K.V. Neverov

Photoinduced dimol luminescence of singlet molecular-oxygen in solutions of photosensitizers

Abstract A delayed luminescence caused by photosensitized formation of dimols of singlet molecular oxygen has been observed in aerobic solutions of different photosensitizers in CCl 4 , hexafluorobenzene and freon 113. Its intensity is proportional to the square of the intensity of excitation and its lifetime is equal to half of the singlet-oxygen lifetime. Removal of oxygen or addition of singlet-oxygen quenchers leads to a strong decrease of this luminescence. At equal intensities of the singlet-oxygen generation, the intensity of this delayed light emission varies by 8 orders of magnitude for solutions of different photosensitizers. If the principal maximum of the photosensitizer fluorescence is at λ⩾703 nm, the emission spectrum coincides with the fluorescence spectrum of the photosensitizer. If the maximum of the photosensitizer fluorescence is at λ⩾680 nm, the emission band at 703 nm is dominant and, in some cases, additional bands at 635 nm and at λ⩾730 nm were observed. The data suggest that the 635 and 703 nm emission bands belong to the singlet-oxygen dimols activated in collisions with molecules of the photosensitizers and products of their oxygenation. Other bands belong to molecules of the photosensitizers and products of their photodestruction excited as a result of energy migration from singlet-oxygen dimols.

Assignment for the novel vibronic bands in the spectrum of the photosensitized singlet oxygen luminescence

Abstract Novel bands at 1073 and 1160 nm were detected in the spectrum of the photosensitized singlet oxygen luminescence in air-saturated solutions. The 1073 nm band was observed in carbon tetrachloride, freon 113 and hexafluorobenzene. The data suggest that it accompanies the { 1 Δ g (ν = 1) → 3 Σ g (ν = 0)} transition in thermally activated singlet oxygen molecules. The 1160 nm band was detected in CCI 4 . It probably corresponds to simultaneous vibronic transitions involving singlet oxygen and Cl atoms in solvent molecules.

Carotenogenic response in photosynthetic organisms: a colorful story.

Carotenoids are a diverse group of terpenoid pigments ubiquitous in and essential for functioning of phototrophs. Most of the researchers in the field are focused on the primary carotenoids serving light harvesting, photoprotection, and supporting the structural integrity of the photosynthetic apparatus (PSA) within the thylakoid membranes. A distinct group of the pigments functionally and structurally uncoupled from the PSA and accumulating outside of the thylakoids is called secondary carotenoids. Induction of the biosynthesis and massive accumulation of the latter termed as secondary carotenogenesis and carotenogenic response (CR), respectively, is a major though insufficiently studied stress response discovered in many phototrophic organisms ranging from single-celled algae to terrestrial higher plants. The CR protects cell by means of optical shielding of cell structures vulnerable photodamage, consumption of potentially harmful dioxygen, augmenting sink capacity of photoassimilates, and exerting an antioxidant effect. The secondary carotenoids exhibit a remarkable photostability in situ. Therefore, the CR-based photoprotective mechanism, unlike, e.g., antioxidant enzyme-based protection in the chloroplast, does not require continuous investment of energy and metabolites making it highly suitable for long-term stress acclimation in phototrophs. Capability of the CR determines the strategy of acclimation of photosynthetic organisms to different stresses such as excessive irradiance, drought, extreme temperatures, and salinities. Build-up of the CR might be accompanied by gradual disengagement of ‘classical’ active (energy-dependent) photoprotective mechanisms such as non-photochemical quenching. In addition to that, the CR has great ecological significance. Illustrious examples of this are extremely stress-tolerant ‘snow’ algae and conifer species developing red coloration during winter. The CR has also considerable practical implications since the secondary carotenoids exert a plethora of beneficial effects on human and animal health. The carotenogenic microalgae are the richest biotechnological sources of natural value-added carotenoids such as astaxanthin and β-carotene. In the present review, we summarize current functional, mechanistic, and ecological insights into the CR in a broad range of organisms suggesting that it is obviously more widespread and important stress response than it is currently thought to be.

Characterization of the low-temperature triplet state of chlorophyll in photosystem II core complexes: Application of phosphorescence measurements and Fourier transform infrared spectroscopy

Phosphorescence measurements at 77 K and light-induced FTIR difference spectroscopy at 95 K were applied to study of the triplet state of chlorophyll a ((3)Chl) in photosystem II (PSII) core complexes isolated from spinach. Using both methods, (3)Chl was observed in the core preparations with doubly reduced primary quinone acceptor QA. The spectral parameters of Chl phosphorescence resemble those in the isolated PSII reaction centers (RCs). The main spectral maximum and the lifetime of the phosphorescence corresponded to 955±1 nm and of 1.65±0.05 ms respectively; in the excitation spectrum, the absorption maxima of all core complex pigments (Chl, pheophytin a (Pheo), and β-carotene) were observed. The differential signal at 1667(-)/1628(+)cm(-1) reflecting a downshift of the stretching frequency of the 13(1)-keto C=O group of Chl was found to dominate in the triplet-minus-singlet FTIR difference spectrum of core complexes. Based on FTIR results and literature data, it is proposed that (3)Chl is mostly localized on the accessory chlorophyll that is in triplet equilibrium with P680. Analysis of the data suggests that the Chl triplet state responsible for the phosphorescence and the FTIR difference spectrum is mainly generated due to charge recombination in the reaction center radical pair P680(+)PheoD1(-), and the energy and temporal parameters of this triplet state as well as the molecular environment and interactions of the triplet-bearing Chl molecule are similar in the PSII core complexes and isolated PSII RCs.

Novel data on the spectra of photosensitized singlet-oxygen luminescence in the solution phase: detection of vibrationally excited monomols and dye-activated dimols of singlet oxygen

The spectrum of photosensitized singlet oxygen luminescence has been investigated in air- saturated CCl4, hexafluorobenzene and freon 113 at the wavelengths shorter fundamental at (lambda) < 1200 nm. Novel bands have been detected. The 1073 nm band was observed in all solvents with all photosensitizers used. The data suggest that it accompanies the 1(Delta) g (v equals 1) yields 3(Sigma) g (v equals 0) transition in thermally activated singlet oxygen molecules. The 1160 nm band was detected in CCl4. It probably corresponded to simultaneous vibronic transitions involving singlet oxygen and Cl-atoms in solvent molecules. The spectra and intensity of luminescence in the visible region strongly depended on the chemical structure and fluorescence properties of the photosensitizers. When the main fluorescence maximum of a photosensitizer was shorter than 630 nm, the 635, 703 and 780 nm, luminescence bands corresponding to the singlet-oxygen dimols were observed. When main fluorescence maxima of photosensitizers or products of their photodestruction were at (lambda) < 700 nm, the luminescence spectra corresponded to fluorescence of photosensitizers and products of their photodestruction. The data suggest that this luminescence is emitted by singlet oxygen dimols activated in collisions with photosensitizer molecules and by molecules of the photosensitizers and products of their destruction excited as a result of energy migration from singlet oxygen dimols. Some photosensitizers as metal-free tetra-4-tret-butyl-phthalocyanine and bis(tri-n-hexylsiloxy)silicon-2,3-naphthalocyanine are extremely strong amplifiers of the dimol emission. This shows that phthalocyanines and naphthalocyanines might be used for luminescence detection of monomols and dimols of singlet oxygen in systems of biomedical importance.