Exciton interactions of chlorophyll tetramer in water-soluble chlorophyll-binding protein BoWSCP.

The exciton interaction of four chlorophyll a (Chl a) molecules in a symmetrical tetrameric complex of the water-soluble chlorophyll-binding protein BoWSCP was analyzed in the pH range of 3-11. Exciton splitting ΔE = 232 ± 2 cm-1 of the Qy band of Chl a into two subcomponents with relative intensities of 78.1 ± 0.7 % and 21.9 ± 0.7 % was determined by a joint decomposition of the absorption and circular dichroism spectra into Gaussian functions. The exciton coupling parameters were calculated based on the BoWSCP atomic structure in three approximations: the point dipole model, the distributed atomic monopoles, and direct ab initio calculations in the TDDFT/PCM approximation. The Coulomb interactions of monomers were calculated within the continuum model using three values of optical permittivity. The models based on the properties of free Chl a in solution suffer from significant errors both in estimating the absolute value of the exciton interaction and in the relative intensity of exciton transitions. Calculations within the TDDFT/PCM approximation reproduce the experimentally determined parameters of the exciton splitting and the relative intensities of the exciton bands. The following factors of pigment-protein and pigment-pigment interactions were examined: deviation of the macrocycle geometry from the planar conformation of free Chl; the formation of hydrogen bonds between the macrocycle and water molecules; the overlap of wave functions of monomers at close distances. The most significant factor is the geometrical deformation of the porphyrin macrocycle, which leads to an increase in the dipole moment of Chl monomer from 5.5 to 6.9 D and to a rotation of the dipole moment by 15° towards the cyclopentane ring. The contributions of resonant charge-transfer states to the wave functions of the Chl dimer were determined and the transition dipole moments of the symmetric and antisymmetric charge-transfer states were estimated.

Chlorophyll a Dimers Bound in the Water-Soluble Protein BoWSCP Photosensitize the Reduction of Cytochrome c.

When bound to water-soluble proteins of the WSCP family, chlorophyll molecules form dimers structurally similar to the "special pair" of chlorophylls (bacteriochlorophylls) in photosynthetic reaction centers. Being exposed to red light (λ ≥ 650 nm) in oxygen-free solutions, chlorophyll a dimers harbored by BoWSCP holoproteins (from Brassica oleracea var. botrytis) have sensitized the reduction of cytochrome c. According to absorption and circular dichroism spectroscopy data, the photochemical process did not significantly impair the structure of chlorophyll a molecules as well as their dimers harbored by BoWSCP protein. Adding tris(hydroxymethyl)aminomethane as an electron donor for chlorophyll recovery stimulated the photoreduction of cytochrome c.

[Water Soluble Chlorophyll-Binding Proteins of Plants: Structure, Properties and Functions].

Water soluble chlorophyll-binding proteins (WSCPs) of higher plants differ from most proteins containing chlorophyll orbacteriochlorophyll in that they are soluble in watr and are neither embedded in the lipid membrane nor directly involved in the process of photosynthesis. Chlorophyll molecules in WSCPs ensembles are packed in dimers within the hydrophobic zone of the protein matrix, similar to the structure of a chlorophyll "special pair" in the reaction centers of phototrophs. This fact together with the detected photosensitizing activity of WSCPs makes it possible to consider these proteins as a promising object for modelling the evolutionary prototypes of the photosynthetic apparatus, as well as for developing the artificial solar energy converters. There are two classes of proteins in the WSCP family, class I and class II the representatives of these classes have a weak degree of homology in the primary structure, but a high degree of similarity in the tertiary and quaternary structure. One of the features of class I WSCPs is photoconversion, that is, to change the structure and spectral properties of the chromophore under the action of light. The functions of WSCPs in the plant are thought to be associated with stress protection.

[Mechanism of photosensitized luminescence of singlet oxygen dimols in air-saturated pigment solutions].

