Chlorophyll Molecules Research Articles

Light-harvesting in photosystem I

This review focuses on the light-harvesting properties of photosystem I (PSI) and its LHCI outer antenna. LHCI consists of different chlorophyll a/b binding proteins called Lhca’s, surrounding the core of PSI. In total, the PSI-LHCI complex of higher plants contains 173 chlorophyll molecules, most of which are there to harvest sunlight energy and to transfer the created excitation energy to the reaction center (RC) where it is used for charge separation. The efficiency of the complex is based on the capacity to deliver this energy to the RC as fast as possible, to minimize energy losses. The performance of PSI in this respect is remarkable: on average it takes around 50 ps for the excitation to reach the RC in plants, without being quenched in the meantime. This means that the internal quantum efficiency is close to 100 % which makes PSI the most efficient energy converter in nature. In this review, we describe the light-harvesting properties of the complex in relation to protein and pigment organization/composition, and we discuss the important parameters that assure its very high quantum efficiency. Excitation energy transfer and trapping in the core and/or Lhcas, as well as in the supercomplexes PSI-LHCI and PSI-LHCI-LHCII are described in detail with the aim of giving an overview of the functional behavior of these complexes.

AVALIAÇÃO DA DEFICIÊNCIA DE NITROGÊNIO NO FEIJOEIRO USANDO UM MEDIDOR PORTÁTIL DE CLOROFILA

O nitrogênio (N) é um dos principais nutrientes para o feijoeiro, sendo sua deficiência bastante comum. Como o nitrogênio também participa da constituição da molécula de clorofila, a avaliação da deficiência de N pela planta pode ser determinada pela mensuração indireta do teor de clorofila. Este trabalho teve como objetivo avaliar a possibilidade de utilização do medidor de clorofila na determinação da deficiência de N. O trabalho teve 5 tratamentos (0, 50, 100, 150 e 200 kg ha-1 de N), com 10 repetições, totalizando 50 vasos com 8 dm³ de material de solo. Foram feitas 10 leituras por vaso aos 30, 40 e 50 DAE (dias após emergência). Os coeficientes de regressão linear simples e de segundo grau foram significativos para todas as épocas estudadas neste trabalho, entretanto, a falta de ajuste também foi significativa, exceto para o comportamento dos valores obtidos aos 50 DAE. De modo geral, os valores obtidos com o medidor de clorofila tenderam a aumentar com o acréscimo da quantidade de N aplicado e com o avanço do estádio de desenvolvimento da planta, indicando o potencial da utilização deste equipamento para discriminar as diferentes doses de nitrogênio aplicadas. Foi possível observar que as diferentes doses de nitrogênio aplicadas influenciaram os valores do medidor de clorofila.

Exploring and tuning the anchorage of chlorophyllin molecules on anionic clays

In experiments modeling photosynthesis, well dispersed chlorophyll molecules are required. A solution could be to anchor them on some host. In this work, new hybrid materials are synthesized using hydrotalcite and a chlorophyll derivative: chlorophyllin. Chlorophyllin is incorporated and dispersed in hydrotalcite through simultaneous precipitation or during hydrotalcite reconstruction. The effect of microwave irradiation on the crystallization step is discussed. In all cases stable materials are obtained, chlorophyllin molecules are isolated and bonded through hydrotalcite OH groups.

Role of Formation of Statistical Aggregates in Chlorophyll Fluorescence Concentration Quenching

Using extensive Monte Carlo simulations, a comprehensive investigation has been carried out on the phenomenon of chlorophyll fluorescence concentration quenching. Our results reveal that statistical aggregations of chlorophylls act mainly as trapping sites for excitation energy and lead to fluorescence quenching. Due to transition dipolar-dipolar interactions between the chlorophylls within a statistical aggregate, the associated oscillator strength changes in comparison to a monomer, and excited energy states show splitting. Further, as the lower energy states are more likely associated with lower oscillator strengths, the fluorescence intensity is observed to decrease. Due to the rapid energy transfer between chlorophyll molecules after photoexcitation, the excitonic energy can easily reach a statistical aggregate, where trapping of the exciton and its subsequent decay occur. With an increase in the chlorophyll concentration, the probability of statistical aggregation increases, thereby accentuating the fluorescence quenching effect.

