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Existence et nature des formes agrégées de chlorophylle a dans les solvants binaires: II. — Étude des Spectres de fluorescence
... That such knowledge is of importance is evident by the same effects that medium composition has on both the chlorosomes (3) and Bchl c and d aggregates in aqueous media (4) or on chloroplast fragments (5) and Chl a aggregates in aqueous solvents (6). While the formation of Chl aggregates in various organic solvent-water mixtures is well documented (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), little is known about the contribution of the structure and properties of the medium for the assembly and stabilization of different Chl aggregate organizations in aqueous media. A study on the relationship between the Chl aggregation and solvent structure and properties can lead to an elucidation of the mechanism for Chl assembly and dissolution in aqueous media and consequently in the photosynthetic antenna complexes. ...
... (2) these fluorescing molecules are the monomeric Chl a because the excitation is at 430 nm, i.e. not at the wavelength longer than the Soret absorption maximum, a region for excitation of aggregated Chl a (20); (3) it is well known that aggregated forms of Chl a in dilute polar solvent-water mixtures are practically nonfluorescent or nearly so at room temperature (9,12) and the possible part of their fluorescence (Յ1% with respect to the monomeric ones) is included in the error limits connected with the calculation of the relative quantum yields (relative error at the highest mole fractions of water Ϯ10-15%). Figure 1 demonstrates that the effect of water on the relative quantum yield of Chl a can be divided into three zones. ...
The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, and then various solvent-water mixture parameters for the f1/2 values were compared. On the basis of their comparison, it is concluded that the hydrogen-bonding ability and the dipole-dipole interactions (function of the dielectric constant) of the solvent-water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the non-aqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer-->aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740-760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667-670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f1/2 values--methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran--yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. J. Biophys. 25, 261-274, 1997). It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl-water interface and bulk liquid.
... Anyway, this result can be accounted as evidence that for 1.5 h greening leaves the irreversible fluorescence rise to the M2 FTC region reflects the release of free Chl a molecules from denatured PPC. Chl a molecules are probably released to the surrounding thylakoid lipid phase because in water they form practically nonfluorescing aggregates (48). The fact that Chl a-containing PPC denature at the temperature range of the fluorescence rise also was supported by the measurements of pronounced changes in their absorption spectra. ...
The origin of heat-induced chlorophyll fluorescence rise that appears at about 55–608C during linear heating of leaves, chloroplasts or thylakoids (especially with a reduced content of grana thylakoids) was studied. This fluorescence rise was earlier attributed to photosystem I (PSI) emission. Our data show that the fluorescence rise originates from chlorophyll a (Chl a) molecules released from chlorophyll-containing protein complexes denaturing at 55–608C. This conclusion results mainly from Chl a fluorescence lifetime measurements with barley leaves of different Chl a content and absorption and emission spectra measurements with barley leaves preheated to selected temperatures. These data, supported by measurements of liposomes with different Chl a/lipid ratios, suggest that the released Chl a is dissolved in lipids of thylakoid membranes and that with increasing Chl a content in the lipid phase, the released Chl a tends to form low-fluorescing aggregates. This is probably the reason for the suppressed fluorescence rise at 55–608C and the decreasing fluorescence course at 60–758C, which are observable during linear heating of plant material with a high Chl a/lipid ratio (e.g. green leaves, grana thylakoids, isolated PSII particles).
... Anyway, this result can be accounted as evidence that for 1.5 h greening leaves the irreversible fluorescence rise to the M2 FTC region reflects the release of free Chl a molecules from denatured PPC. Chl a molecules are probably released to the surrounding thylakoid lipid phase because in water they form practically nonfluorescing aggregates (48). The fact that Chl a-containing PPC denature at the temperature range of the fluorescence rise also was supported by the measurements of pronounced changes in their absorption spectra. ...
