Properties of hybrid complexes composed of photosynthetic reaction centers from the purple bacterium Rhodobacter sphaeroides and quantum dots in lecithin liposomes.

Quantum dots (QDs) can absorb ultraviolet and long-wavelength light energy much more efficiently than natural light-harvesting proteins and transfer the excitation energy to photosynthetic reaction centers (RCs). Inclusion into liposomes of RC membrane pigment-protein complexes combined with QDs as antennae opens new opportunities for using such hybrid systems as a basis for artificial energy-transforming devices that potentially can operate with greater efficiency and stability than devices based only on biological components. RCs from Rhodobacter sphaeroides and QDs with fluorescence maximum at 530 nm (CdSe/ZnS with hydrophilic covering) were embedded in lecithin liposomes by extrusion of a solution of multilayer lipid vesicles through a polycarbonate membrane or by dialysis of lipids and proteins dispersed with excess detergent. The dimensions of the resulting hybrid systems were evaluated using dynamic light scattering and by transmission cryoelectron microscopy. The efficiency of RC and QD interaction within the liposomes was estimated using fluorescence excitation spectra of the photoactive bacteriochlorophyll of the RCs and by measuring the fluorescence decay kinetics of the QDs. The functional activity of the RCs in hybrid complexes was fully maintained, and their stability was even increased.

Examination of chlorophyll fluorescence decay kinetics in sulfur deprived algae Chlamydomonas reinhardtii.

Chlorophyll fluorescence decay kinetics was measured in sulfur deprived cells of green alga Chlamydomonas reinhardtii with a home made picosecond fluorescence laser spectrometer. The measurements were carried out on samples either shortly adapted to the dark ('Fo conditions') or treated to reduce Qa ('Fm conditions'). Bi-exponential fitting of decay kinetics was applied to distinguish two components one of them related to energy trapping (fast component) and the other to charge stabilization and recombination in PS 2 reaction centers (slow component). It was found that the slow component yield increased by 2.0 and 1.2 times when measured under 'Fo' and 'Fm conditions', respectively, in sulfur deprived cells as compared to control ones. An additional rapid rise of the slow component yield was observed when incubation was carried out in a sealed bioreactor and cell culture turned to anaerobic conditions. The obtained results strongly indicate the existence of the redox control of PS 2 activity during multiphase adaptation of C. reinhardtii to sulfur deficiency stress. Probable mechanisms responsible for the observed increased recombinant fluorescence yield in starved cells are discussed.