Visualization of excitation energy transfer processes in plants and algae

Development of the time-resolved fluorescence spectroscopy in the pico-second time range and its application to the energy transfer processes in many photosynthetic organisms is reviewed here. This method enabled visualization of energy transfer processes by three-dimensional expression of fluorescence spectra and discrimination of kinetic components and spectral components. The second generation of the ultrafast fluorescence spectroscopy is the femto-second (fs) fluorescence up-conversion, and this has enabled analyses of the transfer processes from carotenoids to chlorophylls with a resolution of less than 100 fs. For future progress, a further development of the spectroscopy is indispensable as well as structural data at atomic resolution. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanisms of excitation trapping in reaction centers of purple bacteria

The contradiction between two groups of experimental data, which fails to be resolved within the framework of the widely accepted model of excitation migration and trapping (at least in case of purple bacteria), is discussed in the introduction to this review. Three directions of studies intended to resolve this conflict are reviewed in the three further sections: II. Exciton models; III. Water-polarization (water-latch) mechanism of excitation trapping; IV. Quantum-mechanical models. The maximum efficiency of these models in resolving the contradiction mentioned above was assessed. The advantages and disadvantages of the mechanisms described in sections II, III, and IV are discussed in the last section of this review. It is concluded that none of these mechanisms taken alone is able to solve this problem. Therefore, the fundamental problem of the primary excitation conversion in reaction centers remains unsolved and requires additional experimental research.     

Transfer of excitation energy in photosynthesis: some thoughts

The aim of the present paper is to aid biologists understand the complex physical problems of intramolecular energy transfer, in particular, between antenna (bacterio) chlorophyll molecules in vivo.The author has attempted, in the first part of the paper, to explain complicated processes of excitation transfer in a language understandable to readers with knowledge in fundamentals of general physics, but not in molecular optics.The second part of this paper is a critical review relevant to the specifics of physical theories and their applicability to the problem of energy transfer in antenna (bacterio) chlorophylls ((B) Chls) to reaction centers (RCs) in the photosynthetic organisms.Abbreviations PSU photosynthetic unit - RC reaction center - Chl chlorophyll - BChl bacteriochlorophyll - _r intrinsic radiactive lifetime - _fl fluorescence lifetime - _fl fluorescence quantum yield - S^* singlet excited state of a molecule     

The photosystem I trimer of cyanobacteria: molecular organization, excitation dynamics and physiological significance

The photosystem I complex organized in cyanobacterial membranes preferentially in trimeric form participates in electron transport and is also involved in dissipation of excess energy thus protecting the complex against photodamage. A small number of longwave chlorophylls in the core antenna of photosystem I are not located in the close vicinity of P700, but at the periphery, and increase the absorption cross-section substantially. The picosecond fluorescence kinetics of trimers resolved the fastest energy transfer components reflecting the equilibration processes in the core antenna at different redox states of P700. Excitation kinetics in the photosystem I bulk antenna is nearly trap-limited, whereas excitation trapping from longwave chlorophyll pools is diffusion-limited and occurs via the bulk antenna. Charge separation in the photosystem I reaction center is the fastest of all known reaction centers.     

Energy migration in purple bacteria. The criterion for discrimination between migration- and trapping-limited photosynthetic units

A criterion has been evolved for distinguishing between migration- and trapping-limited photosynthetic units (PSUs). Its application to purple bacteria has proved their PSUs to be of trapping-limited type. It means that any improvements of the molecular structure of their PSUs cannot noticeably increase the overall rate constant of excitation delivery from antenna BChls to reaction centers (RCs).Abbreviations PSUs photosynthetic units - RCs reaction centers - Chl chlorophyll - BChl bacteriochlorophyll - R intermolecular distance, _e - quantum yields of the primary excitation trapping and wasteful losses respectively - _fl excitation and fluorescence lifetimes respectively     

Spectral resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 through direct excitation

