Amphiphilic lauryl ester derivatives from aromatic amino acids: significance of chemical architecture in aqueous aggregation properties

Antioxidant, Antidiabetic and Anticancer Activities of L-Phenylalanine and L-Tyrosine Ester Surfactants: In Vitro and In Silico Studies of their Interactions with Macromolecules as Plausible Mode of Action for their Biological Properties

Background:

Aromatic amino acid-based surfactants have been found to have interesting biological properties such as antibacterial and hemolytic activities. Recently, we have reported the antibacterial activity of a range of ester hydrochloride surfactants derived from L-Phenylalanine and LTyrosine. This study aims at assessing the antioxidant, α-glycosidase inhibitory and cytotoxic activities of a series of L-Phenylalanine and L-Tyrosine ester hydrochlorides. Molecular docking and BSA binding studies were also carried out in order to investigate their potential therapeutic targets.

Methods:

L-Phenylalanine and L-Tyrosine surfactants were tested as potential lipophilic antioxidants using the DPPH and ABTS assays. These surfactants were also tested for their α-glycosidase inhibitory activity using 4-nitrophenyl α -D-glucopyranoside (pNPG) as substrate. Their cytotoxicity effects were screened using HeLa and KB cell lines. Glide version 5.7 as implemented in Schrödinger suite 2013-1, was used for performing docking studies of L-Phenylalanine and L-Tyrosine dodecyl esters. The interaction of the ester hydrochlorides of L-Phenylalanine and L-Tyrosine with bovine serum albumin (BSA) was investigated using fluorometric titration.

Results:

The presence of the phenolic moiety in L-Tyrosine-based surfactants was found to enhance the antioxidant and α-glucosidase inhibitory activities compared to the L-Phenylalanine derivatives. The α- glucosidase and anticancer activities of the phenylalanine surfactants were found to increase with chain length up to C12 above which the activities exhibited a downward trend. In the case of the tyrosine series, an increase in chain length from C8 to C14 was found to decrease the α-glucosidase inhibitory activity and increase the anticancer activity of the surfactants. Binding studies with bovine serum albumin showed that the tyrosine surfactants displayed greater affinity for the serum albumin, owing to the presence of the phenolic group which altered the orientation of the surfactant molecule within the hydrophobic core of BSA.

Conclusion:

L-Tyrosine esters having a phenolic moiety were found to possess enhanced biological activity in terms of both the antioxidant and antidiabetic activities as well as also bind more strongly to Bovine serum albumin. Molecular docking studies of the phenylalanine and tyrosine surfactants of similar chain length with target proteins showed direct correlation with their anticancer and antidiabetic activity. Therefore, the findings show that these aromatic based surfactants derived from L-Tyrosine can act as promising antioxidant, antidiabetic and anticancer agents, and they can also be efficiently transported and eliminated in the body, making them useful candidates for drug designs.

Nuclear Magnetic Resonance Investigation of the Effect of pH on Micelle Formation by the Amino Acid-Based Surfactant Undecyl l-Phenylalaninate

Micelle formation by the anionic amino acid-based surfactant undecyl l-phenylalaninate (und-Phe) was investigated as a function of pH in solutions containing either Na⁺, l-arginine, l-lysine, or l-ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing l-arginine and l-lysine had lower CMC than the corresponding solutions with Na⁺ counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und-Phe micelles with Na⁺ counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, l-arginine, l-lysine, and l-ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und-Phe/l-arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with l-lysine and l-ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that l-arginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, l-lysine and l-ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two-dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two-dimensional NMR also showed that in und-Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle.

