Interactions between retinal and phospholipids in monomolecular films at acid pH

The surface properties of mixed monomolecular films of retinal and phospholipids (p. lipids) are measured as a function of mole fraction at a nitrogen-water interface. An acid pH of 6.0 is maintained in the aqueous phase. Before irradiation the surface potential deltaV for 9-cis retinal, 11-cis retinal, phosphatidyl serine (PS) and phosphatidyl ethanolamine (PE), at pi=12 dyn/cm, are 490 mV, 645 mV, respectively. Before irradiation, A0 for 9-cis and 11-cis are 58 A2 and 48 A2, respectively. Experimentally measured isotherms are compared with theoretically calculated isotherms. In case of mixed films of retinal and PS the experimental isotherms are greater than theoretical, while mixed films of retinal and PE are smaller than theoretical. A maximum value for the difference between theoretical and experimental areas are obtained at (retinal)/(p. lipid)=0.1. Retinal and p. lipid do not appear to form a Schiff base, charge transfer or any other type of complex at pH 6. A eutectic type mixture between retinal and p. lipid may occur on the surface. A light induced change in deltaV of -130 mV is observed in the case of 11-cis and PE. The significance of these findings with respect to visual excitation is considered.