Cationic amino acid-based surfactants are known to interact with the lipid bilayer of cell membranes resulting in depolarization, lysis and cell death through a disruption of the membrane topology. A range of cationic surfactant analogues derived from L-Phenylalanine (C1-C20) and L-Tyrosine (C8-C14) esters have been synthesized and screened for their antibacterial activity. The esters were more active against gram positive than gram negative bacteria. The activity increased with increasing chain length, exhibiting a cut-off effect at C12 for gram positive and C8/C10 for gram negative bacteria. The cut-off effect for gram negative bacteria was observed at a lower alkyl chain length. The CMC was correlated with the MIC, inferring that micellar activity contribute to the cut-off effect in antibacterial activity. The interaction of the cationic surfactants with the phospholipid vesicles (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) in the presence of 1-anilino-8-naphthalene sulfonate (ANS) and 1,6-diphenyl-1,3,5-hexatriene (DPH) as fluorescence probes showed that an increase in ionic interaction causes an increase in antibacterial activity. Increase in hydrophobic interaction increases the antibacterial activity only to a certain chain length, attributing to the cut-off effect. Therefore, both electrostatic and hydrophobic interactions, involving the polar and nonpolar moieties are of paramount importance for the bactericidal properties.

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A series of single-chained N-acyl tyrosine surfactants with varying chain lengths (C 10 -C 18 ) and degree of unsaturation, as well as an N-acyl Gemini tyrosine surfactant with chain length C 12 , were synthesized, and the structures were confirmed using spectral analysis. The effect of chain length and level of unsaturation on the physicochemical and antibacterial properties of the N-acyl tyrosine surfactants was evaluated. The C 12 derivative displayed the optimum antibacterial activity among the single chain surfactants, and the presence of double bond in the oleoyl derivative enhanced the antibacterial activity over its saturated analogue. The N-acyl Gemini surfactant displayed the highest antibacterial activity among the series and also showed greater micelle forming ability than its single chain analogue. Mixed micellar behavior of the N-acyl Gemini surfactant with conventional cationic (CTAB) and anionic (SDS) surfactants in aqueous solution was studied. The negative value of the interaction parameter β 12 observed for the N-acyl Gemini in binary mixture with CTAB surfactant indicated a synergistic interaction within the mixed micellar system. However, the binary mixture with SDS displayed antagonistic behavior. The binary mixture of N-acyl Gemini surfactant with CTAB displayed better antibacterial activity and foaming properties than with SDS mixtures. Optimum antibacterial activity was observed for N-acyl Gemini surfactant with mole ratio 0.4 to 0.6 in the CTAB binary mixture, at which the lowest ocular irritation index was observed. Overall, the study showed that the Gemini surfactant in combination with the conventional surfactant CTAB can be used as potential ingredients in detergent and pharmaceutical formulations. FIGURE 7 Variation of CMC with mole fraction of surfactant 8 in (A) SDS and (B) CTAB mixed systems. CMC indicates critical micelle concentration; CTAB, cetyl trimethyl ammonium bromide; SDS, sodium dodecyl sulfate JOONDAN ET AL.

A series of surfactants derived from L-Proline, the free amine esters, the ester hydrochlorides and the quaternary ammonium compounds with varying chain lengths (C 8 -C 14 ) were synthesised. The physicochemical and biological properties were determined in both single and sodium dodecyl sulphate (SDS) mixed systems with a view of enhancing the properties of the individual surfactants as potential ingredients in detergent formulations. The mode of action of the proline surfactants were investigated by their ability to form mixed micelles with the phospholipid 1,2-dimyristoyl-sn-glycero-3phosphatidylcholine (DMPC). The presence of a quaternary ammonium moiety and an increase in alkyl chain length were found to enhance the antibacterial activity of the proline QUAT derivatives. The SDS-C 14 QUAT mixed system displayed good antibacterial activity with optimum activity at mole fractions a QUAT: 0.4 and 0.6. The antibacterial activity of the mixed system was found to be governed by the monomers rather than the micelles. The SDS-C 14 QUAT mixed system also showed moderate irritancy which makes them potential candidates as detergents.

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