Synthesis And Properties Of Amino Acid-based Surfactants

Amino acid-based surfactants consist of amino acid moiety as the polar headgroup attachedto one or more alkyl chains. Since they are well known for being biocompatible,biodegradable, high surface active and environment-friendly, they have drawn great attractionin diverse fields such as drug delivery, cosmetic, food formulation. Due to limited categoriesand unclear structure-activity relationship, three kinds of novel amino acid-based surfactantswere designed and synthesized. And their surface activity, aggregate behavior, and theinteraction with bovine serum albumin of amino acid-based surfactants were studied.(1) Chiral surfactants,(2S)-2-(hydroxymethyl)-1-methyl-1-alkyl pyrrolidinium bromides(L-CnPB, n=12,14,16), were synthesized by the reaction of N-methyl-L-prolinol with longchain alkyl bromides. Their surface activity, thermodynamic properties and aggregationbehaviors were investigated by surface tension, electrical conductivity, steady-statefluorescence, dynamic light scattering (DLS) and the transmission electron microscopy(TEM). It was found that with increasing the length of alky chain from12to16, the CMCvalues decreased from8.95to0.538mmol/L, while γCMCincreased from33.9to32.6mN/m.the micropolarity of aggregates in aqueous was not changed nearly. The thermodynamicparameters of micellization showed that allΔG θmandΔH θmwere negative, allΔSθ mwere positive,and the values ofTΔSθ mwere much smaller thanΔH θm, which implied that the micellizationprocess is an enthopy-driven, exothermic and spontaneous process. DLS and TEM resultsshowed that L-CnPB at10CMC forms slightly sphere micelles with10-40nm inhydrodynamic diameter.(2) Three chiral L-lysine based gemini surfactants with different spacer length [C12-m-C12]Na2(m=2,4,6) were synthesis using Nε-lauroyl-L-lysine and diacyl chloride; twolysine based oligomeric surfactants2C12LyNa2and3C12LyNa3were synthesized usingcyanuric chloride and Nε-lauroyl-L-lysine. For [C12-m-C12]Na2, with increasing the length ofthe space from2to6, the CMC values increased from0.0945to0.355mmol/L, while γCMCincreased from28.6to37.4mN/m. The micropolarity of aggregates in aqueous increasedslightly. For nC12LyNan, with increasing degree of oligomerization, the CMC valuesdecreased from7.59to0.0631mmol/L, while γCMCdecreased from39.3to35.1mN/m. Themicropolarity of aggregates in aqueous decreased. The values of the minimum average areaper surfactant molecule (Amin) for2C12LyNa2and3C12LyNa3are slightly larger than two timesthat of the monomeric surfactant C12LyNa.3C12LyNa3and2C12LyNa2tend to formpremicellar aggregates in solution at a sufficiently low concentration with attributed to thecombination of the hydrophobic effect. The thermodynamic parameters of micellization of thelysine-based surfactants indicated that the micellization process is an enthopy-driven,exothermic and spontaneous process. DLS and TEM showed that the lysine based oligomericsurfactants at10CMC can spontaneously self-assemble to form aggregates in aqueoussolutions, the particle size was among100-400nm; the particle size for C12LyNa was among40-100nm; the particle size for3C12LyNa3was among70-250nm. C12LyNa forms slightlyentangled rod-like micelles.[C12-m-C12]Na2,2C12LyNa2and3C12LyNa2can self-assembleinto regular spherical or ellipsoid assemblies. The circular dichroism (CD) indicated theformation of chiral helical aggregates above the CMC for [C12-m-C12]Na2and2C12LyNa2.(3) Three chiral L-cysteine-based trimeric surfactants with different hydrophobic chainwere using the fatty acyl chloride, L-cysteine and1,3,5-triacryloylhexahydro-1,3,5-triazine.With the increase in the alkyl chain length from8to12, the CMC values decreased from4.82to0.116mmol/L, while γCMCincreased from33.9to36.2mN/m. The micropolarity ofaggregates in aqueous decreased slightly. The CMC value of the3C12CyNa3was two timesthat of the monomeric surfactant C12CyNa. The thermodynamic parameters of micellization of3CnCyNa3indicated that the micellization process is an enthopy-driven, exothermic andspontaneous process. DLS and TEM showed that the3CnCyNa3can spontaneously self-assemble to form spherical or ellipsoid assemblies aggregates with100-250nm inhydrodynamic diameter. The CD indicated the formation of chiral helical aggregates abovethe CMC for3CnCyNa3.(4) The interaction between bovine serum albumin and a series of amino-acid surfactantswere studied by the fluorescence. From endogenous fluorescence spectroscopy results, we canknow that the interaction order with BSA is L-C16PB>L-C14PB>L-C12PB;[C12-2-C12]Na2>[C12-4-C12]Na2>[C12-6-C12]Na2,3C12LyNa3>2C12LyNa2,3C12CyNa3>3C10CyNa3>3C8CyNa3.