INTERACTION OF A POLY(DIMETHYLSILOXANE) WITH TRIGLYCERIDES IN MONOMOLECULAR FILMS AND APPLICATION TO LIPASE KINETICS

It has been known for some time that poly(dimethylsiloxanes) (PDMS) ca n form monolayers al the air-water interface. In this paper we show th at formation of mixed monolayers of PDMS with fatty acids, di- and tri glycerides is also possible. At the air-water interface silicone can i nteract and increase the retention of the fatty acids, which otherwise easily desorb into the aqueous subphase. Unlike fatty acids, long acy l chain tri- and diglycerides did not desorb into the aqueous subphase and their interaction with silicone was manifested by a condensing ef fect and a change in the collapse pressure of the mixed monolayers. Ge otrichum candidum lipase (GCL) showed a similar level of binding to si licone monolayer as to an air-water interface. Al low surface pressure (7.5 mN/m) the lipase hydrolyzed dicaprin in mixed 1,2-sn-dicaprin-PD MS monolayers at a rate 1.5-fold lower than the desorption rate of the capric acid from mixed monolayers of capric acid and PDMS. Compressio n of the dicaprin-PDMS monolayer to 20 mN/m led to a 15-fold increase in lipase activity (70% of the activity observed for monolayer of pure dicaprin compressed to the same surface pressure), indicating an incr ease in dicaprin's surface concentration and its accessibility to the enzyme. The lipase activity profile determined titrimetrically against varying amounts of glycerides in mixed glyceride-PDMS emulsions (at c onstant 5% oil-phase concentration) resembles the lipase activity-surf ace pressure profile obtained with the monolayer method when using pur e glycerides. This suggests that the location of glycerides in condens ed mixed monolayers with PDMS should be similar to that in mixed glyce ride-PDMS emulsions. By changing the molar fraction of tristearin and tripalmitin in the mixture with PDMS, it was possible to regulate the quality of the interface in emulsions and to optimize the enzymatic hy drolysis of these solid triglycerides.