Computational Study on the Dependence of Electronic Transition Energies of Porphin, Chlorin, Mg-Chlorin and Chlorophyll a on an External Charge

SummaryIn order to understand the diversity in electronic transitionenergy of the chlorophyll molecules in biological lightharvesting complexes, we studied model systems of fourporphyrin derivatives, porphin, chlorin, Mg-chlorin, and Chla interacting with an external positive point charge locatedabove the tetrapyrollic ring plane. We calculated molecularorbitals and electronic transition energies to S1 and S2 statesfor the four molecules interacting with the point charge byusing TD-DFT method. The transition energies calculatedby the same computational method in the absence of thecharge are in reasonably good correlation to experimentaldata and then we examined the quantum chemical charac-teristics of electronic transitions of the porphyrin derivativesinfluenced by the external point charge. The transition energies of non-metallic derivatives arefound to be generally blue-shifted by the interaction with anexternal positive charge due to stabilization of occupiedmolecular orbitals. In metal-containing derivatives, Mg-chlorin and Chl a, the atomic orbital of the central Mg atomhas been found to play an important role in electronictransition in the presence of an external charge. Due to thestrong interaction of the atomic orbital with the charge, themolecular orbital in mixing with the atomic orbital is muchstabilized by the presence of nearby positive charges andthen the transition energy to S1 state can be red-shifted orblue-shifted from that of isolated molecules depending onthe detailed location of the external charge. Furthermore, afew new transition bands have been shown to emerge as aresult of the interaction with the charge. This may lead to abroadening mechanism of absorption line shape of chloro-phyll molecules in biological environments where variousdistributions of external charges are often found. Acknowledgments. This work was supported by a researchgrant from Chungbuk National University in 2011.References

Theoretical Study of Electron Transfer Rate from Phylloquinones to Iron-Sulfur Cluster (Fx) in Photosystem I

Photosystem I (PSI) is one of the two multi-subunit, pigment-protein complexes that drive the initial processes of light utilization in oxygenic photosynthesis. The PSI complex has approximately 100 packed chlorophyll (Chl) molecules that form the antenna and functional redox cofactors comprising the reaction center (RC). The latter cofactors are arranged in two pseudo-symmetric branches, starting from P700, and then split into two branches with the ec2 and ec3 chlorophylls and phylloquinone (A1) on each branch, and then rejoin at the iron-sulfur cluster FX. All of these are bound by the heterodimer of the PsaA and PsaB subunits, while the terminal FA and FB clusters are bound by PsaC. Electron transfer (ET) along the two branches has been studied by various investigators, and it was established that both branches are active in electron transfer, but that electron transfer to FX was ∼10-fold faster from PhQB than from PhQA.New results from electron paramagnetic resonance showed that the orientations of the PhQ radicals are different which affect the strength of hydrogen bonding between PhQ and the protein backbone. We are investigating these different rates of electron transfer and orientation of PhQ radicals through molecular dynamics simulation and time dependent density functional theory (TD-DFT) methods.

