An irradiation density dependent energy relaxation in plant photosystem II antenna assembly.

Plant photosystem II (PSII) is a multicomponent pigment-protein complex that harvests sunlight via pigments photoexcitation, and converts light energy into chemical energy. Against high light induced photodamage, excess light absorption of antenna pigments triggers the operation of photoprotection mechanism in plant PSII. Non-photochemical energy relaxation as a major photoprotection way is essentially correlated to the excess light absorption. Here we investigate the energy relaxation of plant PSII complexes with varying incident light density, by performing steady-state and transient chlorophyll fluorescence measurements of the grana membranes (called as BBY), functional moiety PSII reaction center and isolated light-harvesting complex LHCII under excess light irradiation. Based on the chlorophyll fluorescence decays of these samples, it is found that an irradiation density dependent energy relaxation occurs in the LHCII assemblies, especially in the antenna assembly of PSII supercomplexes in grana membrane, when irradiation increases to somewhat higher density levels. Correspondingly, the average chlorophyll fluorescence lifetime of the highly isolated BBY fragments gradually decreases from ~1680 to ~1360 ps with increasing the irradiation density from 6.1×10(9) to 5.5×10(10) photon cm(-2) pulse(-1). Analysis of the relation of fluorescence decay change to the aggregation extent of LHCIIs suggests that a dense arrangement of trimeric LHCIIs is likely the structural base for the occurrence of this irradiation density dependent energy relaxation. Once altering the irradiation density, this energy relaxation is quickly reversible, implying that it may play an important role in photoprotection of plant PSII.

Singlet oxygen phosphorescence lifetime imaging based on a fluorescence lifetime imaging microscope.

The feasibility of singlet oxygen phosphorescence (SOP) lifetime imaging microscope was studied on a modified fluorescence lifetime imaging microscope (FLIM). SOP results from the infrared radiative transition of O2(a(1)Δg → X(3)Σg(-)) and O2(a(1)Δg) was produced in a C60 powder sample via photosensitization process. To capture the very weak SOP signal, a dichroic mirror was placed between the objective and tube lens of the FLIM and used to divide the luminescence returning from the sample into two beams: the reflected SOP beam and the transmitted photoluminescence of C60 (C60-PL) beam. The C60-PL beam entered the scanner of the FLIM and followed the normal optical path of the FLIM, while the SOP steered clear of the scanner and directly entered a finely designed SOP detection channel. Confocal C60-PL images and nonconfocal SOP images were then simultaneously obtained by using laser-scanning mode. Experimental results show that (1) under laser-scanning mode, the obstacle to confocal SOP imaging is the infrared-incompatible scanner, which can be solved by using an infrared-compatible scanner. Confocal SOP imaging is also expected to be realized under stage-scanning mode when the laser beam is parked and meanwhile a pinhole is added into the SOP detection channel. (2) A great challenge to SOP imaging is its extraordinarily long imaging time, and selecting only a few interesting points from fluorescence images to measure their SOP time-dependent traces may be a correct compromise.

Long-lifetime and asymmetric singlet oxygen photoluminescence from aqueous fullerene suspensions.

The photoexcited aqueous fullerene (C60) suspension was shown to exhibit an asymmetric photoluminescence (PL) spectrum, which, different from the symmetric spectrum observed previously in C60 solutions or suspensions, still stems from the characteristic phosphorescence of singlet oxygen (O2(a(1)Δ)) owing to its dependence on oxygen concentration. In contrast to the microsecond-level lifetime of O2(a(1)Δ) in water solutions, that in our C60 suspensions was measured at room temperature to be relatively long, about 2-3 ms, which is ~1000 times longer than the value reported by Bilski et al. The physical mechanism for the asymmetric O2(a(1)Δ) PL from C60 suspensions was studied in depth, indicating that it in fact originates from O2 molecules trapped in the C60 lattice within the suspended C60 aggregates (nC60). This mechanism, which can explain well our above results, was further validated by the nC60's high-resolution transmission electron microscopy (HRTEM) images with lattice fringes and the experimental temperature dependence of O2(a(1)Δ) lifetimes in nC60 suspensions. Our findings suggest that the bulk-phase O2(a(1)Δ) in aqueous nC60 suspensions results from the diffusion of the O2(a(1)Δ) generated within the interior of nC60 aggregates.

Study on gas phase collisional deactivation of O2(a1Δg) by alkanes and alkenes.

Systematic measurements were made on the deactivation rate constants (k(Δ)) of O(2)(a(1)Δ) by homologous series of gaseous n-alkanes and 1-alkenes by using our recently developed quasi-static method. The results indicate that the k(Δ) values for alkanes are in direct proportion to the number of C-H bonds (N(CH)) in the molecules, while those for alkenes are not, but being still linear with N(CH), which is in good agreement with Schmidt's E-V energy transfer model. The direct proportion and linearity relationship, respectively, for alkanes and alkenes were well explained in terms of the type and number of their C-H stretching vibrational modes, together with their corresponding vibrational constants. The physical mechanism for the linearity and additivity in Schmidt's model was also discussed in detail. In addition, the k(Δ) values for alkanes were found to be evidently smaller than those for alkenes with the same number of carbon atoms (n) for n < 4, while the situation is quite the contrary for n > 4, which was also rationalized in terms of E-V energy transfer mechanism, together with their respective C-H stretching vibrational modes.