Concentration Dependent Specific Rotations of Chiral Surfactants: Experimental and Computational Studies

Recent experimental studies have shown unexpected chiroptical response from some chiral surfactant molecules, where the specific rotations changed significantly as a function of concentration. To establish a theoretical understanding of this experimentally observed phenomena, a novel methodology for studying chiral surfactants via combined molecular dynamics (MD) and quantum mechanical (QM) calculations is presented. MD simulations on the +10 000 atom surfactant systems have been performed using MD and QM/molecular mechanics (MM) approaches. QM calculations performed on MD snapshots coupled with extensive analysis on lauryl ester of phenylalanine (LEP) surfactant system indicate that the experimentally observed variation of specific rotation with concentration may be due to the conformational differences of the surfactant monomers in the aggregates. Though traditional MM simulations did not show significant differences in the conformer populations, QM/MM simulations using the forces derived from the PM6 method did predict conformational differences between aggregated and nonaggregated LEP molecules, which is consistent with experimental data. Additionally the electrostatic environment of charged surfactants may also be important, since dramatic changes in the Boltzmann populations of surfactant monomers can be noted in the presence of an electric field generated by the chiral ionic aggregates.

Nanocarriers of solid lipid from micelles of amino acids surfactants coated with polymer nanoparticles

Polymer nanoparticle coated micelle assemblies of lauryl ester of tyrosine (LET) act as potential nanocarriers for the model solid lipid stearyl alcohol. The coating is afforded by a simple methodology of heterophase polymerization reaction of styrene or the mixture of styrene and butyl acrylate at a mole ratio of 0.8:0.2 in the presence of 200 mM LET in water. On the contrary, the polymer nanoparticles produced under similar conditions in the presence of a structurally similar surfactant, lauryl ester of phenyl alanine (LEP), failed to act as nanocarrier. The micelle templates of LET and LEP favored polymerization under controlled conditions as observed from the near monodisperse distribution of molecular weight and size of the polymers. The particle size distribution of poly(styrene) (PS) and poly(styrene-co-butyl acryalte) (PS-co-PBA) nanoparticles from LET was smaller at 24 and 20 nm in comparison to those from LEP. The encapsulation efficiency of polymer nanoparticles from LET surfactant is explained on the basis of difference in the coating of micelle assemblies, which we believe must be arising due to difference in the solubilization site of the monomers in the surfactant micelles before polymerization reaction. The solubilization of the model monomer, benzene at different regions, varying between shell and core of LET and LEP micelles is established from (1)H nuclear magnetic resonance spectra. The evidence for the coating of micelle assemblies from surface tension measurements and the encapsulation of stearyl alcohol in the polymer nanoparticle dispersions from LET drawn from transmission electron microscopy, differential scanning calorimetry, and thermogravimetric analysis is discussed.

pH-responsive aggregation states of chiral polymerizable amphiphiles from L-tyrosine and L-phenyl alanine in water

Sodium salts of maleamic acid derivatives of lauryl ester of tyrosine (MTNa) and phenyl alanine (MPNa) in water exhibited strong pH-responsive behaviors of viscosity and specific conductivity that originate from the concentration and pH dependence of their aggregation states. The aggregates were characterized by a novel spin-probe-partitioning electron paramagnetic resonance (SPPEPR) method and dynamic light scattering (DLS). Results of high-precision fitting of the second-harmonic EPR spectra of the small spin probe di-tert-butyl nitroxide (DTBN) in these aggregates together with viscosity, conductivity, and DLS showed that, at pH ~ 7.54, MTNa formed micelles and MPNa vesicles and MTNa exhibited a pH-induced micelle to vesicle transition as pH was lowered toward 6. MTNa, at pH ~ 7.54, formed small micelles at low concentrations that transformed to long worm-like micelles for concentrations ≥ 0.05 M, accompanied by a 30-fold increase in solution viscosity. The hydrodynamic radii from DLS confirmed the presence of small micellar aggregates of radius ~ 2 nm in MTNa at pH ~ 7.54 at the lower concentrations, with coexisting micelles (~2 nm) and vesicles (~50 nm) at pH near 6.5, vesicles (radii ~ 70 nm) at pH near 6, and large vesicles (85 nm) in MPNa at pH ~ 7.60. Both MTNa and MPNa precipitated upon reduction of pH below 6 and below 7, respectively. The rate of transfer of DTBN between the aqueous phase and the aggregate was calculated from the high-field Lorentzian linewidths of the electron paramagnetic resonance (EPR) spectra. The activation energy for the transfer determined from the temperature dependence of the rate of transfer is 12.7 kJ/mol for MTNa vesicles (pH ~ 6) and 20.6 ± 1.3 kJ/mol for MPNa (pH ~ 7.60). The pH-induced transformations were reversible.

