Paramagnetic fluorescence quenching in chlorophyll A containing vesicles: evidence for the localization of chlorophyll

Aggregation/disaggregation of chlorophyll a in model phospholipid-detergent vesicles and micelles

The photosynthetic pigments of higher plants exist in complex oligomeric states, which are difficult to study in vivo. To investigate aggregation processes of chlorophyll a (Chl a), we used an in vitro reconstitution procedure, with this pigment incorporated into liposomes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), micelles and pre-micelle media of the detergent n-dodecyltrimethylammonium chloride (DTAC), and mixed, spontaneous, DMPC-DTAC vesicles and micelles. Chl a oligomers were characterized by UV-visible absorption, steady-state and time-resolved fluorescence, and fluorescence lifetime imaging microscopy. Equivalent diameters of the colloidal structures were obtained by fluorescence correlation spectroscopy. In DMPC liposomes and DMPC-DTAC vesicles and micelles, three fluorescence lifetimes indicated the coexistence of Chl a monomers (≈5 ns) and oligomers (≈1-2 to ≈0.1 ns). The increase in DTAC amount, in the mixed system, induces a progressive solubilization of DMPC liposomes (from vesicles to micelles) and simultaneous disruption of Chl a aggregates; in pure DTAC micelles, mostly monomers were found. The present work aims for a better understanding of chlorophyll-chlorophyll (Chl-Chl), Chl-lipid, and Chl-detergent interactions in spontaneous colloidal micro- and nanostructures.

Triggerable plasmalogen liposomes: improvement of system efficiency

A photoactivated liposome release system that is generally applicable for triggered release of encapsulated hydrophilic materials is described. This approach to phototriggered release, derived from the known effects of plasmalogen photooxidation on membrane permeability in whole cells and model membrane systems, relies on producing a lamellar phase change or increase in permeability upon cleaving its constitutive lipids to single-chain surfactants using 630-820 nm light to sensitize the photooxidation of the plasmalogen vinyl ether linkage. Semi-synthetic plasmenylcholine liposomes containing encapsulated calcein and a membrane-bound sensitizer, such as zinc phthalocyanine, tin octabutoxyphthalocyanine, or bacteriochlorophyll a, were prepared by extrusion. Irradiation of air-saturated liposome solutions enhanced membrane permeability toward calcein and Mn2+, and promoted membrane fusion processes compared to non-irradiated or anaerobic controls. Bacteriochlorophyll a sensitization produced the fastest observed photoinitiated release rate from these liposomes (100% calcein release in less than 20 min; 800 nm irradiation at 300 mW); the observed release rate was two orders of magnitude slower for egg lecithin liposomes prepared and irradiated under identical experimental conditions. Liposome aggregation, interlipidic particle formation, and membrane fusion between adjoining liposomes was observed by 31P-NMR, freeze-fracture/freeze-etch TEM, and cryo-TEM as a function of irradiation time. The use of near-infrared sensitizers and the capacity of photolyzed plasmenylcholine liposomes to undergo membrane fusion processes make photodynamic therapy with these liposome-borne sensitizers an attractive adjunct to biochemical targeting methods.

Mixing of single-chain amphiphiles in two-chain lipid bilayers. 2. Characteristics of chlorophyll a and alpha-tocopherol incorporation in unilamellar phosphatidylcholine vesicles

As a step toward the elucidation of the biological significance of the isoprenic chains found ubiquitously in single-chain lipids involved in electron and energy transfer of chloroplasts and mitochondria, we undertook a comparative study of the incorporation of chlorophyll a (Chl a) and alpha-tocopherol (alpha T) in unilamellar phosphatidylcholine (PC) vesicles. We observed that while Chl alpha is added to the PC bilayers in a simple, almost linear way, the inclusion of alpha T is a sigmoid-like function characterized first by a slow variation of alpha T incorporation followed by a rather steep increase till saturation occurs at initial mol% alpha T of approx. 5%. Owing to the likeness of the isoprenic chains of Chl a and alpha T the simplest interpretation of the data is that the different mixing behaviours of the two lipids in PC vesicles are due to the special arrangement of the tetrapyrrole macrocycle and the chromanol ring in the lipid bilayers. However, no satisfactory explanation of the mechanisms involved can be given as yet. One possibility is that the inclusion of Chl a and alpha T in the membranes results from adsorption differences brought about by the extent of penetration of the molecule into the monolayers of the unilamellar vesicles.

Influence of lecithin liposomes on chlorophyllase-catalyzed chlorophyll hydrolysis: comparison of intramembraneous and solubilized Phaeodactylum chlorophyllase

1.Chlorophyllase-catalyzed chlorophyll hydrolysis is greatly enhanced by the addition of divalent cations (Mg2+) combined with a reducing agent (dithiothreitol, ascorbate). A similar effect is obtained by the addition of lecithin. In the presence of lecithin, dithiothreitol has only slight or no influence on chlorophyll hydrolysis. Mg2+ eliminates the activating effect of lecithin. 2. In the absence of Mg2+ + dithiothreitol or of lecithin, Triton X-100 has a slight activating effect on chlorophyllase-catalyzed chlorophyll hydrolysis, but only at low concentrations (0.01--0.02%). In the presence of Mg2+ and dithiothreitol or of lecithin, Triton X-100 (greater than or equal to 0.02%) inhibits this reaction. 3. Whereas chlorophyllase combines with chlorophyll, no combination of chlorophyllase and lecithin could be detected. 4. Solubilized chlorophyllase is stabilized by its substrate, chlorophyll. Enzyme stabilization is eliminated by lecithin, whereas in the absence of chlorophyll, denaturation is somewhat increased by dithiothreitol. 5. No clear difference was found between the actions of intramembraneous and solubilized chlorophyllase. The results suggest that chlorophyllase is situated within membranes in such a way that the active group protrudes into the aqueous medium surrounding the membrane. 6. A hypothesis explaining the activating effects which Mg2+ combined with a reducing agent and lecithin have upon chlorophyllase-catalyzed chlorophyll hydrolysis is presented.

Monitoring the location profile of fluorophores in phosphatidylcholine bilayers by the use or paramagnetic quenching

Spin probes differing in the position of their paramagnetic centre are used to quench the fluorescence of pyrene derivatives and chlorophylls incorporated into dimyristoyl phosphatidylcholine membranes. Pyrene butyric acid and pyrene decanoic acid with known orientation relative to the membrane surface are investigated. The quenching efficiency of fatty acid spin probes is dependent on the position of the nitroxide radical group in the fatty acid chain. Using this short range interaction we developed a spectroscopic method to chlorophyll-containing vesicles, we were able to characterize the orientation of the porphyrin ring within the membrane. Moreover, the chlorophyll fluorescence is also quenched by a water-soluble spin label. Therefore the porphyrin ring appears to be orientated in the polar head group region of the lipid layer, but not to be protruding out into the water phase. This conclusion is confirmed by the use of pyrene derivatives. Fluorescence quenching by a water-soluble spin label within the lipid matrix is observed even in the rigid state of the membrane. Fluorescence lifetime measurements suggest the existence of two different quenching mechanisms: (1) a static quenching occurring below the lipid phase transition temperature, and (2) an additional dynamic quenching taking place in the fluid state of the lipid bilayer.