Radka Vladkova

Chlorophyll a Self-assembly in Polar Solvent–Water Mixtures†

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.

Action and target sites of nitric oxide in chloroplasts

Nitric oxide (NO) is an important signalling molecule in plants under physiological and stress conditions. Here we review the influence of NO on chloroplasts which can be directly induced by interaction with the photosynthetic apparatus by influencing photophosphorylation, electron transport activity and oxido-reduction state of the Mn clusters of the oxygen-evolving complex or by changes in gene expression. The influence of NO-induced changes in the photosynthetic apparatus on its functions and sensitivity to stress factors are discussed.

Effects of exogenous 24-epibrassinolide on the photosynthetic membranes under non-stress conditions

In the present work the effects of exogenous 24-epibrassinolide (EBR) on functional and structural characteristics of the thylakoid membranes under non-stress conditions were evaluated 48 h after spraying of pea plants with different concentrations of EBR (0.01, 0.1 and 1.0 mg.L(-1)). The results show that the application of 0.1 mg.L(-1) EBR has the most pronounced effect on the studied characteristics of the photosynthetic membranes. The observed changes in 540 nm light scattering and in the calorimetric transitions suggest alterations in the structural organization of the thylakoid membranes after EBR treatment, which in turn influence the kinetics of oxygen evolution, accelerate the electron transport rate, increase the effective quantum yield of photosystem II and the photochemical quenching. The EBR-induced changes in the photosynthetic membranes are most probably involved in the stress tolerance of plants.

Photoelectron transport ability of chloroplast thylakoid membranes treated with NO donor SNP: Changes in flash oxygen evolution and chlorophyll fluorescence

The nitric oxide (NO) donor sodium nitroprusside (SNP) is frequently used in plant science in vivo. The present in vitro study reveals its effects on the photosynthetic oxygen evolution and the chlorophyll fluorescence directly on isolated pea thylakoid membranes. It was found that even at very low amounts of SNP (chlorophyll/SNP molar ratio∼67:1), the SNP-donated NO stimulates with more than 50% the overall photosystem II electron transport rate and diminishes the evolution of molecular oxygen. It was also found that the target site for SNP-donated NO is the donor side of photosystem II. Compared with other NO-donors used in plant science, SNP seems to be the only one exhibiting stimulation of electron transport through photosystem II.

Bilayer structural destabilization by low amounts of chlorophyll a

The present study shows that small admixtures of one chlorophyll a (Chla) molecule per several hundred lipid molecules have strong destabilizing effect on lipid bilayers. This effect is clearly displayed in the properties of the L(alpha)-H(II) transformations and results from a Chla preference for the H(II) relative to the L(alpha) phase. Chla disfavors the lamellar liquid crystalline phase L(alpha) and induces its replacement with inverted hexagonal phase H(II), as is consistently demonstrated by DSC and X-ray diffraction measurements on phosphatidylethanolamine (PE) dispersions. Chla lowers the L(alpha)-H(II) transition temperature (42 degrees C) of the fully hydrated dipalmitoleoyl PE (DPoPE) by approximately 8 degrees C and approximately 17 degrees C at Chla/DPoPE molar ratios of 1:500 and 1:100, respectively. Similar Chla effect was recorded also for dielaidoyl PE dispersions. The lowering of the transition temperature and the accompanying significant loss of transition cooperativity reflect the Chla repartitioning and preference for the H(II) phase. The reduction of the H(II) phase lattice constant in the presence of Chla is an indication that Chla favors H(II) phase formation by decreasing the radius of spontaneous monolayer curvature, and not by filling up the interstitial spaces between the H(II) phase cylinders. The observed Chla preference for H(II) phase and the substantial bilayer destabilization in the vicinity of a bilayer-to-nonbilayer phase transformation caused by low Chla concentrations can be of interest as a potential regulatory or membrane-damaging factor.

Zero-phonon transitions of chlorophyll a in mature plant leaves revealed by spectral hole-burning method at 5 K

Abstract After monochromatic exposition in the 680–700 nm region persistent holes are formed in the fluorescence excitation spectra of green maize leaves. These narrow (up to 1 cm −1 ) holes correspond to zero-phonon lines whose intensity may reach 45% of the total fluorescence. Thus the fluorescence bands F 635 and F 695 , originating mainly from Photosystem II (Rijgersberg, C.P., Amesz, J. Thielen, A.P.G.M. and Swager, J.A. (1979) Biochim. Biophys. Acta 545, 473–482), are essentially of purely electronic nature. Unlike chlorophyll a in glassy matrices, the pigment in greening leaves cannot be completely phototransformed under high-dose Kr + laser irradiation at 676.4 nm.