Unprecedented relationship between the size of spherical chiral micellar aggregates and their specific optical rotations

Transmission electron microscopy (TEM) images and fluorescence quenching methods indicated that lauryl ester of L-phenylalanine (LEP) and lauryl ester of L-tyrosine (LET) form spherical chiral micelles in the 50-200 mM range and their size increases with concentration. The number of molecules present in these spherical chiral aggregates varied from 80 to 160 for LEP and 80-100 for LET. The specific optical rotation, representing circular birefringence, for LEP at 405 nm and 32 °C is found to increase linearly from 37 deg cc g(-1) dm(-1) for an isolated molecule to 56 deg cc g(-1) dm(-1) for ∼200 nm size aggregate. A similar trend was found for temperatures up to 70 °C and at other visible wavelengths. A linear relation between specific optical rotation and the size of aggregate is also observed for LET. Circular dichroism, as measured in both the visible and infrared wavelength regions, however did not reveal any concentration dependent changes. The unique sensitivity uncovered for specific optical rotation as a function of the size of spherical chiral aggregates is unprecedented and opens new areas of enquiry for physical chemists.

Formation of self-aggregated structures of different types in water of chiral polymerizable amphiphiles from L-tyrosine and L-phenylalanine

Sodium salts of maleamic acid derivatives from lauryl esters of L-tyrosine (MTNa) and L-phenylalanine (MPNa) were synthesized and characterized. The aggregated structures of MTNa and MPNa in water were investigated, employing several independent methods. MPNa showed secondary aggregated structures in contrast to MTNa at concentrations of >1 × 10(-3) M. The results from dynamic light scattering, transmittance, conductivity, and viscosity measurements suggested the formation of aggregated structures of different types in MTNa and MPNa solutions. The measured fluorescence anisotropy (r) at 0.180 of the fluoroprobe, 1,6-diphenyl-1,3,5-hexatriene (DPH), and the d spacing of 38 Å from small-angle X-ray diffraction (SAXD) experiments confirmed the bilayer structures in MPNa. Scanning electron microscope (SEM) images provided the morphological features. The emulsion produced using MPNa solution was more stable. The confocal fluorescence microscopy image of the emulsion from MPNa confirmed the entrapment of water-soluble dye, rhodamine. The models of MTNa and MPNa molecules and the aggregated structures are presented.

1H NMR spectroscopic investigations on the conformation of amphiphilic aromatic amino acid derivatives in solution: effect of chemical architecture of amphiphiles and polarity of solvent medium

In this study, the conformation of the amphiphilic lauryl esters of L-tyrosine (LET) and L-phenylalanine (LEP) in water and dimethyl sulfoxide is established. The alkyl chain protons of LEP in D(2)O appear at δ 1.010-1.398 and show an upfield shift and large line width, suggesting the proximity of the phenyl ring to the alkyl chain in contrast to that of LET. Quite interestingly, in DMSO-d(6), the (1)H NMR spectra of LET and LEP show a strong similarity that is suggestive of an orientation that positions the aromatic ring and aliphatic chain away from each other. These results are substantiated with two-dimensional nuclear Overhauser enhancement spectroscopy (2D NOSEY). Theoretical molecular models of the conformation at the interface corroborate the experimental findings. Investigations of the solvent polarity and chemical structure-dependent conformation are discussed.