Combined surface pressure-interfacial shear rheology studies of the interaction of proteins with spread phospholipid monolayers at the air–water interface

The adsorption of two model proteins, catalase and lysozyme, to phospholipid monolayers spread at the air–water interface has been studied using a combined surface pressure-interfacial shear rheology technique. Monolayers of 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl- sn-glycero-3-[phospho- rac-(1-glycerol)] (DPPG) and DPPC:DPPG (7:3) were spread on a phosphate buffer air–water interface at pH 7.4. Protein solutions were introduced to the subphase and the resultant changes in surface pressure and interfacial storage and loss moduli were recorded with time. The results show that catalase readily adsorbs to all the phospholipid monolayers investigated, inducing a transition from liquid-like to gel-like rheological behaviour in the process. The changes in surface rheology as a result of the adsorption of catalase increase in the order DPPC < DPPC:DPPG < DPPG. Lysozyme behaves in a similar manner beneath a DPPG monolayer, but shows no measurable differences when injected beneath DPPC or the DPPC:DPPG (7:3) mixed monolayer. It is proposed that DPPG monolayers are more susceptible to penetration by adsorbing protein molecules. The interaction between DPPG and lysozyme is further enhanced due to electrostatic interactions between the negatively charged DPPG and the positively charged lysozyme.