pH-Sensitive surfactants from lysine: Assessment of their cytotoxicity and environmental behaviour

Three different sets of cationic surfactants from lysine have been designed and synthesised. The structure, the number of cationic charges on the polar head and the type of cationic charges were systematically varied in these compounds. The first group consists of three monocatenary surfactants with one lysine as the cationic polar head with one cationic charge. The second group consists of three monocatenary surfactants with two amino acids as cationic polar head with two positive charges. Finally, four gemini surfactants were synthesized in which the spacer chain and the number and type of cationic charges have been regulated. In these work we have studied the influence of the pH on the cationic charge of these molecules and the toxicity of these compounds against two types of human cells: fibroblasts and erythrocytes. We have also determined the effect of the pH on the haemolysis of these compounds. Moreover the aquatic toxicity as well as the biodegradation has been evaluated. The obtained pKa values for these surfactants go from 8.8 to 4.8, these means that they have weak acid properties; consequently the cationic character depends on the pH. The critical micelle concentration of these surfactants was dependent only on the hydrophobic character of the molecules. Overall, all the cationic surfactants synthesized from lysine showed low haemolytic character. The haemolytic activity also depended strongly on the type of structure (monocatenary or geminal) and on the location of the cationic charge. The monocatenary compounds showed higher HC50 values than the Gemini ones, and the haemolytic activity increased significantly in compounds with the positive charge on the trimethylated amino group of the lysine. Finally, all prepared compounds can be considered ready biodegradable surfactants. The percentage of biodegradation is mainly affected by the chemical structure type and not by the cationic charge intensity.