Phenylalanine and Tryptophan-Based Surfactants as New Antibacterial Agents: Characterization, Self-Aggregation Properties, and DPPC/Surfactants Vesicles Formulation

Abstract

Cationic surfactants based on phenylalanine (CnPC3NH3Cl) and tryptophan (CnTC3NH3Cl) were synthesized using renewable raw materials as starting compounds and a green synthetic procedure. The synthesis, acid-base equilibrium, aggregation properties, and antibacterial activity were investigated. Conductivity and fluorescence were used to establish critical micelle concentrations. Micellization of CnPC3NH3Cl and CnTC3NH3Cl occurred in the ranges of 0.42–16.2 mM and 0.29–4.6 mM, respectively. Since those surfactants have some acidic character, the apparent pKa was determined through titrations, observing increasing acidity with increasing chain length and being slightly more acidic with the phenylalanine than the tryptophan derivatives. Both families showed promising antibacterial efficacy against eight different bacterial strains. Molecular docking studies against the enzyme peptidoglycan glycosyltransferase (PDB ID:2OQO) were used to investigate the potential binding mechanism of target surfactant molecules. According to small angle X-ray scattering (SAXS) results, the surfactants incorporate into DPPC (Dipalmitoyl Phosphatidyl Choline) bilayers without strong perturbation up to high surfactant concentration. Some of the C12TC3NH3Cl/DPPC formulations (40%/60% and 20%/80% molar ratios) exhibited good antibacterial activity, while the others were not effective against the tested bacteria. The strong affinity between DPPC and surfactant molecules, as determined by the DFT (density functional theory) method, could be one of the reasons for the loss of antibacterial activity of these cationic surfactants when they are incorporated in vesicles.

Antimicrobial and Anesthetic Niosomal Formulations Based on Amino Acid-Derived Surfactants

“…Both samples containing DPPC showed the typical band corresponding to phospholipid bilayers (with the maxima around 0.12 nm −1 and 0.15 nm −1 for L/C/D/P 14 (2:1:5:2) and L/C/D/P 14 (5:1:2:2), respectively), but more clearly in the sample with a higher DPPC content. The bilayer corresponding to the sample richer in DPPC showed parameter values closer to those of pure DPPC (in particular, Z H was close to the value obtained for pure DPPC [ 10 ]) and better fitting result compared to the one with a lower content (with the Z H value reduced because of the increased proportion of shorter-chain molecule P 14 ). This was expected, since we could consider the bilayer to be DPPC with the introduction of some perturbation due to the additives.…”

“…Comparing the antimicrobial activity of CnPN(CH 3 ) 3 with that of pH-sensitive surfactants with a similar chemical structure [ 10 ], it is observed that all these cationic amphiphiles exhibit a common broad spectrum of antibacterial activity. In this regard, the advantage of these phenylalanine derivatives is that the cationic charge does not depend on the pH, so they are expected to maintain their activity in all pH media.…”

“…It has been observed that these liposome systems can improve the stability and antimicrobial activity of these antibiotic compounds [ 26 ]. In the literature, works regarding the preparation of liposomes with antimicrobial activity using amino acid-based surfactants and phospholipids can be found [ 10 , 27 ]. Our group has also reported the properties of antimicrobial cationic vesicles based on arginine surfactants [ 24 ].…”

“…The phenylalanine-based surfactants used in this work were prepared following the synthetic procedure showed in Figure 2 . C n PC 3 NH 2 was synthesized using the process outlined in our previous work [ 10 ]. In brief, the free amine group of the phenylalanine methyl ester was acylated with dodecyl or tetradecyl acid chloride; then, the C n POMe compounds were dissolved in diaminopropane at 60 °C to obtain the corresponding C n PC 3 NH 2 .…”

“…The cationic charge of these compounds was situated on a protonated amine group, and, consequently, their cationic character depended on the pH. It was found that these new surfactants formed unilamellar vesicles with DPPC mixtures [ 10 ]. Surfactants with a C 12 –C 14 alkyl chain exhibited good antimicrobial activity; moreover, vesicles containing 60–80% of surfactants maintained their antimicrobial effectivity.…”