We have measured fluorescence spectra from Photosystem I (PS I) on a PS II-less mutant of the cyanobacterium Synechocystis sp. PCC 6803 at room temperature as a function of excitation wavelength. Our data show a gradual enhancement of long-wavelength fluorescence at 710 nm as the excitation wavelength is increased from 695 to 720 nm. This verifies the presence of low-energy chlorophylls (LE Chls), antenna Chls with energy levels below that of the primary electron donor, P700. The change in fluorescence with excitation wavelength is attributed to the finite time it takes for equilibration of excitations between the bulk and LE Chls. The spectra were deconvoluted into the sum of two basis spectra, one an estimate for fluorescence from the majority or bulk Chls and the other, the LE Chls. The bulk Chl spectrum has a major peak at 688 nm and a lower amplitude vibrational band around 745 nm and is assumed independent of excitation wavelength. The LE Chl spectrum has a major peak at 710 nm, with shoulders at 725 and 760 nm. The relative amplitude of emission at the vibrational side bands increases slightly as the excitation wavelength increases. The ratio of the fluorescence yields from LE Chls to that from bulk Chls ranges from 0.3 to 1.3 for excitation wavelengths of 695 to 720 nm, respectively. These values are consistent with a model where the LE Chls are structurally close to P700 allowing for direct transfer of excitations from both the bulk and LE Chls to P700.     

Characterization of excitation energy trapping in photosynthetic purple bacteria at 77 K

We have studied the energy-transfer dynamics in chromatophores of Rhodobacter sphaeroides and Rhodospirillum rubrum at 77 K, with functional charge separation. Using low-intensity picosecond absorption recovery, we determined that transfer between the energetically low-lying antenna component BChl896 and the special pair of the reaction center occurs with a time constant of 37 ps in Rb. sphaeroides and 75 ps in R. rubrum. Assuming that a Förster energy-transfer mechanism applies to the process, this allows us to estimate the distance between BChl896 in the B875 complex and the special pair P870 in the reaction center to range between 26 and 39 Å in Rb. sphaeroides. Such a distance indicates that the BChl896 pigment and the special pair of the reaction center are at the minimum separation allowed by the size and shape of the reaction center and the light-harvesting polypeptides.     

Growth control and ppGpp synthesis in Brevibacterium flavum cells at various medium mixing rates and aeration intensities

Under the conditions of various aeration and medium mixing intensities (even at a constant medium oxygen partial pressure value), the variations in the RNA synthesis as well as the growth rate of lysine producing Brevibacterium flavum strains are inversely correlated with the ppGpp concentration in the cells. An increase in the ppGpp synthesis and a decrease in the RNA content in the cells was observed in the cases of a low cell energy charge value (lower than 0.6). This took place in the cases of bacterial cultivation at a low or very high medium aeration and mixing intensity. Hence, the energy production in the cells, the ppGpp synthesis and the growth control mechanism in Corynebacteria may be regarded as connected processes.     

Primary photophysical and photochemical processes in visual excitation

The color of visual pigments is experimentally shown to be controlled by excited state effects. These effects which define the primary absorption of light by rhodopsin are considered together with results obtained from emission and picosecond spectroscopy. In addition, the molecular changes induced in rhodopsin when a photon is absorbed are analyzed using resonance Raman spectroscopy. The molecular changes observed are compared in bacterial and photoreceptor rhodopsins. This comparison yields a unique explanation for the biological role of the cis-trans isomerization in visual transduction.Presented at the EMBO-Workshop on Transduction Mechanism of Photoreceptors, Jülich, Germany, October 4–8, 1976     

Fluorescence from low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 at physiological temperatures

Fluorescence spectra from Photosystem I (PS I) are measured from 25 to –5 °C on a PS II-less mutant of the cyanobacterium Synechocystis sp. PCC 6803. Emission from antenna chlorophylls (Chls) with energy levels below that of the reaction center, or low-energy Chls (LE Chls), is resolved verifying their presence at physiological temperatures. The 25°C spectrum is characterized by peaks at 688 and 715 nm. As temperature decreases, fluorescence at 688 nm decreases while at 715 nm it increases. The total fluorescence yield does not change. The temperature dependent spectra are fit to a sum of two basis spectra. At 25°C, the first basis spectrum has a major peak at 686 nm and a minor peak at 740 nm. This is attributed to fluorescence from the majority or bulk antenna Chls. The second basis spectrum has a major peak at 712 nm, with shoulders at 722 and 770 nm. It characterizes fluorescence from a small number of LE Chls. A progressive shift to the red in the fluorescence spectra occurs as the temperature is decreased. The temperature dependence in the relative amount of fluorescence from the bulk and LE Chls is fit using a two-component energy transfer model at thermal equilibrium.     

