Chemoenzymatic synthesis of anionic alkyl glycosides

Abstract: Growing concerns for environmental issues and product safety for users have shifted research interests towards the use of renewable resources and climate-neutral processes. The enzymatic approach having high selectivity under mild reaction conditions is promising, and the nature-originating products are well accepted. Alkyl glycosides (AGs) consisting of fatty alcohol and sugar are nonionic surfactants applied in many fields, because of their mildness and biodegradability. However, to broaden their applications and replace some of the currently used surfactants, which negatively affect the environment, there is a need to modify the structure of AGs. In this thesis, the focus is on the synthesis of new derivatives of AGs. The work includes three parts: 1. Production of anionic AGs; 2. Elongation of anionic AGs; 3. Characterization of modified products. In the first part, the method employed was the laccase/TEMPO oxidation of primary alcohol groups of AGs such as octyl glucoside (OG), dodecyl maltoside (DDM), hexadecyl maltoside (HDM), dodecyl maltooctaoside (DDMO) to corresponding acids. The influencing factors, including enzyme and TEMPO concentration, were investigated. The reaction showed complete conversion of AGs and high yields at high enzyme and TEMPO concentrations. However, inactivation of laccase by oxidized TEMPO was observed as well as the formation of by-product ketones. Another disadvantage of the process was the inevitable degradation of glycosides, which however decreases at high TEMPO concentration, except for AGs with long carbohydrate chain, such as DDMO, where the depolymerization became severe. Interestingly, TEMPO can convert any form of AGs, such as monomers, micelles, rod-like and lamellar structures, to give a conversion of 100% without the assistance of co-solvent. The fact that the presence of carboxylic groups in AGs improve the solubility of AGs, especially long alkyl AGs, opens up for their application in aqueous solution. In the second part, the aim was to produce anionic AGs with an oligomeric head group, which is expected to give an improved product with lower cellular toxicity. The extension of the carbohydrate chain was achieved on octyl glucuronic acid (OG-COOH) and dodecyl maltoside diuronic acid (DDM-2COOH), using cyclodextrin glycosyltransferase enzymes (CGTases) as catalyst at pH 5.6. At this pH, these AGs exist mainly in ionized form, which caused a significant decrease in reaction rate as compared to the corresponding underivatized AGs. An increase in pH from 4 to 6 caused about 25 and 10 times decreases in the initial rates for DDM-2COOH and OG-COOH, respectively. The preference of CGTases for the neutral form over the ionized form of AGs was confirmed by molecular modeling of the enzyme itself and docking experiments with the potential acceptor substrates. A negative charge in the carbohydrate chain could lead to electrostatic repulsion with the negatively charged catalytic base, Asp 356, disturbing the reaction. In the last part, the surfactant behaviour was studied. The interfacial properties of anionic AGs appeared to be highly pH-dependent. Increasing pH caused increased CMC (critical micelle concentration) values of OG-COOH, resulting in low foamability, which is the opposite to the behavior of OG. This can be due to the increased repulsive interactions between negatively charged head groups of OG-COOH, which were formed at elevated pH values. In addition, the size of micelles was studied at different salt concentrations. The results showed that the micellar size increased with an increase in salt concentration. The comparison of the size of micelles formed by OG and OG-COOH indicated that the OG-COOH micellar size is smaller and less influenced by salt and surfactant concentrations than that of OG.