S. S. Brody

THE MECHANISM OF THE FLAVIN SENSITIZED PHOTODESTRUCTION OF INDOLEACETIC ACID

Abstract— A detailed in vitro study was made of the flavin sensitized photoinactivation of indoleacetic acid, using primarily riboflavin as sensitizer. The dependence of the quantum yield on reactant concentrations, pH, presence of oxygen, viscosity, temperature, KI concentration, and solvent was determined. The involvement of a limiting dark reaction was demonstrated, using an intermittent light technique. The results are consistent with a mechanism involving a metastable state of riboflavin as the photochemically reactive species. The calculated rate constant for intersystem crossing to this state was found to be 2.5 times 108/sec. Riboflavin, in the metastable state, is believed to oxidize indoleacetic acid to indolealdehyde, with subsequent recovery of riboflavin by autoxidation. The maximum quantum yield of the photoinactivation of IAA is 0.71, indicating a highly efficient process, approaching 100% when energy loss due to riboflavin fluorescence is taken into account. Both carotenoids and pure chlorophyll‐a were found to be inactive as sensitizers.

Role of various carotenoids in mediating electron transfer sensitized by chlorophyll and pheophytin

Numerous studies demonstrating the photoconductive properties of bilayer lipid membranes containing chlorophyll have appeared [l]. The conjugated bond system found in carotenoids may play a major role in mediating photoconductivity [2,3]. Here we investigate the role of the functional groups on the ionone ring of carotenoids in modifying the photosensitivity of chlorophyll bilayer membranes. sium ferrocyanide (reducing side) and 10 mM potassium ferricyanide (oxidizing side). Both phases contained 10 mM KC1 buffered at pH 7.8 (10 mM potassium phosphate). For illumination (632.8 mm) a 5 mW He-Ne laser (Spectra Physics, model 150) was used.

SPECTRAL CHANGES IN CHLOROPLASTS DURING AGING

The spectral properties of photosynthetic and accessory pigments of chloroplasts can change significantly over the course of an experiment which may last a couple of hours. As chloroplasts age at room temperature, there are decreases in absorbance at 708, 678, 510, 484, 465 and 432 nm. Chlorophylls absorbing at long wavelength (708 nm) and carotenoids (510 nm) are most sensitive to the aging process. The decrease in absorbance at 510 nm could indicate that aging results in a decrease in electrical potential across the thylakoid membrane.

ASYMMETRIC AND SYMMETRIC BIMOLECULAR MEMBRANES CONTAINING PHOTOSYNTHETIC PIGMENTS

Although chlorophyll a has been incorporated into black lipid films (Tien, 1968; Van and Tien, 1970; Berns, 1974), chlorophyll containing bimolecular membranes have not yet been made from “solventfree” (or low solvent containing) monolayers (Montal, 1974b). The advantages of bimolecular membranes over black lipid films have been described elsewhere (Montal, 1974b), in particular, the former have a more clearly defined chemical composition and readily allow the formation of asymmetric membranes. The ubiquitous co-occurrence of chlorophyll a, carotenoids, non-pigmented lipids and proteins in the photosynthetic apparatus of higher plants makes it especially desirable to work with such asymmetric membranes. In the present note we report both the formation and the relative [see below] electrical properties of symmetric, asymmetric, and mixed bimolecular membranes of a-lecithin, chlorophyll a, p-carotene and HP700 (a chlorophyll-protein-detergent complex).

