Surface Reactions of Chlorophyll a Monolayers at a Water-Air Interface. Photochemistry and Complex Formation

Monolayers of chlorophyll a were spread at a water-air interface in a Wilhelmy plate film balance, and surface isotherms were measured under various conditions. On aqueous phosphate buffer at pH 7.8 the area per molecule (extrapolated to zero pressure) was 122 Ų. With tetraethylammonium chloride or methyl viologen dissolved in the subphase, the area per molecule increased with the substrate concentration in accordance with a Langmuir-type adsorption isotherm. This increase in area was attributed to complex formation. Equilibrium constants for complex formation between chlorophyll and substrates were calculated to be 7.2 X 102 and 2.6 X 10³ M^-1 for methyl viologen and tetraethylammonium chloride, respectively. Mixed monolayers of chlorophyll a and cytochrome c or egg albumin were also prepared. These exhibited nonideal behavior, in that the area of the mixed monolayers is greater than the sum of the areas of the individual components. Either a complex is formed between chlorophyll and protein, or the protein undergoes a conformational change or a reorientation on the surface in the presence of the pigment, so as to enlarge its surface area. Upon illumination in air, chlorophyll monolayers undergo photooxidation, which results in an increase in the area per molecule to 203 Ų when all the pigment is oxidized. The quantum yield for this reaction when the monolayer is compressed to 17 dyn/cm was calculated to be 0.06. This is 12 times greater than the yield for photooxidation of a dilute solution of chlorophyll a in benzene. For expanded monolayers (initial pressure of 2 dyn/cm), the reaction was at least seven times less efficient than for the compressed systems, which suggests that energy transfer is important in the photochemistry of these monolayers. For mixed monolayers of chlorophyll a and β-carotene in air, the photooxidation of chlorophyll was inhibited, either because chlorophyll sensitized the photooxidation of carotene or because carotene quenches the triplet state of chlorophyll.