•State transitions take place under various light conditions.•State transitions achieve a conserved level at a variable rate.•State transition stops as soon as OCP-mediated quenching appearance.•Appearance, level and rate of the quenching depends on light intensity.•Red-form OCPs undergo a deactivation under light.State transition and non-photochemical fluorescence quenching in cyanobacteria are short-term adaptations of photosynthetic apparatus to changes in light quality and intensity, however, the kinetic details and relationship are still not clear. In this work, time-dependent 77 K fluorescence spectra were monitored for cyanobacterium Synechocystis PCC 6803 cells under blue, orange and blue–green light in a series of intensities. The characteristic fluorescence signals indicated state transition taking place exclusively under 430–450 or 580–600 nm light or 480–550 nm light at the intensities ⩽150 μE m−2 s−1 to achieve a conserved level with variable rate constant. Under 480–500 nm or 530–550 nm light at the intensities ⩾160 μE m−2 s−1, state transition took place at first but stopped as soon as the fluorescence quenching appeared. The dependence of appearance, induction period, level and rate constant for the quenching on light intensity suggests that a critical concentration of photo-activated OCPs is necessary and may be achieved by a dynamic equilibrium between the activation and deactivation under light.

•Two hypocrellin derivatives 3 and 4 were designed and synthesized for PDT of AMD.•They exhibited the maximal absorption around yellow-orange light.•4 exhibited more photodynamic activity than 3 or its parent in vitro or in vivo.•The vascular leakage of 4 was close to its parent.•4 is more suitable for PDT of AMD than 3.The phototherapeutic window from 600 to 900 nm is necessary for photodynamic therapy (PDT) of solid tumors, but may not suitable for PDT of some microvascular diseases, like age-related macular degeneration (AMD), because of its deep penetration. Moreover, absorption of some neighboring photoreceptors should be avoided for PDT of AMD. Considering these, yellow-orange light may be a proper phototherapeutic window of AMD. Herein, two novel amino-alkyl-sulfonic acid-substituted hypocrellin B derivatives, 3 and 4 were designed and synthesized. They exhibited the maximal absorption at yellow-orange light, and possessed higher PDT activity than 2, proved by the in vitro or in vivo experiments. Besides, 4 showed much higher PDT activity than 3, ascribed to its higher cellular uptake suggested by its optimized amphiphilicity and liposome–mimic results. And the vascular leakage of 4 was close to 2. Consequently, 4 has great potential for PDT of AMD or other superficial diseases.

For photodynamic therapy (PDT) treatment of microvascular diseases, drugs are delivered via blood circulation and the targets are vasculature endothelial cells, for which the contradictory requirements of hydrophilicity and lipophilicity of the drugs have been achieved by liposome preparations. Herein, it is demonstrated that the drug delivery and target affinity are achieved by a single chemical compound, hypocrellin B (HB) derivative 6 selected from three novel aminoalkanesulfonic acid HB derivatives, 5–7. 6 exhibits a much higher PDT activity (IC50 = 22 nM) on human gastric carcinoma BGC823 cells than HB, while it has no cellular toxicity in the dark. On the basis of estimation of the clinically required concentration according to relative PDT activity and clinical criteria, it can be predicted that 6 is directly deliverable to and PDT effective on target cells. The enhanced red absorption and superhigh photoactivity suggest that 6 is more powerful for PDT of tumors than HB.

The mechanisms of oxygen evolution and carbon fixation in oxygenic organisms depend on the equal distribution of excitation energy to photosystems I and II, which is regulated by a mechanism referred to as light-state transition. In this work, a novel mechanism, energy spillover from PS I to PS II referred to as “inverse spillover”, was revealed besides “mobile phycobilisome (PBS)” and the “spillover” of energy from PS II to PS I in cyanobacteria. Under continuous illumination with blue light, time-dependent 77-K fluorescence spectra demonstrated heterogeneous kinetics for the PBS and photosystem components, indicating that inverse spillover and mobile PBS work successively to regulate the excitation to a balanced distribution in cyanobacterial cells under blue light. Inverse spillover and mobile PBS occur under both 100 and 300 μmol m−2 s−1 blue-light conditions but they are accelerated under the latter.

The two subunits of R-phycocyanin from Polysiphonia urceolata were isolated and renatured. The renatured subunits were characterized by electrophoresis, molecular weights and spectra. The blue-shifted spectra, fluorescence recovery and restoring of the energy transfer suggested correct refolding of the subunits. The molecular properties of the subunits in potassium phosphate buffer (KPB) were investigated in detail. The total fluorescence yields (QT) of the β subunit declined while the energy transfer efficiency (ET) in the β subunit was promoted with the increase of KPB concentration. On the other hand, both QT and ET were enhanced with the increasing of the subunit concentrations. Based on the structural information, the fluorescence quenching in high concentrations of KPB was ascribed to less rigid chromophores caused by the weakening of the hydrogen-bond interaction network, while the enhancement of the fluorescence and ET was due to the aggregation of the subunits in the ionic solvent. Aggregation was confirmed by cysteine-assisted promotion of renaturation yield and stability, as well as equilibrium unfolding tests. Optimal conditions were proposed for the refolding/unfolding studies, under which the subunits were mainly monomeric. Compared to that in C-PC, the blue-shifted spectrum of PCB in R-PC is suggested to bring larger energy transfer efficiency, probably due to the necessity of the light harvesting for P. urceolata living in deep water.