Reaction center and antenna processes in photosynthesis at low temperature

Around 1960 experiments of Arnold and Clayton, Chance and Nishimura and Calvin and coworkers demonstrated that the primary photosynthetic electron transfer processes are not abolished by cooling to cryogenic temperatures. After a brief historical introduction, this review discusses some aspects of electron transfer in bacterial reaction centers and of optical spectroscopy of photosynthetic systems with emphasis on low-temperature experiments.Abbreviations (B)Chl (bacterio)chlorophyll - (B)Phe (bacterio)pheophytin - FMO Fenna-Matthews-Olson - LH1, LH2 light harvesting complexes of purple bacteria - LHC II, CP47 light harvesting complexes of Photosystem II - P, P870 primary electron donor - RC reaction center     

Heat and water rate transfer processes in the human respiratory tract at various altitudes

The process of the respiratory air conditioning as a process of heat and mass exchange at the interface inspired air-airways surface was studied. Using a model of airways (Olson et al., 1970) where the segments of the respiratory tract are like cylinders with a fixed length and diameter, the corresponding heat transfer equations, in the paper are founded basic rate exchange parameters-convective heat transfer coefficient h(c)(W m(-2) degrees C(-1)) and evaporative heat transfer coefficient h(e)(W m(-2)hPa(-1)). The rate transfer parameters assumed as sources with known heat power are connected to airflow rate in different airways segments. Relationships expressing warming rate of inspired air due to convection, warming rate of inspired air due to evaporation, water diffused in the inspired air from the airways wall, i.e. a system of air conditioning parameters, was composed. The altitude dynamics of the relations is studied. Every rate conditioning parameter is an increasing function of altitude. The process of diffusion in the peripheral bronchial generations as a basic transfer process is analysed. The following phenomenon is in effect: the diffusion coefficient increases with altitude and causes a compensation of simultaneous decreasing of O(2)and CO(2)densities in atmospheric air. Due to this compensation, the diffusion in the peripheral generations with altitude is approximately constant. The elements of the human anatomy optimality as well as the established dynamics are discussed and assumed. The square form of the airways after the trachea expressed in terms of transfer supposes (in view of maximum contact surface), that a maximum heat and water exchange is achieved, i.e. high degree of air condition at fixed environmental parameters and respiration regime.     

Determination of the antenna heterogeneity of Photosystem II by direct simultaneous fitting of several fluorescence rise curves measured with DCMU at different light intensities

Several methods for determination of the antenna heterogeneity of Photosystem II from fluorescence rise curves measured with DCMU have been developed so far. Using these methods, two, three or four types of Photosystem II with respect to the antenna heterogeneity were determined. However, the accuracy of some of these methods is under debate. Here, we present a new method for the determination of the antenna heterogeneity of Photosystem II. The method is based on direct simultaneous fitting of several fluorescence rise curves measured with DCMU at different intensities of light excitation. As several curves measured under different light conditions are fitted simultaneously by the same model, reliability and accuracy in determination of model parameters increase. Our method was applied to two plant materials with different structure of the thylakoid membrane: wheat leaves and cells of green alga Chlamydomonas reinhardtii. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of insulin on anabolic processes and growth in hypothyroid rats at various age periods

Studies have been made on a possibility of compensation of growth retardation in hypothyroid rats by insulin. In 30-day rats, compensation is not complete, although hypothyroidism is less significant as compared to that in animals which received merkazolil together with insulin. Further administration of insulin to rats up to the 90th day, resulted in a significant increase in the growth rate of skeletal muscles, body mass, protein content of skeletal muscles, as well as glycogen content of the liver and skeletal muscles. Thyroid insufficiency may be compensated by insulin alone, i.e. without simultaneous administration of thyroid hormones.     