The origin of heat-induced chlorophyll fluorescence rise that appears at about 55–60°C during linear heating of leaves, chloroplasts or thylakoids (especially with a reduced content of grana thylakoids) was studied. This fluorescence rise was earlier attributed to photosystem I (PSI) emission. Our data show that the fluorescence rise originates from chlorophyll a (Chl a ) molecules released from chlorophyll-containing protein complexes denaturing at 55–60°C. This conclusion results mainly from Chl a fluorescence lifetime measurements with barley leaves of different Chl a content and absorption and emission spectra measurements with barley leaves preheated to selected temperatures. These data, supported by measurements of liposomes with different Chl a /lipid ratios, suggest that the released Chl a is dissolved in lipids of thylakoid membranes and that with increasing Chl a content in the lipid phase, the released Chl a tends to form low-fluorescing aggregates. This is probably the reason for the suppressed fluorescence rise at 55–60°C and the decreasing fluorescence course at 60–75°C, which are observable during linear heating of plant material with a high Chl a /lipid ratio ( e.g. green leaves, grana thylakoids, isolated PSII particles).
... That such knowledge is of importance is evident by the same effects that medium composition has on both the chlorosomes (3) and Bchl c and d aggregates in aqueous media (4) or on chloroplast fragments (5) and Chl a aggregates in aqueous solvents (6). While the formation of Chl aggregates in various organic solvent-water mixtures is well documented (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), little is known about the contribution of the structure and properties of the medium for the assembly and stabilization of different Chl aggregate organizations in aqueous media. A study on the relationship between the Chl aggregation and solvent structure and properties can lead to an elucidation of the mechanism for Chl assembly and dissolution in aqueous media and consequently in the photosynthetic antenna complexes. ...
The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, and then various solvent–water mixture parameters for the f1/2 values were compared. On the basis of their comparison, it is concluded that the hydrogen-bonding ability and the dipole–dipole interactions (function of the dielectric constant) of the solvent–water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the nonaqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer → aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740–760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667–670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f1/2 values—methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran—yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. J. Biophys. 25, 261–274, 1997). It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl–water interface and bulk liquid.
Visible absorption and fluorescence spectroscopy, togheter with circular dichroism were employed to investigate the equilibrating chlorophyll a species formed in solution at different water/acetonitrile content and different pigment concentrations. Changes in chlorophyll a auto-aggregation occurring in water containing acetonitrile in the range 0.1-33% (v/v) were observed and related to the bulk properties of the medium. A 713 nm absorbing species, occurring in solution for water concentrations exceeding 99% was characterized. The influence of the ionic strength of the solution was also subject of the present study, together with the time evolution of the different aggregates.
The Chlorophyll a (Chl a) is the main pigment in plant photosynthesis, whose relevance resides in its functional duality. The Chl a, indeed, acts as antenna, collecting and funneling light, and as photoreactive center, where the energy transduction takes place. The different roles played by this molecule are closely related to its molecular organization due to stereospecific interactions involving the Mg atom and the C=O group of the Chl a with water or side chains (N-H, S-H, O-H) of natural aminoacids [1].
Polarographic and wavelength-selected fluorescence excitation methods are employed in this study of Chl a aggregation in water containing trace amounts of acetone. These methods have been used for the characterization of a hitherto unreported Chl a aggregate. Two voltammetric adsorption peaks attributable to the presence of at least two hydrated Chl a aggregates are observed at Chl a concentrations lower than 1.4 × 10−6 M. These peaks merge into a single band as the Chl a concentration exceeds 1.2 × 10−6 M. The coverage area of 7.6 nm2 per aggregate thus observed suggests the formtion of a micelle-like aggregate complex, with an apparent contact coverage area about three times that of a Chl a monomer. The observation of this new Chl a aggregate by polarographic measurements is corroborated by the observation of a corresponding absorption band at 718 nm and by the wavelength-selected excitation, at 457.9 nm, of a Chl a aggregate fluorescence band at 725 nm.
The lecithin organogels are employed in order to immobilize the chlorophyll a (Chl a) between the organic and the aqueous phases. The Chl a aggregation has been investigated by employing IR and visible absorption and fluorescence spectroscopy.The 2,6-dichlorophenolindophenol photoreduction has been followed with and without carotenoids used as sensitizer. The involvement of the dihydrate dimer as photocatalyst of the reaction has been assumed.