Luminescence of singlet oxygen dimols (1O2)2 with the main spectral maximum at 703-706 nm and much weaker bands at 640 and 770-780 nm was studied using mechanical phosphoroscopes in aerobic solutions of the nonfluorescent photosensitizer phenalenone in CCl4 and C6F6 at relatively low radiant power. The spectrum of this luminescence resembles that of dimol luminescence, which we had detected previously in solutions of porphyrins and other compounds. It was shown that, in phenalenone solutions, the mechanism of dimol luminescence involves reaction of two 1O2 molecules and one ground-state pigment molecule. It is most likely that light is emitted by the dimol-pigment contact complexes formed as a result of collisions of 1O2 with metastable, probably triplet intermediates that result from reactions of 1O2 with pigment molecules. It is proposed that this luminescence mechanism is of general importance for many organic and biologically important systems where singlet oxygen is generated. However, a comparison with the literature data suggests that the luminescence of this type can be detected at relatively low rates of 1O2 generation. At high singlet oxygen generation rates, dimol luminescence with the main maximum at 635-637 nm dominates, which is likely caused by direct collisions of 1O2 molecules.

[Phosphorescence analysis of the chlorophyll triplet state in preparations of photosystem II]. / Fosforestsentnyi analiz obrazovaniia tripletnogo sostoianiia khlorofilla v preparatakh fotosistemy II.

The low-temperature (77 K) phosphorescence of chlorophyll (Chl) in the reaction centres (D1D2-cyt b559-particles) and the core complexes of photosystem II isolated from higher plants was studied. Two phosphorescence spectral bands with the emission maxima at 950 and 977 nm, excitation maxima at 666 and 675-680 nm, and the lifetimes equal to 2 and 1.5 ms, respectively, were registered. The data indicate that the phosphorescence corresponds to the triplet Chl a molecules spatially separated from carotenoids. In samples treated by potassium ferricyanide and frozen under illumination by red light, the intensities of both bands were reduced, but the decrease of the short-wavelength 950-nm band was much more pronounced. This allows an assumption that the short-wavelength phosphorescence belongs to Chl a molecules, which are more accessible for ferricyanide because they are located on the surface of the chlorophyll-protein complexes, whereas the long-wavelength phosphorescence is emitted by the Chl molecules located inside the D1D2 heterodimer and therefore, is more protected by protein macromolecules.

Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids.

Evidence is presented, by means of both fluorescence and action spectroscopy, that a small, spectroscopically heterogeneous population of both Chl a and Chl b molecules is present in isolated spinach thylakoids and is active in photoinhibition. The broadness of the action spectrum suggests that degraded or incompletely assembled pigment-protein complexes may be involved.

Photophysical studies of pheophorbide a and pheophytin a. Phosphorescence and photosensitized singlet oxygen luminescence.

The triplet states of pheophorbide a and pheophytin a were studied in several environments by direct measurement of the phosphorescence of the pigments and photosensitized singlet oxygen (1O2) luminescence. The spectra, lifetimes and quantum yields of phosphorescence and the quantum yields of 1O2 generation were determined. These parameters are similar for monomeric molecules of both pigments in all the environments studied. Aggregation of the pigment molecules leads to a strong decrease in the phosphorescence and 1O2 luminescence intensities, which is probably due to a large decrease in the triplet lifetime and triplet quantum yield in the aggregates. The results obtained for pheophorbide a and pheophytin a are compared with those previously reported for chlorophyll alpha. The data suggest that the photodynamic activity of the pigments in living tissues is probably determined by the monomeric pigment molecules formed in hydrophobic cellular structures. Aggregated molecules seem to have a much lower activity.

[Sensibilized dimer luminescence of singlet molecular oxygen in solutions]. / Sensibilizirovannaia dimernaia liuminestsentsiia singletnogo molekuliarnogo kisloroda v rastvorakh.

With the use of mechanical phosphoroscope the "universal" delayed emission has been found in aerobic solutions of different sensitizers in CCl4. The spectrum of this emission has the main maximum at 703 nm. The luminescence intensity is proportional to the square of the intensity of the exciting light. Removal of oxygen or addition of 10% of acetone led to disappearance of the luminescence. At equal intensities of singlet oxygen generation relative intensities of the 1272 and 703 nm bands differed by several orders of magnitude in solutions of different sensitizers. The energy migration from the molecules responsible for the luminescence to bacteriopheophytin and phtalocyanine has been observed. The luminescence is interpreted as dimol emission of solvated singlet molecular oxygen activated by sensitizer molecules.