Systematic Analysis of the Demetalation Kinetics of Zinc Chlorophyll Derivatives Possessing Different Substituents at the 3-Position: Effects of the Electron-Withdrawing and Electron-Donating Strength of Peripheral Substituents

Removal of the central metal from chlorophyll (Chl) molecules is biologically important in terms of production of the primary electron acceptors in photosystem-II photosynthetic reaction centers and the early stage in Chl degradation. The physicochemical properties on demetalation of chlorophyllous pigments are useful in the understanding of such reaction mechanisms in photosynthetic organisms. Here we analyzed the demetalation kinetics of a series of Zn-Chl derivatives with a systematic variation in the electron-withdrawing and -donating substituents at the 3-position of the chlorin macrocycle under acidic conditions to elucidate thoroughly the substitution effects on the demetalation properties of chlorophyllous pigments. Dehydrogenation of the aliphatic group (CH(2)CH(3) → CH═CH(2) → C≡CH) at the 3-position slowed the removal of the central zinc from the chlorin macrocycle. The gradual decrease in the demetalation rate constants of the three zinc chlorins originates from differences in the electron-withdrawing strength of the ethyl, vinyl, and ethynyl groups directly linked to the chlorin π macrocycle. Reduction of the 3(1)-carbonyl groups significantly increased the demetalation rate constants, and the relative ratios of the demetalation rate constants of the zinc chlorins possessing a carbonyl group to those possessing the corresponding hydroxy group were analogous in the cases of 3-formyl- and 3-acetyl-zinc chlorins. The demetalation rate constants of the seven Zn-Chl derivatives possessing various electron-withdrawing and -donating groups exhibited good correlation with the Hammett σ parameters of the 3-position substituents.

LIGHT ASSISTED IN-VIVO MICROWAVE SENSING FOR ELECTRICAL CHARACTERIZATION OF PROKARYOTES

This paper reports an in vivo characterization technique to characterize dielectric properties of living tissues and bio-molecules at microwave frequency using cavity perturbation technique, where a slot ring resonant sensor has been used, that works at 8 GHz and has been designed to enumerate the effective dielectric constant of Spirulina platensis and chlorophyll molecule. Observed value of the dielectric constant of Spirulina platensis was 8 ± 0.04 in the absence of light and 14.575 ± 0.145 in the presence of light. Molecular polarizability of chl a molecule was 5.07 ± 0.05 × 104 Å3. Experimentally calculated local electric field actually experienced by chl a molecule was 14.197 ± 0.003 V/m for applied field of 9.79 V/m across the slot ring, dipole moment of chl a molecule was 2.175 ± 0.005 × 105 Debye and total polarisation produced due to these molecules was 1.545 ± 0.005 C/m 2. Observed relaxation time of chl a molecule was 8.09 ± 0.18 × 10-9 s . The proposed sensing method can be an alternate to spectral characterisation technique, generally used to characterize light sensitive bio-molecules and can also be extended to characterize light sensitive bio-molecules in plant cells.

The impact of environmental factors on the measurement of the normalized difference vegetation index

The level of nitrogen supply of a plant population can be quickly measured with non-destructive optical measurement devices and the differentiated determination of nitrogen shortage and the replenishment of nitrogen can also be carried out. The level of nitrogen supply is based on the fact that the chlorophyll content of crops is in close correlation with nitrogen content and that the amount of chlorophyll can be easily measured on the basis of the light absorption of chlorophyll molecules. The successfulness of optical measurements can be influenced by the change of weather parameters; therefore, it is important to know the correlations between measurement results and weather parameters when it comes to practical use.The GreenSeeker Model 505 measurement device determines the relative chlorophyll content in the form of the Normalized DifferenceVegetation Index (NDVI) calculated on the basis of the intensity of the reflected red and infrared rays of light from the crop population. The measurements were performed in alfalfa population with 10 replications at five measurement heights and four measurement times. The weather parameters were measured by a weather station located in the middle of the alfalfa population and the correlations between the meteorological data and the NDVI values were examined. During the statistical evaluation of the results, it was established that the NDVI measurement is primarily influenced by the relative humidity of the air, secondly by air temperature and thirdly by wind speed. Relative humidity was in strong correlation with the NDVI values which were also influenced by the measurement height and time. Regression was not significant in the case of 20 cm measurement height, but the measurements above 40 cm height showed significant correlations. The correlation was shown to be strong at each measurement time, but the influence of humidity was the lowest at 11:00 and 14:00.

