Arkadiusz Ptak

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.

Interactions between chlorophyll a and β-carotene in nematic liquid crystals

Abstract Fluorescence lifetime in the ps range, polarized absorption, polarized fluorescence spectra and delayed luminescence time resolved spectra were measured for chlorophyll a solutions with and without β-carotene addition in nematic liquid crystal. Photoacoustic spectra of the same samples at various frequencies of light modulation were also taken. The frequency dependence of the photoacoustic spectra suggests that part of the excitation is converted into heat in a slow process (with a decay time of the order of ms). The lifetime results suggest that at used concentration some aggregation of the chlorophyll a occurs. The chlorophyll a molecules interact strongly with the β-carotene forming some nonfluorescent or weakly fluorescent aggregates characterized by having various thermal deactivation yields and orientations in anisotropic matrix when compared to those of separated pigments. It seems that the aggregated forms of the chlorophylls are partially disrupted as a result of the their interaction with the β-carotene. Singlet excitation of β-carotene is not transferred to the fluorescent form of chlorophyll a . Delayed (in μs time range) luminescence of chlorophyll a is quenched by β-carotene. This luminescence is located in the same spectral region as prompt fluorescence. Interactions between chlorophyll a and β-carotene depend on the degree of pigment orientation and their aggregation.

Effect of Humidity on Nanoscale Adhesion on Self-Assembled Thiol Monolayers Studied by Dynamic Force Spectroscopy

The adhesion force between silicon nitride tips of an atomic force microscope and different self-assembled thiol monolayers (SAMs) was measured at different loading rates and humidity. SAMs were formed from HS(CH(2))(n)CH(3) with n = 6, 8, 9, 10, 15 and HS(CH(2))(n)OH with n = 6, 9, 11, 16. With a special setup, the loading rate could be increased to 10(7) nN s(-1). For the interaction with two-dimensional crystalline CH(3)-terminated SAMs (n > or = 8), two regimes can be distinguished. At loading rates below 10(4)-10(5) nN s(-1), the adhesion force increased proportional to the logarithm of the loading rate. Adhesion is most likely dominated by van der Waals attraction. At higher loading rates, the adhesion forces increased steeper with the logarithm of the loading rate. The specific process limiting separation is not yet identified. On OH-terminated SAMs, the adhesion force was approximately 6 times higher than on the CH(3)-terminated SAMs, even at low humidity. This can partially, but not fully, be explained by hydrogen bridges forming between the hydroxyl groups of the monolayer and silanol groups of the tip. For relative humidity above 10%, the capillary force further increased the adhesion force, which reached a maximum at values of relative humidity between 40% and 80%. Adhesion force versus loading rate (F(ad) versus r(F)) curves increased roughly linearly over the whole range of loading rates. The slope depended on the humidity, and it is correlated with the absolute strength of the capillary force.

Quantitative characterization of nanoadhesion by dynamic force spectroscopy.

We present a method for the characterization of adhesive bonds formed in nanocontacts. Using a modified atomic force microscope, the nanoadhesion between a silicon nitride tip and a self-assembled monolayer of 1-nonanethiol on gold(111) was measured at different loading rates. Adhesion force-versus-loading rate curves could be fitted with two logarithmic terms, indicating a two step (two energy barrier) process. The application of the Bell-Evans model and classical contact mechanics allows the extraction of quantitative information about the effective adhesion potential and characterization of the different components contributing to nanoadhesion.

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.

Photoelectrical properties of green bacteria cells and cell fragments located in electrochemical cell

Abstract Photopotential action spectra as well kinetics of their generation and decay for green bacteria Prosthecochloris aestuarii cells and cell fragments were measured. The sample was located between two transparent electrodes, both semiconducting or one semiconducting and second one metallic. The electric properties of the cells in darkness and under illumination were also established. In the photopotential action spectrum the maximum about 750 nm, responsible for the bacteriochlorophyll c oligomers absorption, is absent. It shows that bacteriochlorophyll c molecules located in chlorosome oligomers are not taking part in the photopotential generation. From the action spectra it follows that predominantly desaggregated bacteriochlorophyll c and bacteriopheophytin, and may be also some chlorophyll a -like pigments are responsible for a photopotential generation. In the 670 nm region the desaggregated bacteriochlorophyll c as well the bacteriopheophytin absorption are located, whereas the Soret band is characteristic rather for the bacteriopheophytin, but some contribution from the desaggregated bacteriochlorophyll c cannot be excluded. With the sample pheophytinization the photopotential amplitudes increase. The complex kinetic of the photopotential generation suggests the superposition of at least two processes: the fast one, occurring just after light absorption, which probably is due to charge separation in the reaction centers and the second one—slower—may be related to some diffusion of the charge generated in the antenna pigments through the cell membrane. Due to slower photopotential decay and higher concentrations of the antenna pigments than that of the reaction centers, the contributions from the desaggregated antenna pigments, at used in the experiments frequencies of light modulation, predominant in the photopotential action spectra. For each sample the sign of the photopotential signal changes with the change of the side of illumination of the electrochemical cell.

