Nonionic surfactants: a key to enhance the enzyme activity at cationic reverse micellar interface

The primary objective of the present study is to understand how the different nonionic surfactants modify the anisotropic interface of cationic water-in-oil (W/O) microemulsions and thus influences the catalytic efficiency of surface-active enzymes. Activity of Chromobacterium viscosum lipase (CV-lipase) was estimated in several mixed reverse micelles prepared from CTAB and four different nonionic surfactants, Brij-30, Brij-92, Tween-20, and Tween-80/water/isooctane/n-hexanol at different z ([cosurfactant]/[surfactants]) values, pH 6 (20 mM phosphate), 25 °C across a varying range of Wo ([water]/[surfactants]) using p-nitrophenyl-n-octanoate as the substrate. Lipase activity in mixed reverse micelles improved maximum up to ∼200% with increasing content of non-ionic surfactants compared to that in CTAB probably due to the reduced positive charge density as well as plummeted n-hexanol (competitive inhibitor of lipase) content at the interfacial region of cationic W/O microemulsions. The highest activity of lipase was observed in CTAB (10 mM) + Brij-30 (40 mM)/isooctane/n-hexanol)/water system, k2 = 913 ± 5 cm3 g-1 s-1. Interestingly, this observed activity is even higher than that obtained in sodium bis (2-ethyl-1-hexyl) sulfosuccinate (AOT)/n-heptane reverse micelles, the most popular W/O microemulsion in micellar enzymology. To ascertain the influence of non-ionic surfactants in improving the activity of surface-active enzymes is not limited to lipase only, we have also investigated the catalytic activity of Horseradish peroxidase (HRP) in different mixed W/O microemulsions. Here also following the similar trend as observed for lipase, HRP activity enhanced up to 2.5 fold with increasing concentration of nonionic surfactants. Finally, the enzyme activity was correlated with the change in the microenvironment of mixed reverse micelles by steady-state fluorescence study using 8-anilino-1-napthalenesulphonic acid (ANS) as probe

Hydrogelation through self-assembly of Fmoc-peptide functionalized cationic amphiphiles: potent antibacterial agent

The present work reports a new class of antibacterial hydrogelators based on anti-inflammatory N-fluorenyl-9-methoxycarbonyl (Fmoc) amino acid/peptides functionalized cationic amphiphiles. These positively charged hydrogelators were rationally designed and developed by the incorporation of a pyridinium moiety at the C-terminal of Fmoc amino acid/peptides, because the pyridinium-based amphiphiles are a known antibacterial agent due to their cell membrane penetration properties. The Fmoc amino acid/peptide-based cationic amphiphiles efficiently gelate (minimum gelation concentration ∼0.6−2.2%, w/v) water at room temperature. Judicious variation of amino acid and their sequences revealed the architectural dependence of the molecules on their gelation ability. Several microscopic techniques like field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used to obtain the visual insight of the morphology of the gel network. A number of spectroscopic techniques like circular dichroism, FTIR, photoluminescence, and XRD were utilized to know the involvement of several noncovalent interactions and participation of the different segments of the molecules during gelation. Spectroscopic results showed that the π−π interaction and intermolecular hydrogen bonding are the major responsible factors for the self-assembled gelation process that are oriented through an antiparallel β-sheet arrangement of the peptide backbone. These Fmoc-based cationic molecules exhibited efficient antibacterial activity against both Gram-positive and Gram-negative bacteria

Single-walled nanotube/amphiphile hybrids for efficacious protein delivery: rational modification of dispersing agents

Carbon nanotube (CNT)/amphiphile hybrids with remarkable stability and cell viability under biologically relevant conditions can be obtained by rational modification of the molecular structure of amphiphilic dispersing agents (see scheme; PEG = poly(ethylene glycol)). The CNT/amphiphile hybrids are dispersible in water and efficiently shuttle proteins across mammalian cell membranes

In situ synthesized Ag nanoparticle in self-assemblies of amino acid based amphiphilic hydrogelators: development of antibacterial soft nanocomposites

The present work reports the development of a new class of antibacterial soft-nanocomposites by in situ synthesis of silver nanoparticle (AgNP) within the supramolecular self-assemblies of amino acid (tryptophan/tyrosine) based amphiphilic hydrogelators. Interestingly, the nanoparticle synthesis does not require the use of any external reducing/stabilizing agents. The nanocomposites were characterized by UV-vis spectra, transmission electron microscopy (TEM) images, X-ray diffraction spectroscopy (XRD) and thermo gravimetric analysis (TGA). Encouragingly, these soft nanocomposites showed excellent antibacterial activity against both Gram-positive and Gram-negative bacteria whereas the amphiphiles alone were lethal only toward Gram-positive bacteria. Judicious combination of bactericidal AgNP within the self-assemblies of inherently antibacterial amphiphilic gelators led to the development of soft nanocomposites effective against both type of bacteria. The head group charge and structure of the amphiphiles were altered to investigate their important role on the synthesis and stabilization of AgNP and also in modulating the antibacterial activity of the nanocomposites. The antibacterial activities of soft nanocomposites comprising amphiphiles with cationic head group were found to be more efficient than the anionic soft nanocomposites. Interestingly, these nanocomposites have shown considerable biocompatibility to mammalian cell, NIH3T3. Furthermore, the well-known tissue engineering scaffold, agar-gelatin film infused with these soft nanocomposites allowed normal growth of mammalian cells on its surface while being lethal toward both Gram-positive and Gram-negative bacteria