MEASURING SUNFLOWER NITROGEN STATUS FROM AN UNMANNED AERIAL VEHICLE-BASED SYSTEM AND AN ON THE GROUND DEVICE

Abstract. Precision agriculture recognizes the inherent spatial variability associated with soil characteristics, land morphology and crop growth, and uses this information to prescribe the most appropriate management strategy on a site-specific basis. To reach this task, the most important information related with crop growth is nutrient status, weed infestation, disease and pet affectation and water management. The application of fertilizer nitrogen to field crops is of critical importance because it determines plant's gro wth, vigour, colour and yield. Furthermore, nitrogen has been observed as a nutrient with high spatial variability in a single field, related to its high mobility. Some previous works have shown that is possible to measure crop nitrogen status with optical instruments. Since most leaf nitrogen is contained in chlorophyll molecules, there is a strong relationship between leaf nitrogen and leaf chlorophyll content, which is the basis for predicting crop nitrogen status by measuring leaf reflectance. So, sensors that can easily monitor crop nitrogen amount throughout the growing season at a high resolution to allow producers to reach their production goals, will give useful information to prescribe a crop management on a site-specific basis. Sunflower is a crop which is taking importance again because it can be used both for food and biofuel purposes, and it is widely cultivated in the South of Spain and other European countries.The aim of this work was to compare an index related with sunflower nitrogen status, deduced from multispectral images taken from an Unmanned Aerial Vehicle (UAV), with optical data collected with a ground-based platform.An ADC Lite Tetracam digital cam was mounted on a md4-200 Microdrones to take pictures of a sunflower field during the crop season. ADC Lite Tetracam is a single sensor digital camera designed for capture of visible light wavelength longer than 520 nm and near-infrared wavelength up to 920 nm. The md4-200 Microdrones is an UAV which can be programmed to follow a route defined by several way-points and actions. The ground-based device was a Pacific Vision, Inc. multispectral radiometer. Four images with both systems were taken during the crop season and an index related with nitrogen crop status was calculated from them and compared in a sunflower field that had four irrigation treatments and eight nitrogen application rates, resulting in 32 plots of 7 m by 3.4 m, with a plant density of 7.1 plants m-2. Calculated Normalized Difference Vegetation Index (NDVI) from both measurement systems was a good indicator of nitrogen applied, but the UAV-based system provided a better estimate than ground-based system because in the first system was possible to eliminate the soil and shadows for calculating the index..

Primary steps of electron and energy transfer in photosystem I: Effect of excitation pulse wavelength

Time-resolved differential spectra of photosystem I complex were obtained by the "pump-probe" technique with 25-fs pulses with maxima at 670, 700, and 720 nm. The ratio between the number of excited chlorophyll molecules of the antenna and of the reaction center was shown to depend on spectral characteristics of the pump pulses. In all cases, an ultrafast (<150 fs) formation of the primary radical pair P700(+)A(0)() was recorded. However, on excitation by pulses with maxima at 670 or 700 nm, detection of the charge separation was masked by the much more intensive bleaching at the chlorophyll Q(y) band due to excitation of the bulk antenna chlorophylls. We show that triggering the charge separation by 25-fs pulses centered at 720 nm allows to detect more clearly kinetics of formation of the primary and secondary ion-radical pairs. The findings help to explain possible reasons for discrepancies of kinetics of primary steps of electron transfer detected in different laboratories.