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.

FLUORESCENCE LIFETIMES OF ORIENTED GREEN BACTERIA CELLS, CELL FRAGMENTS AND ORIENTED BACTERIOCHLOROPHYLL C MOLECULES

Abstract The decays of the polarized components of fluorescence (excited at 410 nm by a 70 fs laser flash) of whole bacteria Prosthecochloris aestuarii cells, their fragments and isolated bacteriochlorophyll c molecules located in isotropic and stretched polymer films have been measured and analysed using exponential components which in the literature data are assigned to monomers, small aggregates and oligomers of bacteriochlorophyll c. The amplitudes of the lifetime components depend only slightly on the light polarization but they change strongly with a change in the spectral region of fluorescence observation and the type of sample. Two spectral regions of fluorescence emission have been observed: at 680 nm (bacteriochlorophyll c and bacteriopheophytin monomers) and at 750 nm (bacteriochlorophyll c oligomers). The longest decay component (from 5.6 to 7.4 ns for various samples and light polarizations) is similar to that observed for monomer bacteriochlorophyll c in various solutions. The middle component (1.1–2.7 ns) is probably due to small bacteriochlorophyll c aggregates, whereas the shortest one (ranging from 0.001 to 0.37 ns depending on the type of sample, polarization of light, etc.) is related to bacteriochlorophyll c oligomers. Some components exhibit negative amplitudes, showing that the emission is generated by excitation transfer from other pigment forms. The amplitude of oligomer component emission is low in bacteria and bacteria fragments in stretched film, whereas it is high for artificial oligomers. This is because in organisms the excitation is efficiently transferred to bacteriochlorophyll a complexes emitting in a longer-wavelength range, whereas in the model system oligomers of bacteriochlorophyll c are the final acceptors of excitation. In bacteria the stretching of a polymer matrix not only orients the whole object but also, to some extent, perturbs the natural arrangement of pigment and the disaggregated part of oligomers to monomers and smaller aggregates. Therefore in stretched polymer we observe the middle component of decay. The intensities of various polarized components of emission measured for the same sample are different. The coefficients of the anisotropy of emission a short time (about 100 fs) after excitation and averaged over the whole decay time for anisotropic and isotropic samples have been calculated. In some cases a strong depolarization of fluorescence during the decay time is observed; in others the anisotropy only changes slowly in this time. The results are compared with literature data for lifetimes and anisotropy of fluorescence of bacteriochlorophyll c in organisms and in artificial aggregates.

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.

Fluorescence polarization studies of B-phycoerythrin oriented in polymer film.

Polarized steady‐state fluorescence and fluorescence excitation spectra as well as time‐resolved fluorescence for B‐phycoerythrin (B‐PE) from red algae, Porphyridium cruentum, embedded in polyvinyl stretched films were measured. The lifetimes of polarized fluorescence were analyzed using exponential components and fractal models. The interactions between various chromophores of the pigment‐protein complexes investigated were discussed. The anisotropy of fluorescence excitation spectra differs from the anisotropy of absorption spectra and depends on the wavelength of observation. This shows that differently oriented chromophores take part in various paths of excitation energy transfer (ET) or change their excitation into heat with various efficiencies (or both). Also, analysis of time‐resolved fluorescence measured in various spectral regions gives different polarized components of emission. Fractal analysis of lifetimes, done under supposition of the Foerster resonance ET mechanism, suggests different arrangements of energy donors and acceptors for molecules absorbing in different spectral regions. It shows that several fractions of differently oriented “forms” of chromophores exhibiting different spectral properties occur in B‐PE complexes. Small changes in the orientation of the chromophores can be followed by modification of the path of excitation energy migration. Based on the results obtained a new reorientational mechanism of the State 1 → State 2 transition was proposed: Even small conformational modifications of biliproteins, which could be caused in vivo by the change in the conditions of preillumination of bacteria, are able to modify the path of excitation ET. Such a reorientation may be responsible for the change in the partition of biliprotein excitation energy between photosystem II (PSII) and PSI (State 1 → State 2 transition). The proposed mechanism needs further verification by the investigation of whole bacteria cells.