Nanostructure formation in aqueous solution of amphiphilic copolymers of 2-(N,N-dimethylaminoethyl)methacrylate and alkylacrylate: Characterization, antimicrobial activity, DNA binding, and cytotoxicity studies

Three amphiphilic random copolymers poly(2-(dimethylaminoethyl)methacrylate-co-alkylacrylate) (where, alkyl = hexyl, octyl, dodecyl) with 16 mol% hydrophobic substitution were synthesized. Surface tension, viscosity, fluorescence probe, dynamic light scattering (DLS), as well as transmission electron microscopic (TEM) techniques were utilized to investigate self-assembly formation by the hydrophobically modified polymers (HMPs) in pH 5. Formation of hydrophobic domains through inter-polymer chain interaction of the copolymer in dilute solution was confirmed by fluorescence probe studies. Average hydrodynamic diameter of the copolymer aggregates at different polymer concentration was measured by DLS studies. The copolymer with shorter hydrophobic chain exhibits larger hydrodynamic diameter in dilute solution, which decreased with either increase of concentration or increase of hydrophobic chain length. TEM images of the dilute solutions of the copolymers with shorter as well as with longer hydrophobic chain exhibit spherical aggregates of different sizes. The antimicrobial activity of the copolymers was evaluated by measuring the minimum inhibitory concentration value against one Gram-positive bacterium Bacillus subtilis and one Gram-negative bacterium Escherichia coli. The copolymer with the octyl group as pendent hydrophobic chain was found to be more effective in killing these microorganisms. The interaction of the cationic copolymers with calf-thymus DNA was studied by fluorescence quenching method. The polymer-DNA binding was found to be purely electrostatic in nature. The hydrophobes on the polymer backbone were found to have a significant influence on the binding process. Biocompatibility studies of the copolymers in terms of cytotoxicity measurements were finally performed at different concentrations of the HMPs to evaluate their potential application in biomedical fields

Dipeptide-based low-molecular-weight efficient organogelators and their application in water purification

The development of new low-molecular-weight gelators for organic solvents is motivated by several potential applications of gels as advanced functional materials. In the present study, we developed simple dipeptide-based organogelators with a minimum gelation concentration (MGC) of 6–0.15 %, w/v in aromatic solvents. The organogelators were synthesized using different L-amino acids with nonpolar aliphatic/aromatic residues and by varying alkyl-chain length (C-12 to C-16). The self-aggregation behavior of these thermoreversible organogels was investigated through several spectroscopic and microscopic techniques. A balanced participation of the hydrogen bonding and van der Waals interactions is crucial for efficient organogelation, which can be largely modulated by the structural modification at the hydrogen-bonding unit as well as by varying the alkyl-chain length in both sides of the hydrophilic residue. Interestingly, these organogelators could selectively gelate aromatic solvents from their mixtures with water. Furthermore, the xerogels prepared from the organogels showed a striking property of adsorbing dyes such as crystal violet, rhodamine 6G from water. This dye-adsorption ability of gelators can be utilized in water purification by removing toxic dyes from wastewater

Counterion dependent hydrogelation of amino acid based amphiphiles: switching from non-gelators to gelators and facile synthesis of silver nanoparticles

With the growing importance of hydrogels in scientific applications, the search for low molecular weight hydrogelators (LMWH), with simultaneous logical structural correlation is continuously increasing. In the present work, counterion variation of amino acid based amphiphiles was done to qualitatively evaluate its contribution towards hydrogelation. Further changes in the molecular skeleton of the amino acid amphiphile were done along with counterion variation to establish the importance of π–π interactions of aromatic planar ring in hydrogelation. An efficient conversion of a non-gelator to gelator molecule was achieved simply by changing the counterions to aromatic carboxylates. Role of the counterion in the mechanism of gelation process through the self-assembly of amino acid based amphiphiles has been discussed. The formation of supramolecular structures during hydrogelation was investigated by FESEM, CD, FT-IR, luminescence, 2D-NOESY and rheological studies. Interestingly, the L-tryptophan containing amphiphile hydrogelators were further utilized for synthesis of Ag nanoparticles under mild conditions without any need for high temperature, alkaline medium and external reducing agent. The nanoparticles obtained were characterized by UV-Vis, TEM, AFM and XRD experiments