Chlorophyll a (Chl a) aggregation in alcohol was examined as a function of the type of alcohol, Chl a concentration and water content. The sample preparation procedure was observed to play an important role in determining whether the dihydrate dimer or the monohydrate dimer was formed.The characterization of the different species formed in alcohol on addition of water was carried out using absorption, fluorescence and circular dichroism spectroscopic techniques.The results were confirmed by adsorptive stripping cathodic voltammetry and differential pulse polarography.
Abstract—Absorption and fluorescence spectra of Chl-a in ethanol, ethanol/water mixtures and aqueous sodium dodecyl sulphate are reported. Contrary to a previous report, it is found that Chl-a is not solubilized in the anionic detergent as a microcrystalline form.
Polarographic techniques are employed in the investigation of Chl a dihydrate formation and aggregation in binary solvent mixtures of water/acetone. The variation of the relative intensities of two observed polarographic peaks, at 2 −1100 and −1200 mV, with the composition of the binary solvent supports a shift in the equilibrium in favor of the formation of chlorophyll a dihydrate, Chl a · 2H2O, from the acetone solvate, Chl a · Ac, as the water concentration is increased. It is concluded that the equilibrium further shifts towards the dimerization of Chl a · 2H2O at water concentrations exceeding 40% (volume). A third polarographic peak at −1434 mV, observed in 50:50 (v/v) water/acetone solutions at Chl a concentrations greater than 1·10−5 M, is attributed to the oligomer of Chl a dihydrate, (Chl a·2H2O)n. From the amount of electricity calculated from the polarographic peaks, the coverage areas, 259 and 597 Å2, are respectively obtained for the adsorbed monomer, Chl a·Ac, and dimer, (Chl a·2H2O)2, at the electrode surface.
In the presence of a surfactant that does not ligate Mg, chlorophyll is adsorbed to polyethylene particles swollen with tetradecane principally as the infrared-absorbing, highly polymeric species Chl 740. Examples of such surfactants are quaternary ammonium salts and soaps. However, when surfactants of opposite charge are present together, in this case dodecylpyridinium iodide and Na butyrate or myristate, chlorophyll may exist entirely in a dispersed form absorbing around 666 nm. Absorption and fluorescence spectral data show that much of the dispersed pigment is still associated, but as dimeric and perhaps short oligomeric species. It is concluded from fluorescence lifetime analysis that most of the observed fluorescence comes from a small population of chlorophyll that is probably monomeric and isolated; fluorescence of more highly associated species decays with a wide range of lifetimes. The capacity for photochemical sensitization of these particles with dispersed chlorophyll is similar to that of particles with ligating surfactants examined earlier. Structures are suggested for chlorophyll association in which Mg is ligated by water hydrogen-bonded to a carboxylate group, while the dodecylpyridinium counterion lies near the chlorophyll ring. The effect of the two surfactants is synergistic. Overall, spectra of dispersed chlorophyll adsorbates resemble most closely those of colloidal chlorophyll suspensions prepared by dilution of solutions in polar organic solvents with water.
The origin of heat-induced chlorophyll fluorescence rise that appears at about 55-60 degrees C during linear heating of leaves, chloroplasts or thylakoids (especially with a reduced content of grana thylakoids) was studied. This fluorescence rise was earlier attributed to photosystem I (PSI) emission. Our data show that the fluorescence rise originates from chlorophyll a (Chl a) molecules released from chlorophyll-containing protein complexes denaturing at 55-60 degrees C. This conclusion results mainly from Chl a fluorescence lifetime measurements with barley leaves of different Chl a content and absorption and emission spectra measurements with barley leaves preheated to selected temperatures. These data, supported by measurements of liposomes with different Chl a/lipid ratios, suggest that the released Chl a is dissolved in lipids of thylakoid membranes and that with increasing Chl a content in the lipid phase, the released Chl a tends to form low-fluorescing aggregates. This is probably the reason for the suppressed fluorescence rise at 55-60 degrees C and the decreasing fluorescence course at 60-75 degrees C, which are observable during linear heating of plant material with a high Chl a/lipid ratio (e.g. green leaves, grana thylakoids, isolated PSII particles).
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