Amino acid surfactants structural schemes

III. STRUCTURAL SCHEMES AND METHODS FOR THE PREPARATION OF AMINO ACID SURFACTANTS... 

The preparation of surfactant molecules in the laboratory is often achieved by the method of modular synthesis [7,8]. This means that the molecule is considered as an assembly of subunits called moduli. In this way we can distinguish in natural surfactants bi- or trimodular structures, as shown in Scheme 1. Based on this principle, the synthetic approaches and strategies are variable. The amino acid by itself may or may not constitute the hydrophilic modulus alternatively, it may play the role of the junction modulus or that of an anchor unit for the hydrophobic group attached to it via a C-C bond, depending on its specific structure. 

One important milestone in our research is the design and development of new amino acid-based surfactants with antimicrobial properties, which mimic natural amphiphilic cationic peptides [42,43]. To this end, Lys and Arg derivatives of long-chain A -acyl, COO-ester, and A-alkyl amide have been prepared. In particular, the A -acylarginine methyl ester derivatives series 1 (Scheme 1) have turned out to be an important class of cationic surface active compounds with a wide bactericidal activity, high biodegradability, and low toxicity profile. We have shown that essential structural factors for their antimicrobial activity include both the length of the fatty residue (akin with their solubility and surface activity) and the presence of the protonated guanidine function [43,44]. 

Anionic surfactants with the general structure RpCONH-X-COONa (e.g., X = -(0112)5-) have been reported in the patent literature. These surfactants are synthesized from the corresponding isopropyl ester and 6-amino-hexanoic acid sodium or ammonium salt (H2N(CH2) 5COOM). Another class of anionic surfactants derived from PFCA derivatives are perfluoroacylbenzenesulfo-nates (Scheme 18.11). This group of surfactants is synthesized by Friedel-Crafts acylation of benzene with a PFCA halide (e.g., perfluorooctanoyl chloride) in the presence of at least one equivalent of a Lewis acid (e.g., anhydrous aluminum chloride). This reaction proceeds smoothly and in good yields at subambient or ambient temperatures. The perfluoroacylbenzene is sulfonated with oleum or sulfur trioxide and neutralized with a base (e.g., sodium hydroxide). 

Aspartic and glutamic acid have also been used frequently as starting material for the preparation of surfactants. The diesters of glutamic acid, obtained by reaction with a long-chain alcohol, have been rendered surface active by grafting a quaternary ammonium group on the a-amino fimction via an acylation reaction [51] (structure 20, Scheme 18). 

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