Danuta Fra̧ckowiak

Photocurrent kinetics (in the microsecond time range) of chlorophyll a, chlorophyll b and stilbazolium merocyanine solutions in a nematic liquid crystal located in an electrochemical cell

Abstract The photosynthetic pigments chlorophyll a and chlorophyll b solutions in a nematic liquid crystal were located between semiconducting and metallic transparent electrodes and illuminated by lamp or laser light. Cells with two semiconducting electrodes were also investigated. Such systems represent models of biological membranes simulating their fluid and oriented structure. The kinetics of photocurrent generation in the time range from microseconds to minutes were measured. The action spectrum of the photocurrent effect was established. The kinetics of the photocurrent generated by laser flash photolysis in the chlorophyll solutions are complex and to some extent similar to the kinetics reported for thylakoids. For comparison, the photocurrent kinetics of stilbazolium merocyanine, a highly protonated dye, were also measured. In this case, on flash illumination, only one photocurrent peak was observed. This peak is due to electron tunnelling to the semiconducting electrode. In the case of the chlorophylls, several effects, such as charge displacement and migration of the ions, are superimposed giving a complex photocurrent generation. The results suggest that, in the case of chloroplasts, the kinetics of photopotential generation may be partially dependent on the membrane structure and fluidity.

Electrochemical cell with bacteriochlorophyll c and chlorophylls a and b in nematic liquid crystal

Abstract The photocurrent generation and current-voltage (I(V)) characteristics of the electrochemical cells consisting of a layer of chlorophyll a, chlorophyll b or bacteriochlorophyll c solution in a nematic liquid crystal sandwiched between two semiconducting (In2O3) electrodes covered by an orienting layer SiOx were investigated. Bacteriochlorophyll c was introduced into the nematic liquid crystal in monomeric and in aggregated predominantly tetrameric forms. The photoelectrical properties of bacteriochlorophyll c in monomeric and aggregated forms are different. All investigated samples exhibit cathodic photocurrent related to the extraction of electrons from the conduction band of the illuminated semiconducting electrode. Amplitudes of generated photocurrents and the shapes of the current-voltage voltammogramic loops, at the same conditions of illumination, are for various chlorophylls different.

Fluorescence of aggregated forms of Chl a in various media

Abstract The fluorescence lifetimes in the picosecond time range of various aggregated forms of chlorophyll a (Chl a ) in different media (wet n -haxane, adsorbed layers and polymer films) were measured. Photographs of Chl a and Chl b poly(vinyl alcohol) (PVA) samples containing pigment oligomers are compared and discussed on the basis of previous and present spectral data concerning chlorophyll aggregation. In the investigated samples, the aggregated forms of Chl a can be divided into three groups: “dry monomer” (isolated from interaction with water), hydrated dimers and oligomers built from such dimers. These three forms have different emission lifetimes and, from photoacoustic spectra, yields of thermal deactivation. The excitation energy transfer from dry monomers to aggregated forms is rather ineffective.

Spectral properties of bacteriochlorophyll c in nematic liquid crystal. Part 1. Monomeric forms of dye

Abstract The spectroscopic features of bacteriochlorophyll c and bacteriopheophytin c in a nematic liquid crystal matrix have been investigated. Absorption, circular dichroism, fluorescence and time resolved delayed luminescence spectra have been measured. The pigment is introduced to the liquid crystal from a dry and from a hydrated chloroform solution. In both cases the pigment is in the monomeric form. Hydration of the solvent and the presence or absence of the central Mg atom affect the interaction of the pigment molecules with the liquid crystal matrix, changing the fluorescence anisotropy. A model for the bacteriochlorophyll c orientation in the liquid crystal is proposed and the averaged angles between the transition moments and the liquid crystal orientation axis are determined. A slow process (in the microsecond range) of radiative deactivation of energy absorbed by the pigments is observed. This delayed emission could be due to pigment ionization and delayed charge recombination and/or thermal activation from the triplet to the excited singlet state.

Photoelectric properties of chlorophyll and carotene solutions in nematic liquid crystal located between semiconducting electrodes.

The photopotential and photocurrent generation for chlorophyll a, beta-carotene and a mixture of these pigments dissolved in nematic liquid crystal and located between transparent semiconducting electrodes were measured. Both pigments exhibit photopotential and photocurrent generation. From the photocurrent amplitudes it follows that the efficiency of electron transfer to a semiconducting electrode from beta-carotene is higher than from chlorophyll alpha. The photocurrent amplitude of the pigment mixture is slightly lower than that calculated as a sum of amplitudes of pigments located in separated cells. This difference can be explained by secondary effects, such as competition between carotene and chlorophyll molecules in a process of adsorption on a semiconducting electrode. Therefore it seems that no charge transfer complexes of chlorophyll and carotene are formed in the investigated model system.

Spectral properties of stilbazolium merocyanines – potential sensitizers in photodynamic therapy and diagnosis: Part I. Merocyanines in model systems

Abstract Spectral and photochemical properties of seven stilbazolium merocyanines of different chain length and side groups attached, dissolved in ethanol or methanol solutions as well as in polyvinyl alcohol water solutions and films, were investigated. The absorption, fluorescence emission and fluorescence excitation spectra as well as steady state photoacoustic and laser induced optoacoustic spectroscopy signals were measured. Triplet states are, because of their slow decay time, usually very effective in photochemical reaction; therefore dyes with efficient triplet states generation are suitable for destruction of illuminated diseased cells. The laser induced optoacoustic measurements were used to evaluate the efficiency of generation and decay times of dye triplet states. The quenching of the dye triplet state by oxygen was also studied. The results show that properties of merocyanines and their interactions with macromolecules depend strongly on the type of side groups attached. The dyes with NO 2 groups attached exhibit low yield of fluorescence therefore they could be useful rather in photodynamic therapy (PDT) than for diagnostic purposes. Some of the merocyanines investigated exhibit high yields of triplet state generation and, therefore, should be effective in photodynamic reactions. Further investigations will be carried out with the same dyes on healthy and diseased cells in order to show whether it is possible, on the grounds of spectral and photochemical properties of dye molecules in simple model systems, to predict the efficiencies of the dye incorporation into cells and the yield of photodynamic reactions.

