Solution Behavior and Interaction of Pepsin with Carnitine Based Cationic Surfactant: Fluorescence, Circular Dichroism, and Calorimetric Studies.

The present work reports the pH-induced conformational changes of pepsin in solution at room temperature. The conformational change makes the protein surface active. The protein was found to be present in the partially denatured state at pH 8 as well as at pH 2. The fluorescence probe and circular dichroism (CD) spectra suggested that the most stable state of pepsin exists at pH 5. The binding affinities of pepsin in its native and denatured states for a D,L-carnitine-based cationic surfactant (3-hexadecylcarbamoyl-2-hydroxypropyl)trimethylammonium chloride (C16-CAR) were examined at very low concentrations of the surfactant. The thermodynamics of the binding processes were investigated by use of isothermal titration calorimetry. The results were compared with those of (3-hexadecylcarbamoylpropyl)trimethylammonium chloride (C16-PTAC), which is structurally similar to C16-CAR, but without the secondary -OH functionality near the headgroup. None of the surfactants were observed to undergo binding with pepsin at pH 2, in which it exists in the acid-denatured state. However, both of the surfactants were found to spontaneously bind to the most stable state at pH 5, the partially denatured state at pH 8, and the alkaline denatured state at pH 11. Despite the difference in the headgroup structure, both of the surfactants bind to the same warfarin binding site. Interestingly, the driving force for binding of C16-CAR was found to be different from that of C16-PTC at pH ≥ 5. The steric interaction of the headgroup in C16-CAR was observed to have a significant effect on the binding process.

Cationic vesicles of a carnitine-derived single-tailed surfactant: physicochemical characterization and evaluation of in vitro gene transfection efficiency.

Spontaneous vesicle formation in water by single-chain surfactants is rare. Here we show that in aqueous solution, a single-chain cationic surfactant 3-(dodecylcarbamoyl-2-hydroxypropyl)-trimethylammonium chloride (C12-CAR) derived from D,L-carnitine spontaneously forms vesicles with hydrodynamic diameters in the range of 30-70 nm. A detailed self-assembly study of the C12-CAR surfactant was performed for the first time by use of a combination of techniques, including surface tension, conductivity, fluorescence probe, dynamic light scattering and transmission electron microscopy. The cationic surfactant was found to have a reasonably low critical aggregation concentration (3.4±0.2 mM) in water at 298 K. The vesicles were observed to be stable at the physiological temperature (310 K) over a long period of time. Although the vesicles formed were found to be unstable and exhibit vesicle-to-tubule transition in the presence of salt, in the presence of 10 mol% cholesterol the stability of vesicles is enhanced. When compared with lipofectamine-2000, the C12-CAR was found to act as an effective gene transfection agent in COS-1 cell line. The surfactant-DNA complex was also found to be nontoxic toward CHO cells.

A smart supramolecular hydrogel of N(alpha)-(4-n-alkyloxybenzoyl)-L-histidine exhibiting pH-modulated properties.

Six L-histidine-based amphiphiles, N(alpha)-(4-n-alkyloxybenzoyl)-L-histidine of different hydrocarbon chain lengths, were designed, synthesized, and examined for their ability to gelate water. Four of members of this family of amphiphiles were observed to form thermoreversible hydrogels in a wide range of pH at room temperature. The structural variations were characterized by critical gelation concentration, gelation time, gel melting temperature (T(gs)), rheology, and electron microscopy. Among the amphiphiles, the n-octyl derivative showed better gelation ability in the studied pH range. The amphiphiles were found to have T(gs) higher than body temperature (37 degrees C) showing their stability. Also, relatively higher yield stress (>1000 Pa) values of the hydrogels show their higher strength. The effective gelator molecules self-assemble into fibrous structures. Scanning electron microscopic picture of the hydrogels revealed large ribbons with right-handed twist. Small-angle XRD and circular dichroism spectroscopy were also employed to characterize the hydrogels. It was observed that pi-pi stacking, hydrophobic interaction, amide hydrogen bonding, and solubility factor contribute to the stability and strength of the hydrogels.

Birefringent physical gels of N-(4-n-alkyloxybenzoyl)-L-alanine amphiphiles in organic solvents: the role of hydrogen-bonding.

A new class of amphiphiles, N-(4-n-alkyloxybenzoyl)-L-alanine was designed and synthesized. These amphiphiles have been shown to form thermoreversible gels in organic solvents such as aromatic hydrocarbons, cyclohexane, and chlorinated hydrocarbons at room temperature. The effects of amide functionality, chain length of the hydrocarbon tail, and the chirality of the head group of the amphiphiles on the ability to promote gelation in organic solvents have been studied. The n-tetradecyl derivative showed the best gelation ability, whereas the amphiphile with DL-alanine as the head group formed weak organogels. The 4-dodecyloxybenzoic-1-carboxyethyl ester derivative in which the amide group is replaced by an ester group also formed weak organogels at a slightly lower temperature (293 K). The gelation number and the gel melting temperature of the gelators in different solvents were determined. The rheological measurements suggested that the organogels of n-tetradecyl derivatives are stronger than those of amphiphiles containing n-dodecyl chains. Also the organogels of the amphiphiles, except the one with an ester group, were found to have gel-to-sol transition temperatures, T(gs), higher than room temperature (approximately 303 K), which increased with the increase of chain length and total concentration of the gelator. SEM pictures of the gels show fibrous structures. Small-angle XRD and optical microscopy were also employed to characterize the gels. The organogels of alanine derivatives, except that of 4-dodecyloxybenzoic-1-carboxyethyl ester, showed optical birefringence. The mechanism of gelation was studied using (1)H NMR and FTIR spectroscopy. Hydrogen-bonding between -CO(2)H groups as well as pi-pi interactions were found to be important for the gelation process.

Salt-induced vesicle to micelle transition in aqueous solution of sodium N-(4-n-octyloxybenzoyl)-L-valinate.

The self-organization of a single-tailed amino acid based chiral surfactant sodium N-(4-n-octyloxybenzoyl)-L-valinate (SOBV) has been studied in water. A number of techniques like surface tension, fluorescence probe, dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM) have been utilized for characterization of the self-assemblies. The amphiphile forms large spherical vesicles of 400-600 nm diameters in dilute aqueous solution. However, the vesicles get transformed into spherical micelles with increase of surfactant concentration or upon addition of relatively low amount (20 mM) of NaCl or KCl. This is the first example of salt-induced vesicle to micelle transition (VMT) in a single surfactant system. The vesicles are stable in the temperature range of 30-70 degrees C. Cleavage of intermolecular hydrogen bonds among the amide groups in the presence of salt appears to be the plausible cause for the VMT.