Effects of 24-epibrassinolide Pre-treatment on UV-B-Induced Changes in the Pigment Content of Pea Leaves

In the present work, the effects of 24-epibrassinolide (EBR) on the UV-Binduced changes in the pigment content of pea leaves were studied. Control (non-EBR-treated) and EBR-treated plants were irradiated with UV-B for 3 h and pigment analysis was performed after 24 and 48 h. The results show that EBR spraying of plants 48 h prior to UV-B exposure alleviates its detrimental effect on chlorophyll a and b (Chl a and Chl b) content in comparison with control pea leaves. An increase in carotenoids (Car) and UV-B absorbing compounds was also observed at low dose of UV-B radiation. For the first time, it is shown that UV-B damage effect on control leaves is accompanied by a significant (more than 50%) increase in their pheophytin a (Pheo a) content 48 h after the UV-B exposure and that the EBR pre-treatment prevents the increase of Pheo a content in UV-B irradiated leaves. In addition, it is demonstrated that EBR application modifies UV-B-induced alterations of energy distribution between the main pigment-protein complexes in pea thylakoid membranes.

Chlorophyll a is the crucial redox sensor and transmembrane signal transmitter in the cytochrome b6f complex. Components and mechanisms of state transitions from the hydrophobic mismatch viewpoint

The cytochrome (cyt) b6f complex is involved in the transmembrane redox signaling that triggers state transitions in cyanobacteria and chloroplasts. However, the components and molecular mechanisms are still unclear. In an attempt to solve this long-standing problem, we first focused on the unknown role of a single chlorophyll a (Chla) in cyt b6f with a new approach based on Chla structural properties. Various b6f X-ray crystal structures were analyzed to identify their differences, which correlate with differences in Chla molecular volume. We found that the distance of the Rieske [2Fe-2S] cluster to Chla correlates with the distance between a pair of residues at the Qo-site and the distance between a pair of residues at the opposite membrane side. These correlations were accompanied by the rotation of a key peripheral residue and by changes in the hydrophobic thickness of cyt b6f. Parallel analysis of cyt bc1 crystal structures allowed us to conclude that Chla acts as the crucial redox sensor and transmembrane signal transmitter in b6f for changes in the plastoquinone pool redox state. The hydrophobic mismatch induced by the changed hydrophobic thickness of cyt b6f is the driving force for the structural reorganizations of the photosynthetic apparatus during induction and the progression of state transitions in cyanobacteria and chloroplasts. A mechanism for LHCII kinase activation in chloroplasts is also proposed. Our understanding of the dynamic structural changes in bc-complexes during turnover at the Qo-site and state transitions is augmented by the time-sequence ordering of 56 bc crystal structures.

Specific solvation of chlorophyll b: A site-selection study at 5 K

Abstract The site-selection fluorescence excitation spectra of chlorophyll b were recorded in three different types of polar solvent host at 5 K. The influence of specific solvation of chlorophyll b on the S 1 state vibrational modes active in these spectra was studied in the high frequency vibrational region above 1000 cm −1 . The nucleophilic solvation of chlorophylls ( i.e. one or two axial extra ligands at the central magnesium atom), unlike electrophilic interactions (hydrogen bonding), can be detected very successfully by the method used. The maximum down-shift of 20 – 30 cm −1 of the high characteristic vibrational frequencies in the CC stretching region estimated under electronic excitation of chlorophyll b is comparable with the effect of magnesium biligation and is explained in terms of an expansion of the tetrapyrrolic macrocycle core of the molecule. The broad-band fluorescence spectra of chlorophyll b in the same frozen solutions recorded at different laser excitation wavelengths are also presented and discussed.

Spectral Characteristics of Chlorophyll A at Different States of Solvation

The experimental data available suggested the monomeric nature of the cation radicals and triplets of P680 and P700, the primary electron donors in plant photosynthesis1. Interactions of monomeric Chlorophyll a /Chl.a/ electrostatics /for a review, see2 /, are proposed as a factors, responsible for the observed in vivo behavior of these complexes. Chl.a-water interactions have been used for modelling of P680 and P700 /for a review, see3 /. Recent works4,5, have demonstrated that polar solvents can be used for modelling of protein environment and four types of specific solvation, depending on solvents’ empirical parameters /electrophilicity, nucleophilicity and steric factors/, have been distingished. Thus, bisligation and hydrogen-bonding of Chl.a in the case of methanol and only bisligation in the case of pyridine have been shown4.