Cross-linking starch at various moisture contents by phosphate substitution in an extruder

Starch was extruded with sodium trimetaphosphate and sodium hydroxide in a single-screw extruder with barrel and die temperature of 130 °C at three screw speeds of 40, 90, and 140 rpm and three moisture contents of 40, 55, and 70%. Time required for phosphorylation of starch in an extruder was found to be less than 2 min, and cross-linking of starch by phosphorus, which was incorporated into starch, was confirmed by paste viscosity of extruded starch. Cross-linking starch with 2.5% sodium trimetaphosphate did not significantly affect water absorption index, but reduced water solubility index so replacement of the process including mixing of starch with synthetic polymer in the extruder with another process including cross-linking of starch and then mixing with synthetic polymer can result in composite with lower solubility of starch. On the other hand, increasing moisture content of starch reduced both water absorption and solubility index of extrudates.     

Equatorial x-ray diffraction from single skinned rabbit psoas fibers at various degrees of activation. Changes in intensities and lattice spacing.

Equatorial x-ray diffraction patterns were obtained from single skinned rabbit psoas fibers during various degrees of activation under isometric conditions at ionic strength 170 mM and 6-9 degrees C. By direct calcium activation, contraction was homogeneous throughout the preparation, and by using a cycling technique (Brenner, 1983) integrity of the fiber was maintained even during prolonged steady activation. The intensity ratio of the two innermost reflections I11/I10, and the normalized intensities I*10 and I*11 varied linearly with increasing force. Thus the result agreed qualitatively with an earlier finding, obtained from the whole sartorius muscle, that intensity changes in 10 and 11 are directly correlated with isometric force level (Yu et al., 1979). Spacing of the myofilament lattice (d10) was found to decrease with increasing isometric tension. With the filaments in full overlap, maximum shrinkage was 14%. The lattice spacing started to level off when the degree of calcium activation was greater than or equal to 50%, approaching a limit approximately at 380-360 A. This decrease of the lattice spacing indicates that there is a radial force produced by force generating cross-bridges, but the net radial force appears to become insignificant as lattice spacing approaches 380-360 A.     

Cellular processes associated with germ band retraction in Drosophila

Large-scale movements of epithelial sheets are necessary for most embryonic and regenerative morphogenetic events. We have characterized the cellular processes associated with germ band retraction (GBR) in the Drosophila embryo. During GBR, the caudal end of the embryo retracts to its final posterior position. We show using time-lapse recordings that, in contrast to germ band extension, cells within the lateral germ band do not intercalate. In addition, the germ band and amnioserosa move as one coherent sheet, and the amnioserosa strongly shortens along its dorsal-ventral axis. Furthermore, during GBR, the amnioserosa adheres to and migrates over the caudal end of the germ band via lamellipodia. Expression of both dominant-negative and constitutively active RhoA in the amnioserosa disrupts GBR. As RhoA acts on both actomyosin contractility and cell-matrix adhesion, it suggests a role for such processes in the amnioserosa during GBR. The results establish the cellular movements and shape changes occurring during GBR and provide the basis for an analysis of the forces acting during GBR.     

Picosecond fluorescence decay studies on water-stressed pea leaves: energy transfer and quenching processes in photosystem 2

Detached leaves of pea (Pisum sativum) were submitted to water stress at different relative air humidities. The photosynthetic activity of photosystem 2 (PS2) was monitored by time-resolved picosecond chlorophyll (Chl) fluorescence spectroscopy. In the first days the well-known fast Chl fluorescence decay was observed which indicated high PS2 activity. After a few days the average fluorescence decay time τm reached a maximum, depending on the wilting conditions, but always at a relative loss of leaf mass of 80%. After this maximum, τm decreased within a few hours, the fluorescence decay became similar to that one of an intact leaf, but an additional fluorescence decay component with a lifetime of 3.6 ns appeared. At first the primary quinone QA was reduced due to inhibition of the electron transfer to the secondary quinone QB. Simultaneously, water deficiency caused an electron lack at the oxidizing site of PS2. This disabled the primary electron donor of PS2, tyrosine Z, from reducing the oxidized reaction centre of PS2 (P680+). Thus a recombination of P680+-pheophytin-QA- took place, and the energy was lost as heat. With further water stress, QA was decoupled from PS2. The new fluorescence decay component could therefore be assigned to energetically decoupled antenna complexes. This revised version was published online in June 2006 with corrections to the Cover Date.