Lipid sponge phase nanostructures as carriers for enzymes

Abstract: Nonlamellar lipid liquid crystalline phases have many potential applications, such as for drug delivery, protein encapsulation or crystallization. Lipid liquid crystalline sponge phase (L3) has so far not been very much considered in these applications, in spite of apparent advantages in terms of its flexibility and capacity of forming large aqueous pores able to encapsulate large bioactive molecules. In this thesis the potential of these L3 phases as carriers of two important enzymes used in food industry was explored.In the first part of my thesis, a novel lipid system able to form highly swollen L3 phases was investigated. This lipid system was composed of diglycerol monoleate (DGMO), glycerol monoleate (Capmul GMO-50) and polysorbate 80 (P80). Since the L3 phase is not as well characterized as other lipid liquid crystalline phases, a detailed study was performed using scattering and spectroscopic methods. The results showed that the L3 phase is closely related to other bilayer-type of structures, especially inverse bicontinuous cubic phases. The formed L3 phase was found to be easy to disperse in excess water to form sponge-like nanoparticles (L3-NPs). Their structure, interfacial properties and the role of P80 for particle stabilization were determined. Small angle neutron scattering revealed that P80 was mostly located at the lipid-aqueous interface, but also contributed to form the inner L3 structure. This indicated that the highly extended structures observed on the L3-NPs surface by cryo-TEM were formed thanks to P80. This type of structures stabilises the nanoparticles. The interfacial properties of L3-NPs were studied using different surface techniques. Adsorption of the L3-NPs on hydrophilic silica led to spreading of the particles on the surface to form a bilayer-type of structure.The second part of this work demonstrates the applicability of these lipid L3 phases as matrices for enzyme encapsulation. The enzymes investigated here were Aspartic protease (34 kDa) and the dimeric form of β-galactosidase (238 kDa). Both of them are commonly used in the dairy industry and therefore, their immobilization is crucial to improve their stability and control their activity. In this thesis, structural changes of the lipid system, the stability of the encapsulated proteins, the location of the enzyme within the L3 phase and the nature of lipid-enzyme interactions were revealed. The results suggest that both enzymes were successfully entrapped into the L3 phase. Both enzymes maintain a higher activity when encapsulated compared to the free enzyme kept under the same conditions. Furthermore, different techniques proved that the enzymes are located within the L3-NPs, with encapsulation efficiencies of 81 % and 100 % for Aspartic protease and β-galactosidase, respectively. Finally, lipid-enzyme interactions were investigated to explain the efficiency of the encapsulation process. The results suggested that there are attractive interactions between enzymes and the lipid bilayer, where hydrophobic interactions play a major role.