Y.V. Kovalev

Spectral and Kinetic Parameters of Phosphorescence of Triplet Chlorophyll a in the Photosynthetic Apparatus of Plants

Spectral and kinetic parameters and quantum yield of IR phosphorescence accompanying radiative deactivation of the chlorophyll a (Chl a) triplet state were compared in pigment solutions, greening and mature plant leaves, isolated chloroplasts, and thalluses of macrophytic marine algae. On the early stages of greening just after the Shibata shift, phosphorescence is determined by the bulk Chl a molecules. According to phosphorescence measurement, the quantum yield of triplet state formation is not less than 25%. Further greening leads to a strong decrease in the phosphorescence yield. In mature leaves developing under normal irradiation conditions, the phosphorescence yield declined 1000-fold. This parameter is stable in leaves of different plant species. Three spectral forms of phosphorescence-emitting chlorophyll were revealed in the mature photosynthetic apparatus with the main emission maxima at 955, 975, and 995 nm and lifetimes ∼1.9, ∼1.5, and 1.1–1.3 ms. In the excitation spectra of chlorophyll phosphorescence measured in thalluses of macrophytic green and red algae, the absorption bands of Chl a and accessory pigments — carotenoids, Chl b, and phycobilins — were observed. These data suggest that phosphorescence is emitted by triplet chlorophyll molecules that are not quenched by carotenoids and correspond to short wavelength forms of Chl a coupled to the normal light harvesting pigment complex. The concentration of the phosphorescence-emitting chlorophyll molecules in chloroplasts and the contribution of these molecules to chlorophyll fluorescence were estimated. Spectral and kinetic parameters of the phosphorescence corresponding to the long wavelength fluorescence band at 737 nm were evaluated. The data indicate that phosphorescence provides unique information on the photophysics of pigment molecules, molecular organization of the photosynthetic apparatus, and mechanisms and efficiency of photodynamic stress in plants.

The Role of Singlet and Triplet Dioxygen Molecules in the EPR and 1H NMR Spectrum Line Broadening

In the Mo(VI)/H2O2/H2O system, the relaxation time (T1) of protons in a water molecule and in a CH3 group decreases 10 to 30 times under conditions of dismutation of H2O2 with the formation of 1O2(1Δg). It is experimentally found that the overequilibrium concentration of triplet dioxygen cannot be the reason behind a decrease in T1 in the 1H NMR spectra. Neither can it explain the anomalous line broadening in ESR spectra under conditions of 1O2(1Δg) formation in the systems V(V)/H2O2/AcOH and Mo(VI)/H2O2/H2O. Ab initio calculations showed that it is principle possible that the 3O4(3·-g-1Δg) molecule exists in a snake-like form and is formed by the reaction between 3O2(3·-g) and 1O2(1Δg), which is the product of H2O2 decomposition in the systems V(V)/H2O2/AcOH and Mo(IV)/H2O2/H2O. The interaction of 1O2 with the ·OOH radical is exothermic (ΔQ = 2.30 kcal/mol) and leads to the formation of ·OOOOH. It is assumed that the paramagnetic species of type ·OOOOH or 3O4(3A1) that is formed in the reaction might be responsible for the spectral effects observed.