EVIDENCE FOR THE FORMATION OF A CHLOROPHYLL a/ZEAXANTHIN COMPLEX IN LECITHIN LIPOSOMES FROM FLUORESCENCE DECAY KINETICS

The interaction of Chi a with zeaxanthin (Zea), which is an analogue of lutein, has been studied in soya bean lecithin liposomes using the fluorescence of Chi as monitor. The fluorescence emission spectrum at 4.2 K of Chi a showed characteristic changes in the presence of Zea: the emission maximum shifted from 688 nm to 680 nm, and a peak at 731 nm appeared. The fluorescence decay kinetics of Chi a alone could be described by the sum of two exponential components (T1,≅0.8 ns, T2≅2.5 ns). In the presence of Zea a component with a long lifetime, T≅5 ns, appeared with a large relative amplitude (40%). This indicated the formation of a Chl a/Zea complex, in which Chl a/Chl a interaction is negligible, presumably because of strong interaction between Chl a and Zea. The fluorescence anisotropy decay kinetics supported the hypothesis of the formation of a large Chl a containing complex in the presence of Zea. A rotational correlation time, φ≅14 ns at 4°C and φ≅21 ns at 30°C, was found, which is distinctly larger than for samples containing Chl a only. We interpret these results as further evidence for a strong interaction between Chl a and Zea in the hydrophobic environment of the lecithin liposomes. This interaction may also occur in the Chl‐proteins of the Chi alb light‐harvesting complex of plant photosynthesis.

A QUANTITATIVE COMPARISON OF CHLOROPHYLL BILAYERS FORMED WITH AND WITHOUT SOLVENT

Abstract Bimolecular lipid membranes are formed from solutions containing lecithin and chlorophyll‐a (chl‐a) in various molar ratios. Both the Mueller‐Ruding technique as well as the solvent free, Montal‐Mueller technique are used to form bilayers. In both methods, increasing the chl concentration produces greater photosensitivity. The photocurrent/area is about an order of magnitude higher in bilayers formed with the solvent free method, under similar conditions. From the quantum yield calculations, it appears that the higher photocurrent/area obtained with the Montal‐Mueller membranes cannot be explained solely due to the greater concentration of pigment molecules in the solvent free system. The possible role of chl—chl interactions are considered.

LIGHT AND pH INDUCED CHANGES IN THE SURFACE PRESSURE‐AREA ISOTHERMS OF BACTERIORHODOPSIN

Abstract— Pressure‐area isotherms of monolayers of purple membrane fragments were measured at an air‐water interface. The pH of the aqueous phase was varied from 2 to 10. Over the pH range 5 to 9 the area/molecule of bacteriorhodopsin was constant and illumination of the film had no effect on the isotherms.

Chlorophyll a and Cytochrome c at a Heptane-Water Interface

Abstract The surface pressure was measured as a function of area/molecule for chlorophyll a (Chl) and cytochrome c (Cyt) at a heptane-water interface. At a surface pressure of 6 dyn/cm the area per molecule of Chl(A6) = 113 ± 6 Å2, for reduced Cyt c (red. Cyt) the A6 = 5000 ± 300 Å2 and for oxidized Cyt c (ox. Cyt) A6= 4100 ± 250 Å2. Cyt appears to denature at the interface. Irradiation results in a decrease of the A6 for Chl to A6= 100 ± 5 Å2. There appears to be interaction between Chi and red. Cyt in a mixed film no interaction is observed between Chi and ox. Cyt

Studies on fragments of rod outer segments from bovine retinas.

Abstract Vision, Monolayers, Rhodopsin, Photobiology Monolayers of fragments of our rod segments were formed at an air-water interface. The area of these particles was measured as a function of the pH of the aqueous phase. A maximum area was measured at pH 6.5. The film was characterised by measuring the decrease in surface pressure, after compression, and the half time required to reach constant pressure. At a pH between 6 and 7 the decrease in surface pressure was a minimum. Irradiation of films of fragments resulted in a decrease in surface potential of between 30 and 40 mV. The largest decrease occured at a pH between 6 and 6.5. A small decrease in surface pressure was also observed upon irradiation. Mixed films of fragments and phosphatidyl ethanolamine were examined. Irradiation of the mixed film resulted in a 70 mV decrease in surface potential.

Reactions of Retinals in a Model Membrane System

Monomolecular films of 9-cis, 11-cis, 13-cis and all-trans retinal were formed at an air-water interface. Area/molecule and surface potential were measured before, during and after illumination. The initial quantum yield of the photoisomerization of 9-cis retinal was 0.25. Irradiation of a retinal monolayer resulted in 30 to 60 mVolt changes in surface potential. Complexation of retinals with lysine and cysteine were studied.