Influence of gold nanoparticles of varying size in improving the lipase activity within cationic reverse micelles

Herein, we report the effect of gold nanoparticles (GNPs) in enhancing lipase activity in reverse micelles of cetyltrimethylammonium bromide (CTAB)/water/isooctane/n-hexanol. The size and concentration of the nanoparticles were varied and their specific roles were assessed in detail. An overall enhancement of activity was observed in the GNP-doped CTAB reverse micelles. The improvement in activity becomes more prominent with increasing concentration and size of the GNPs (0–52 μM and ca. 3–30 nm, respectively). The observed highest lipase activity (k2=1070±12 cm3 g-1 s-1) in GNP-doped CTAB reverse micelles ([GNP]: 52 μm, ca. 20 nm) is 2.5-fold higher than in CTAB reverse micelles without GNPs. Improvement in the lipase activity is only specific to the GNP-doped reverse micellar media, whereas GNP deactivates and structurally deforms the enzyme in aqueous media. The reason for this activation is probably due to the formation of larger-sized reverse micelles in which the GNP acts as a polar core and the surfactants aggregate around the nanoparticle (‘GNP pool’) instead of only water. Lipase at the augmented interface of the GNP-doped reverse micelle showed improved activity because of enhancement in both the substrate and enzyme concentrations and increased flexibility in the lipase conformation. The extent of the activation is greater in the case of the larger-sized GNPs. A correlation has been established between the activity of lipase and its secondary structure by using circular dichroism and FTIR spectroscopic analysis. The generalized influence of GNP is verified in the reverse micelles of another surfactant, namely, cetyltripropylammonium bromide (CTPAB). TEM, dynamic light scattering (DLS), and UV/Vis spectroscopic analysis were utilized to characterize the GNPs and the organized aggregates. For the first time, CTAB-based reverse micelles have been found to be an excellent host for lipase simply by doping with appropriately sized GNPs

Organogelation and hydrogelation of low-molecular-weight amphiphilic dipeptides: pH responsiveness in phase-selective gelation and dye removal

The search for efficient low-molecular-weight gelators (LMWGs) with possible structure−activity correlation is on the rise. The present work reports a novel set of amphiphilic dipeptide-based carboxylic acids capable of efficiently gelating organic solvents. More interestingly, their sodium salts showed enhanced efficiency in organogelation with the additional ability to gelate water. Electrostatic interactions present in the aggregation of the sodium carboxylates of amphiphilic dipeptides seem to be important because some of the nongelator carboxylic acids turned out to be excellent gelators upon salt formation. The combinations and sequence of the amino acids in the dipeptide moiety were systematically altered to understand the collective importance of the nonpolar aliphatic/aromatic substitution in amino acids in the self-assembling behavior of amphiphiles. Almost a 20-fold enhancement in the gelation ability was observed on reversing the sequence of the amino acid residues, and in some cases, nongelators were transformed to efficient gelators. Spectroscopic and microscopic studies of these thermoreversible organo/hydrogels revealed that balanced participation of the noncovalent interactions including hydrogen bonding and van der Waals interactions are crucial for organo/hydrogelation. These dipeptides selectively gelate organic solvents from their mixtures with water, and the xerogels prepared from these organogels showed time-dependent adsorption of dyes such as crystal violet. The most remarkable feature of these gelators is the pH responsiveness, which was aptly utilized for the pH-dependent phase-selective gelation of either solvent in a biphasic mixture of oil and water. The dissimilar gelation ability of the acid and its salt originating from the pH responsiveness of the amphiphilic dipeptide was employed in the instant removal of large amounts of dyes for wastewater treatment

Antimicrobial activity, biocompatibility and hydrogelation ability of dipeptide-based amphiphiles

The development of new antibiotics is of increasing importance due to the growing resistance power of microbes against conventional drugs. To this end, cationic peptides are emerging as clinically potent antimicrobial agents. In the present study, we have synthesized six dipeptide-based cationic amphiphiles with different head group structures by varying combinations of L-amino acid residues. These amphiphiles showed remarkable growth inhibiting activity on several Gram-positive (minimum inhibitory concentration (MIC) = 0.1–10 μg/mL) and Gram-negative (MIC = 5–150 μg/mL) bacteria as well as on fungus (MIC = 1–50 μg/mL). The inherent antimicrobial efficacies of these cationic dipeptides were influenced by the head group architecture of the amphiphiles. Encouragingly, these amphiphiles selectively attacked microbial cells, while showing biocompatibility toward mammalian cells. The results show that the rational designing of short peptide-based cationic amphiphiles might serve as a promising strategy in the development of antimicrobial agents with greater cell specificities. In addition, the amphiphiles showed water gelation ability at room temperature. The formation of non-covalent supramolecular networks in gelation was established by microscopic and spectroscopic studies