An LED-based fluorometer for chlorophyll quantification in the laboratory and in the field

The chlorophyll content is an important experimental parameter in agronomy and plant biology research. In this report, we explore the feasibility of determining total concentration of extracts containing chlorophyll a and chlorophyll b by chlorophyll fluorescence. We found that an excitation at 457 nm results in the same integrated fluorescence emission for a molecule of chlorophyll a and a molecule of chlorophyll b. The fluorescence yield induced by 457 nm is therefore proportional to total molar chlorophyll concentration. Based on this observation, we designed an instrument to determine total chlorophyll concentrations. A single light emitting diode (LED) is used to excite chlorophyll extracts. After passing through a long-pass filter, the fluorescence emission is assessed by a photodiode. We demonstrate that this instrument facilitates the determination of total chlorophyll concentrations. We further extended the functionality of the instrument by including LEDs emitting at 435 and 470 nm wavelengths, thereby preferentially exciting chlorophyll a and chlorophyll b. This instrument can be used to determine chlorophyll a and chlorophyll b concentrations in a variety of organisms containing different ratios of chlorophylls. Monte-Carlo simulations are in agreement with experimental data such that a precise determination of chlorophyll concentrations in carotenoid-containing biological samples containing a concentration of less than 5 nmol/mL total chlorophyll can be achieved.

Photosynthetic and physiological analysis of the rice high-chlorophyll mutant (Gc)

Chlorophyll (Chl) molecules are essential for harvesting light energy in photosynthesis. A rice high-chlorophyll mutant (Gc) with significantly increased Chl b was identified previously in Zhenshan 97B (Oryza sativa indica). However, the mechanism underlying this higher Chl b content and its effectsonphotosynthetic efficiency are still unclear. Immunoblot and blue native polyacrylamide gel electrophoresis (BN-PAGE) with a second dimension electrophoresis followed by the matrix-assisted laser-desorption/ionization time-of-flight (MALDI-TOF) analysis showed that a few core proteins of photosystem I (PSI) and photosystem II (PSII), and light-harvesting complex II (LHCII) proteins were overexpressed in the mutant plants. Remarkable differences in chloroplast ultrastructure were observed between the wild-type and mutant plants, with the latter having more highly stacked and larger grana. Chl florescence analysis demonstrated that Gc had markedly increased quantum efficiency of photosystem II (ΦPSII), photochemical quenching (qP), non-photochemical quenching (qN) and electron transport rate (ETR). This morphological and physiological adaptation might confer a higher photosynthetic capacity in Gc than the wild-type.

Energy Transfer in Peridinin-Chlorophyll-Protein-Metallic Nanoparticle Hybrid Nanostructure: Effect of Spatial Arrangement

We theoretically investigate dynamics of excitation in a hybrid nanostructure comprising a photosynthetic complex peridinin-chlorophyll-protein (PCP) coupled to a gold spherical nanoparticle. Our model includes the analytical description of radiative and non-radiative relaxation channels of the chlorophylls in PCP, as well as the change of energy transfer rate within the PCP due to the presence of metal scatterer. We show that by measuring the intensity of uorescence from selectively excited chlorophyll molecules in the PCP complex, elementary geometric properties of the system can be deduced.

UVR defense mechanisms in eurytopic and invasive Gracilaria vermiculophylla (Gracilariales, Rhodophyta)

The invasive success of Gracilaria vermiculophylla has been attributed to its wide tolerance range to different abiotic factors, but its response to ultraviolet radiation (UVR) is yet to be investigated. In the laboratory, carpospores and vegetative thalli of an Atlantic population were exposed to different radiation treatments consisting of high PAR (photosynthetically active radiation) only (P), PAR+UV-A (PA) and PAR+UV-A+UV-B (PAB). Photosynthesis of carpospores was photoinhibited under different radiation treatments but photosystem II (PSII) function was restored after 12 h under dim white light. Growth of vegetative thalli was significantly higher under radiation supplemented with UVR. Decrease in chlorophyll a (Chl a) under daily continuous 16-h exposure to 300 µmol photons m(-2) s(-1) of PAR suggests preventive accumulation of excited chlorophyll molecules within the antennae to minimize the generation of dangerous reactive oxygen species. Moreover, an increase in total carotenoids and xanthophyll cycle pigments (i.e. violaxanthin, antheraxanthin and zeaxanthin) further suggests effective photoprotection under UVR. The presence of the ketocarotenoid β-cryptoxanthin also indicates protection against UVR and oxidative stress. The initial concentration of total mycosporine-like amino acids (MAAs) in freshly-released spores increased approximately four times after 8-h laboratory radiation treatments. On the other hand, initial specific MAAs in vegetative thalli changed in composition after 7-day exposure to laboratory radiation conditions without affecting the total concentration. The above responses suggest that G. vermiculophylla have multiple UVR defense mechanisms to cope with the dynamic variation in light quantity and quality encountered in its habitat. Beside being eurytopic, the UVR photoprotective mechanisms likely contribute to the current invasive success of the species in shallow lagoons and estuaries exposed to high solar radiation.