Aggregation of chlorophyll b in model systems

Abstract Chlorophyll b (Chl b ) in model systems occures in several forms. The relationship between these forms and the forms occurring in organisms is not yet clear. Chl b aggregation has been investigated less than that of Chl a . In this paper, several spectral features, e.g. fluorescence lifetimes in the picosecond time range, time-resolved delayed luminescence spectra, photoacoustic spectra and photopotential generation of Chl b embedded in model systems (poly(vinylalcohol) film or nematic liquid crystal), were measured. The decay of the fluorescence of Chl b in polymer films can be analyzed, to a good approximation, on the basis of the following three exponential components: 3300–4300 ps, 400–900 ps and 40–71 ps. These decays are tentatively related to the emission of “dry” monomers and dry and wet dimers and oligomers respectively. The delayed luminescence spectra of the same samples in the microsecond range (at 8–295 K) are located in a similar spectral range to the fluorescence spectra. The intensity ratio between the delayed luminescence and prompt fluorescence is higher in the long-wavelength region, in which the oligomer emission is observed, than in the short-wavelength region, in which the emission of monomers and small aggregates predominates. The yield of thermal deactivation of the aggregated forms is higher than that of the monomers. The differences between the lifetimes of the various forms can be explained by the competition between the emission of prompt fluorescence, thermal deactivation and energy trapping (which is, in part, later deactivated as delayed luminescence). The excitation energy transfer from “dry” monomers to aggregated forms is not very effective. The most effective process of excitation trapping followed by delayed luminescence emission occurs in oligomers of Chl b . On the basis of photopotential generation, the delayed luminescence is due, at least in part, to pigment ionization followed by slow charge recombination. The kinetics of photopotential generation and decay depend on the aggregation of the pigment.

The influence of the presence of lipid on the aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c located in adsorbed layers and monolayers

The photoacoustic spectra and time-resolved delayed luminescence spectra in the microsecond time range were measured for layers of 8,12-diethyl farnesyl bacteriochlorophyll c adsorbed on quartz supports by solvent evaporation and as Langmuir-Blodgett monolayers. Both types of model system were also investigated with the addition of lipid. The data showed a very strong influence of lipid addition on pigment aggregation. In samples with synthetic and natural lipid addition, the pigments were found to be predominantly in the monomeric and dimeric states, whereas in the same type of sample without lipid, the pigments were aggregated to a higher degree. The influence of the presence of lipid on the aggregation of bacteriochlorophyll c in monolayers and adsorbed layers may also suggest that the contact of various pigment molecules with the lipids surrounding the chlorosome may influence the formation of various pigment aggregates in vivo. The synthetic lipid L-alpha-phosphatidylcholine dipalmitoyl and the natural lipid L-alpha-phosphatidylcholine type IVS from soy beans were used. In the latter case, only adsorbed layers were investigated. Our interpretation is preliminary as only one 8,12-diethyl farnesyl bacteriochlorophyll c homologue was present in our systems.

Spectral properties of the photosynthetic reaction units reconstituted from bacterial reaction centres and antenna pigments located in liposomes suspended in buffer or ordered in Langmuir-Blodgett films

Abstract Artificial photosynthetic reaction units reconstituted from photosynthetic bacterial reaction centres and antenna pigments (bacteriochlorophyll a or chlorophyll a ) and built in liposome vesicles were investigated. Samples were suspended in buffer or ordered in Langmuir-Blodgett films. The absorption, photoacoustic and delayed luminescence spectra of the liposomes in suspension and in Langmuir-Blodgett films deposited on quartz plates were measured. As a reference, suspensions or films containing reaction centres or antenna pigments only, both embedded in liposomes, were investigated. The absorption spectra of the samples containing reaction centres and pigment mixtures were the sum of the absorptions of samples containing reaction centres and pigments separately. This result shows that, in the mixed sample, strong interactions due to the formation of ground state aggregates are absent. The photoacoustic and absorption spectra are different because of the different yield of thermal deactivation of various chromphores. The yield of thermal deactivation of the pigments in reaction centres is higher than that of additional artificial antenna molecules. This yield is at least partially due to the rather efficient excitation energy transfer from the antenna to reaction centres competing with the thermal deactivation of excitation. All samples exhibit a delayed luminescence with the spectra located in the spectral regions of the prompt fluorescence of the investigated pigments. A comparison of the intensity of delayed luminescence of the artificial photoreaction unit with the intensities of reference samples demonstrates that excitation energy transfer from the antenna to the chromophores of the reaction centres occurs. In liposome suspensions and in monolayers, the absorption spectra of the samples containing reaction centres and pigment mixtures are similar, but the photoacoustic spectra and delayed luminescence spectra are different. This shows that the mutual interactions between the reaction centre chromophores and antenna pigments in liposomes located in suspensions and in monolayers are different.