Amino acid-based surfactants consist of amino acid moiety as the polar headgroup attached to one or more alkyl chains. Since they are well known for being biocompatible, biodegradable, high surface active and environment-friendly, they have drawn great attraction in diverse fields such as drug delivery, cosmetic, food formulation. Due to limited categories and unclear structure-activity relationship, three kinds of novel amino acid-based surfactants were designed and synthesized. And their surface activity, aggregate behavior, and the interaction 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 long chain alkyl bromides. Their surface activity, thermodynamic properties and aggregation behaviors were investigated by surface tension, electrical conductivity, steady-state fluorescence, dynamic light scattering (DLS) and the transmission electron microscopy (TEM). It was found that with increasing the length of alky chain from 12 to 16, the CMC values decreased from 8.95 to 0.538 mmol/L, while γCMC increased from 33.9 to 32.6 mN/m. the micropolarity of aggregates in aqueous was not changed nearly. The thermodynamic
parameters of micellization showed that allΔGθm and ΔHθm were negative, allΔSθm were positive, and the values of TΔSθm were much smaller than ΔHθm , which implied that the micellization process is an enthopy-driven, exothermic and spontaneous process. DLS and TEM results showed that L-CnPB at 10CMC forms slightly sphere micelles with 10-40 nm in hydrodynamic 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; two lysine based oligomeric surfactants 2C12LyNa2 and 3C12LyNa3 were synthesized using cyanuric chloride and Nε-lauroyl-L-lysine. For [C12-m-C12]Na2, with increasing the length of the space from 2 to 6, the CMC values increased from 0.0945 to 0.355 mmol/L, while γCMC increased from 28.6 to 37.4 mN/m. The micropolarity of aggregates in aqueous increased slightly. For nC12LyNan, with increasing degree of oligomerization, the CMC values decreased from 7.59 to 0.0631 mmol/L, while γCMC decreased from 39.3 to 35.1 mN/m. The micropolarity of aggregates in aqueous decreased. The values of the minimum average area
per surfactant molecule (Amin) for 2C12LyNa2 and 3C12LyNa3 are slightly larger than two times that of the monomeric surfactant C12LyNa. 3C12LyNa3 and 2C12LyNa2 tend to form premicellar aggregates in solution at a sufficiently low concentration with attributed to the combination of the hydrophobic effect. The thermodynamic parameters of micellization of the lysine-based surfactants indicated that the micellization process is an enthopy-driven, exothermic and spontaneous process. DLS and TEM showed that the lysine based oligomeric surfactants at 10CMC can spontaneously self-assemble to form aggregates in aqueous solutions, the particle size was among 100-400 nm; the particle size for C12LyNa was among 40-100 nm; the particle size for 3C12LyNa3 was among 70-250 nm. C12LyNa forms slightly entangled rod-like micelles. [C12-m-C12]Na2, 2C12LyNa2 and 3C12LyNa2 can self-assemble into regular spherical or ellipsoid assemblies. The circular dichroism (CD) indicated the formation of chiral helical aggregates above the CMC for [C12-m-C12]Na2 and 2C12LyNa2.
(3) Three chiral L-cysteine-based trimeric surfactants with different hydrophobic chain were using the fatty acyl chloride, L-cysteine and 1, 3, 5-triacryloylhexahydro-1, 3, 5-triazine. With the increase in the alkyl chain length from 8 to 12, the CMC values decreased from 4.82 to 0.116 mmol/L, while γCMC increased from 33.9 to 36.2 mN/m. The micropolarity of aggregates in aqueous decreased slightly. The CMC value of the 3C12CyNa3 was two times that of the monomeric surfactant C12CyNa. The thermodynamic parameters of micellization of 3CnCyNa3 indicated that the micellization process is an enthopy-driven, exothermic and spontaneous process. DLS and TEM showed that the 3CnCyNa3 can spontaneously self-assemble to form spherical or ellipsoid assemblies aggregates with 100-250 nm in hydrodynamic diameter. The CD indicated the formation of chiral helical aggregates above the CMC for 3CnCyNa3.
(4) The interaction between bovine serum albumin and a series of amino-acid surfactants were studied by the fluorescence. From endogenous fluorescence spectroscopy results, we can know 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.
Surfactants are important products in the field of fine chemicals and have a series of unique application properties such as wetting, penetrating and waterproofing, emulsifying and breaking emulsions, foaming and defoaming. They are widely concerned and used in many fields such as daily chemicals, food, pesticides, pharmaceuticals and oil extraction. At present, most surfactants are made from petroleum-based raw materials, which have problems such as non-renewable raw materials and low chemical safety.
At the same time, residual surfactants after use can cause serious environmental pollution. Therefore, in order to meet the growing environmental awareness and personalized needs of consumers, the development of surfactants with high surface activity, multifunctionality and excellent biocompatibility, improving product safety and reducing their impact on the environment are the current hot spots of surfactant research.
Biomass surface active agents based on renewable substances such as amino acids, sugars, precipitates or rosin have been synthesized by biotechnology or chemical methods, and these surfactants mimic natural amphiphilic molecular structures such as alkyl glycosides, lecithin, cholesterol and proteins to achieve the safety of surfactants for the environment and human body and to realize the greening of products . surfactants, which have the advantages of low irritation, low toxicity, good biocompatibility, biodegradability and environmental compatibility, have been widely noticed and applied in the food, pharmaceutical and cosmetic industries.
Amino acids are broadly defined as organic compounds containing a basic amino group and an acidic carboxyl group, and there are more than 300 types of amino acids in nature. The amino acid surfactants mentioned here are synthesized by selecting one or more of the 20 basic amino acids as raw materials and introducing hydrophobic long chains into the amino group, carboxyl group or active side group.