Alteration of proteins and pigments influence the function of photosystem I under iron deficiency from Chlamydomonas reinhardtii.

BackgroundIron is an essential micronutrient for all organisms because it is a component of enzyme cofactors that catalyze redox reactions in fundamental metabolic processes. Even though iron is abundant on earth, it is often present in the insoluble ferric [Fe (III)] state, leaving many surface environments Fe-limited. The haploid green alga Chlamydomonas reinhardtii is used as a model organism for studying eukaryotic photosynthesis. This study explores structural and functional changes in PSI-LHCI supercomplexes under Fe deficiency as the eukaryotic photosynthetic apparatus adapts to Fe deficiency.Results77K emission spectra and sucrose density gradient data show that PSI and LHCI subunits are affected under iron deficiency conditions. The visible circular dichroism (CD) spectra associated with strongly-coupled chlorophyll dimers increases in intensity. The change in CD signals of pigments originates from the modification of interactions between pigment molecules. Evidence from sucrose gradients and non-denaturing (green) gels indicates that PSI-LHCI levels were reduced after cells were grown for 72 h in Fe-deficient medium. Ultrafast fluorescence spectroscopy suggests that red-shifted pigments in the PSI-LHCI antenna were lost during Fe stress. Further, denaturing gel electrophoresis and immunoblot analysis reveals that levels of the PSI subunits PsaC and PsaD decreased, while PsaE was completely absent after Fe stress. The light harvesting complexes were also susceptible to iron deficiency, with Lhca1 and Lhca9 showing the most dramatic decreases. These changes in the number and composition of PSI-LHCI supercomplexes may be caused by reactive oxygen species, which increase under Fe deficiency conditions.ConclusionsFe deficiency induces rapid reduction of the levels of photosynthetic pigments due to a decrease in chlorophyll synthesis. Chlorophyll is important not only as a light-harvesting pigment, but also has a structural role, particularly in the pigment-rich LHCI subunits. The reduced level of chlorophyll molecules inhibits the formation of large PSI-LHCI supercomplexes, further decreasing the photosynthetic efficiency.

Mutations that Affect the Directionality of Electron Transfer in Photosystem 1

Photosystem 1 (PS1) is one of the two multi‐subunit, pigment‐protein complexes that drive the initial processes of light utilization in oxygenic photosynthesis. The PS1 complex has approximately 100 packed chlorophyll (Chl) molecules that form the antenna and functional redox cofactors comprising the reaction center (RC). The latter cofactors are arranged in two pseudo‐symmetric branches, starting from P700, and then split into two branches with the ec2 and ec3 chlorophylls and phylloquinone (A1) on each branch, and then rejoin at the iron‐sulfur cluster FX. All of these are bound by the heterodimer of the PsaA and PsaB subunits, while the terminal FA and FB clusters are bound by PsaC. Electron transfer (ET) along the two branches has been studied by various investigators, and it was established that both branches are active in electron transfer, but that electron transfer to FX was ~10‐fold faster from PhQB than from PhQA. Recently it was suggested that the initial charge separation event can take place independently within each ec2/ec3 pair (Holzwarth et al. 2010). In order to get a clearer picture of this phenomenon, we have generated new mutations in the PsaA and PsaB subunits near the ec2 Chls. Immunoblots and optical spectroscopy were used to estimate the amount of PS1 accumulated in each mutant. We measured nanosecond optical measurements in whole cells to probe for the directionality of electron transfer in the mutants, and identified some mutations that produce an observable effect on the relative use of the two branches. We are currently investigating these by ultra‐fast pump‐probe spectroscopy.

