the Aβ(1–40) fibers does not only depend on the binding affinity of the chelators with Cu²⁺. Strong self-assembly ability of SDUC and π–π interaction of the conjugate group of CQ also contributes toward the disaggregation of the Cu²⁺-triggered Aβ(1–40) fibers and result in the formation of mixed small aggregates

Self-Assembly of Aβ-Based Peptide Amphiphiles with Double Hydrophobic Chains

Two peptide–amphiphiles (PAs), 2C₁₂–Lys–Aβ­(12–17) and C₁₂–Aβ­(11–17)–C₁₂, were constructed with two alkyl chains attached to a key fragment of amyloid β-peptide (Aβ(11–17)) at different positions. The two alkyl chains of 2C₁₂–Lys–Aβ­(12–17) were attached to the same terminus of Aβ(12–17), while the two alkyl chains of C₁₂–Aβ­(11–17)–C₁₂ were separately attached to each terminus of Aβ(11–17). The self-assembly behavior of both the PAs in aqueous solutions was studied at 25 °C and at pHs 3.0, 4.5, 8.5, and 11.0, focusing on the effects of the attached positions of hydrophobic chains to Aβ(11–17) and the net charge quantity of the Aβ(11–17) headgroup. Cryogenic transmission electron microscopy and atomic force microscopy show that 2C₁₂–Lys–Aβ­(12–17) self-assembles into long stable fibrils over the entire pH range, while C₁₂–Aβ­(11–17)–C₁₂ forms short twisted ribbons and lamellae by adjusting pHs. The above fibrils, ribbons, and lamellae are generated by the lateral association of nanofibrils. Circular dichroism spectroscopy suggests the formation of β-sheet structure with twist and disorder to different extents in the aggregates of both the PAs. Some of the C₁₂–Aβ­(11–17)–C₁₂ molecules adopt turn conformation with the weakly charged peptide sequence, and the Fourier transform infrared spectroscopy indicates that the turn content increases with the pH increase. This work provides additional basis for the manipulations of the PA’s nanostructures and will lead to the development of tunable nanostructure materials

Modulation of Aβ(1–40) Peptide Fibrillar Architectures by Aβ-Based Peptide Amphiphiles

Modulation of the fibrillogenesis of amyloid peptide Aβ(1–40) with two Aβ-based peptide amphiphiles has been studied. Both peptide amphiphiles contain two alkyl chains but in different positions. The two alkyl chains of 2C₁₂–Aβ­(11–17) are attached to the same terminus of Aβ(11–17), while those of C₁₂–Aβ­(11–17)–C₁₂ are separately attached to opposite termini of Aβ(11–17). Thioflavin T fluorescence spectroscopy shows that all the peptide amphiphiles promote the formation of the cross-β-sheet structure of Aβ(1–40) and the aggregation of Aβ(1–40), while 2C₁₂–Aβ­(11–17) does this more efficiently. The atom force microscopy images indicate that the modulations of these two peptide amphiphiles on the Aβ(1–40) aggregation experience two distinct pathways. 2C₁₂–Aβ­(11–17) leads to amorphous aggregates, whereas C₁₂–Aβ­(11–17)–C₁₂ generates short rodlike fibrils. However, Fourier transform infrared spectroscopy suggests that the amorphous aggregates and rodlike fibrils display similar secondary structures. This work suggests that the aggregation ability and the aggregate structures of the peptide amphiphiles significantly affect their interactions with Aβ(1–40) and lead to different morphologies of the Aβ(1–40) aggregates

Association Behaviors of Dodecyltrimethylammonium Bromide with Double Hydrophilic Block Co-polymer Poly(ethylene glycol)-<i>block</i>-Poly(glutamate sodium)

The association behaviors of single-chain surfactant dodecyltrimethylammonium bromide (DTAB) with double hydrophilic block co-polymers poly­(ethylene glycol)-<i>b</i>-poly­(sodium glutamate) (PEG₁₁₃–PGlu₅₀ or PEG₁₁₃–PGlu₁₀₀) were investigated using isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, ζ potential, and particle size measurements. The electrostatic interaction between DTAB and the oppositely charged carboxylate groups of PEG–PGlu induces the formation of super-amphiphiles, which further self-assemble into ordered aggregates. Dependent upon the charge ratios between DTAB and the glutamic acid residue of the co-polymer, the mixture solutions can change from transparent to opalescent without precipitation. Dependent upon the chain length of the PGlu block, the mixture of DTAB and PEG–PGlu diblocks can form two different aggregates at their corresponding electroneutral point. Spherical and rod-like aggregates are formed in the PEG₁₁₃–PGlu₅₀/DTAB mixture, while the vesicular aggregates are observed in the PEG₁₁₃–PGlu₁₀₀/DTAB mixture solution. Because the PEG₁₁₃–PGlu₁₀₀/DTAB super-amphiphile has more hydrophobic components than that of the PEG₁₁₃–PGlu₅₀/DTAB super-amphiphile, the former prefers forming the ordered aggregates with higher curvature, such as spherical and rod aggregates, but the latter prefers forming vesicular aggregates with lower curvature

Coassembly of Poly(ethylene glycol)-<i>block</i>-Poly(glutamate sodium) and Gemini Surfactants with Different Spacer Lengths

The coassembly of poly­(ethylene glycol)-<i>b</i>-poly­(glutamate sodium) copolymer (PEG₁₁₃-PGlu₁₀₀) with cationic gemini surfactants alkanediyl-α,ω-bis-(dodecyldimethylammonium bromide) [C₁₂H₂₅(CH₃)₂N­(CH₂)_<i>S</i>N­(CH₃)₂C₁₂H₂₅]­Br₂ (designated as C₁₂C_<i>S</i>C₁₂Br₂, <i>S</i> = 3, 6, and 12) have been studied by isothermal titration microcalorimetry, cryogenic transmission electron microscopy, circular dichroism, small-angle X-ray scattering, zeta potential, and size measurement. It has been shown that the electrostatic interaction of C₁₂C_<i>S</i>C₁₂Br₂ with the anionic carboxylate groups of PEG₁₁₃-PGlu₁₀₀ leads to complexation, and the C₁₂C_<i>S</i>C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀ complexes are soluble even at the electroneutral point. The complexes display the feature of superamphiphiles and assemble into ordered nanosheets with a sandwich-like packing. The gemini molecules which were already bound with PGlu chains associate through hydrophobic interaction and constitute the middle part of the nanosheets, whereas the top and bottom of the nanosheets are hydrophilic PEG chains. The size and morphology of the nanosheets are affected by the spacer length of the gemini surfactants. The average sizes of the aggregates at the electroneutral point are 81, 68, and 90 nm for C₁₂C₃C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀, C₁₂C₆C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀, and C₁₂C₁₂C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀, respectively. Both C₁₂C₃C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀ and C₁₂C₁₂C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀ mainly generate hexagonal nanosheets, while the C₁₂C₆C₁₂Br₂/PEG₁₁₃-PGlu₁₀₀ system only induces round nanosheets