Photoreactions and Complex Formation of Retinal Monolayers at a Water-Air Interface

Abstract

Monomolecular films of cis or trans isomers of retinal were spread at a water-air interface. Isotherms were measured. The area per molecule (extrapolated to zero pressure) for 9-cis retinal, all-trans retinal, and 13-cis retinal on 10-2 ᴍ phosphate buffer at pH 6 is 56 Å2, 54 Å2 and 45 Å2, respectively. Complexation was observed between 9-cis retinal (9-cis) and β-mercaptoethylamine (MEA) dissolved in the subphase; however, ʟ-lysine does not appear to form a complex wth 9-cis retinal. The area per molecule of the complex as a function of subphase concentration (MEA) was determined. The area per molecule for 9-cis is decreased to 49 Å2 after irradiation with blue light. Irradiation does not appear to break the complex between 9-cis and MEA. The lower limit for the quantum yield for photoisomerization of 9-cis, on phosphate buffer, is 0.06. When 9-cis is complexed with MEA (5 × 10-6 ᴍ) the quantum yield for photoisomerization is lowered, e. g. at 5 × 10-6 ᴍ MEA the yield is 0.03.