Extinction coefficient for red-shifted chlorophylls: Chlorophyll d and chlorophyll f

Both chlorophyll f and chlorophyll d are red-shifted chlorophylls in oxygenic photosynthetic organisms, which extend photon absorbance into the near infrared region. This expands the range of light that can be used to drive photosynthesis. Quantitative determination of chlorophylls is a crucial step in the investigation of chlorophyll-photosynthetic reactions in the field of photobiology and photochemistry. No methods have yet been worked out for the quantitative determination of chlorophyll f. There is also no method available for the precise quantitative determination of chlorophyll d although it was discovered in 1943. In order to obtain the extinction coefficients (ε) of chlorophyll f and chlorophyll d, the concentrations of chlorophylls were determined by Inductive Coupled Plasma Mass Spectrometry according to the fact that each chlorophyll molecule contains one magnesium (Mg) atom. Molar extinction coefficient εchl f is 71.11×103Lmol−1A707nmcm−1 and εchl d is 63.68×103Lmol−1A697nmcm−1 in 100% methanol. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Correlated intermolecular coupling fluctuations in photosynthetic complexes

The functioning and efficiency of natural photosynthetic complexes is strongly influenced by their embedding in a noisy protein environment, which can even serve to enhance the transport efficiency. Interactions with the environment induce fluctuations of the transition energies and couplings between the chlorophyll molecules, and due to the fact that different fluctuations will partially be caused by the same environmental factors, correlations between the various fluctuations will occur. We argue that fluctuations of the couplings should, in general, not be neglected, as these have a considerable impact on population transfer rates, decoherence rates, and the efficiency of photosynthetic complexes. Furthermore, while correlations between transition energy fluctuations have been studied, we provide the first quantitative study of the effect of correlations between coupling fluctuations and transition energy fluctuations, and of correlations between the various coupling fluctuations. It is shown that these additional correlations typically lead to changes in interchromophore transfer rates and population oscillations and can lead to a limited enhancement of the light harvesting efficiency.

Spectroscopy of Photosynthetic Pigment-Protein Complex LHCII

Light-harvesting pigment–protein complex of photosystem II is the most abundant membrane protein in the biosphere, comprising more than half chlorophyll molecules. The protein plays a role of photosynthetic antenna, collecting solar radiation and transferring excitations towards the reaction centers, where electric charge separation takes place. Efficient excitation energy capture and transfer requires unique organization of the complex and unique photophysical properties of the accessory pigments: chlorophylls and carotenoids. LHCII is also a place where extremely harmful singlet oxygen may be generated, under strong illumination conditions. Several physical mechanisms have been found in LHCII, operating to protect the photosynthetic apparatus against light-induced damage, including chlorophyll triplet and singlet excitations quenching by carotenoids. In this paper we discuss the results of our recent studies, carried out with the application of several molecular spectroscopy techniques (electronic absorption, fluorescence, resonance Raman and FTIR), designed to investigate molecular mechanisms responsible for regulation of excitation density in LHCII. Among the most interesting findings are the light-induced molecular configuration changes of the LHCII-bound xanthophylls, leading to conformational rearrangements of the protein. These mechanisms are discussed in terms of excessive excitation quenching in the pigment–protein complex subjected to overexcitation. Such an activity seems to represent a vital regulatory process in the photosynthetic apparatus, at the molecular level, protecting plants against photodegradation.