Enzymatic lipophilization of phenolic compounds and evaluation of their antioxidant properties

The oxidation of unsaturated lipids, which is one of the major causes of foods and cosmetics adulteration, may be limited by the use of antioxidants. Nowadays, phenolic compounds (particularly phenolic acids and flavonoids) generate a growing interest owing to their antioxidant properties. However, the implementation of such polar molecules in lipid-based systems (emulsion or other) is difficult and can lead to a decrease of their efficiency. To solve this issue, one strategy consists in adjusting the polarity of these molecules by the grafting of aliphatic chains of different lengths. The first part of this work has been dedicated to the enzymatic synthesis of a series of chlorogenate and rosmarinate alkyl esters. Regarding chlorogenic acid, the best results were obtained using a two-steps strategy. This strategy involves a preliminary esterification with methanol in presence of a sulfonic resin followed by an enzymatic transesterification (Candida antarctica B lipase) of methyl chlorogenate, with alcohols of 4-16 carbon chain lengths. Under optimal conditions, the initial rates of transesterification were up to two-fold higher than that of direct enzymatic esterification. Furthermore, the two-steps strategy led to overall yields of 61% to 93% against 40% to 60% in the case of direct esterification. With regard to rosmarinic acid, only the direct esterification in the presence of sulfonic resin leads to satisfactory yields (82% to 99%, depending on alkyl chain length). In the second part of this work, we evaluated, from kinetic and stationary point of view, the impact of hydrophobation on the ability of new molecules to scavenge the stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH.). The kinetic data showed that chlorogenic acid was stabilized in its quinone form, while esters tend to form dimers from their semi-quinone form. The results obtained at stationary state indicated that both acids, but also their corresponding esters, have a radical scavenging activity greater than the theoretical capacity attributable to the number of phenolic hydroxyl. These results can be explained considering the existence of regeneration and/ or dimerization mechanisms. In the third part of this work the antioxidant capacity of lipophilic derivatives of chlorogenic and rosmarinic acids was evaluated in emulsified systems by a new methodology called conjugated autoxidizable triene (CAT). The results showed that the increase of alkyl chain length did not necessarily lead to an improvement in the antioxidant capacity as predicted the so called " Polar Paradox". Indeed, maximum antioxidant capacities for chain length of 12 and 8 carbon atoms were observed for fatty esters of chlorogenic and rosmarinic acid, respectively. The main hypotheses proposed to explain this unexpected behaviour were based on the distribution of antioxidants and their oxidized forms in the different phase of the system.