•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.

State transition and blue-green light-induced fluorescence quenching are two short-term processes in cyanobacteria. The details of their kinetics and the relationship between these processes have not been elucidated. In this work, these two processes were studied in the wild-type cyanobacterium Synechocystis PCC 6803 cells as well as in apcD− and apcF− mutants by monitoring their time-dependent 77 K fluorescence responses to blue-green light (430–540 nm) at a series of intensities ranging from 20–800 µE m−2 s−1. The lowest light intensity to induce fluorescence quenching in wild-type cells was 160 µE m−2 s−1 under the selected experimental conditions, while state transition took place at the intensities lower than 160 µE m−2 s−1 at a conservative level, but at variable rates. The quenching level increased at intensities higher than 160 µE m−2 s−1, reaching the maximum level at intensities equal to or higher than 200 µE m−2 s−1. Fluorescence kinetics indicated that both the length of the induction period and time required to reach the maximum level were functions of light intensity. State transitions as well as fluorescence quenching took place in both wild-type and mutant cells, but might involve different mechanisms.

•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.

Transition-metal borides MB0.5 (M = Co, Mo, V) were synthesized by high-speed mechanical ball-milling of the corresponding elemental metals and boron, and investigated as aqueous anode materials. The as-synthesized borides can achieve an excellent discharge capacity, about twice that of their parent transition metals. The metal boride electrodes also exhibit polarizations about 100–300 mV lower than those of their parent metals. The galvanostatic discharge curve of CoB0.5 shows a single discharge voltage plateau as a result of simultaneous electro-oxidation of elemental cobalt and/or amorphous cobalt boride. Both MoB0.5 and VB0.5 show two well-defined voltage plateaus, corresponding to the electro-oxidation of the corresponding metal and boride. These results show that the coexisting transition metal and boride in the metal borides co-activate each other in the ball-milling process, thereby significantly enhancing their electrochemical performances.

Elsinochrome A (EA) possesses the highest singlet-oxygen quantum yield (0.98) amongst the perilenoquinoid pigments and may be suitable as a phototherapeutic drug. However, there have been virtually no studies into its medicinal applications. Based on the analysis of chemical derivatives of hypocrellins (the same family as EA), 5-(3-mercapto-1-propanesulfonic acid)-substituted elsinochrome A (MPEA) with an amphiphilicity was designed and synthesized by considering drug delivery and biological activity requirements. MPEA possesses a water solubility of 5.1 mg mL−1, which is just sufficient to enable dissolution at a clinically acceptable concentration, while its partition coefficient (n-octanol/phosphate buffered saline) of 7 guarantees affinity to biological targets. MPEA could photogenerate semiquinone anion radicals and reactive oxygen species, especially singlet oxygen, at a yield of 0.73, which approaches that for hypocrellin B. Biological tests confirmed that the photodynamic activity of MPEA was as high as 60% of that of its parent EA, which is significantly higher than that of most other photosensitizers.

To satisfy the dual requirements of the fluent transportation in blood and the affinity to the target tissues of vascular diseases, hypocrellin derivatives with optimized amphiphilicity are expected. In this work, 3-amino-1-propanesulfonic acid and 4-amino-1-butanesulfonic acid substituted hypocrellin B, named compounds 1 and 2, were designed, synthesized in high yields and characterized. Besides greatly strengthened red absorption, the maximum solubility of compound 2 in phosphate buffered saline (PBS) is 4.2 mg/mL which is just enough to prepare an aqueous solution for intravenous injection in clinically acceptable concentration, while the partition coefficient between n-octanol and PBS, 5.6, benefits the cell-uptake and biological activity as well. Furthermore, EPR measurements reveal that the photosensitization activities of the two compounds to generate semiquinone anion radicals, superoxide anion radicals and singlet oxygen are a little bit higher than those of taurine substituted hypocrellin B (THB), but the photodynamic activities to human lung cancer A549 cells are several times that of THB, mainly due to increases in lipophilicity and cell-uptake.

For making hypocrellins clinically applicable for phototherapy to vascular diseases, it is mainly focused onto finding a derivative which can be transported fluently in blood system but without serious loss of the inherent activity of its parents. Based on this consideration, a novel 17-3-amino-1-propane-sulfonic acid-HB Schiff-base (NSHB) was designed and synthesized in this work. As expected, NSHB is readily dissolved in phosphate buffered saline (PBS) or any other aqueous solvent in a concentration which is suitable for intravenous injection, while the quite higher partition coefficient (5:1) is beneficial to the affinity to biological targets. Based on EPR measurements, it is proved that the photosensitization activity of NSHB to photo-generate semiquinone anion radicals and superoxide anion radical (O2-) is even higher than its parent HB, while the ability to generate singlet oxygen (1O21O2) is not seriously reduced. In addition, nearly comparable PDT activity to A549 cells for NSHB and HB confirms that the molecular design is successful and NSHB is readily delivered into target tissues via blood circulation after intravenous injection. Furthermore, the quantum yield of 1O21O2 for NSHB is as 12.5 times as that for HB under red light (600–700 nm), which is beneficial to phototherapy to solid tumors.

Light state transition is a physiological function of oxygenic organisms to balance the excitation of photosystem II (PSII) and photosystem I (PSI), hence a prerequisite of oxygen-evolving photosynthesis. For cyanobacteria, phycobilisome (PBS) movement during light state transition has long been expected, but never observed. Here the dynamic behavior of PBS movement during state transition in cyanobacterium Synechocystis PCC6803 is experimentally detected via time-dependent fluorescence fluctuation. Under continuous excitation of PBSs in the intact cells, time-dependent fluorescence fluctuations resemble “damped oscillation” mode, which indicates dynamic searching of a PBS in an “overcorrection” manner for the “balance” position where PSII and PSI are excited equally. Based on the parallel model, it is suggested that the “damped oscillation” fluorescence fluctuation is originated from a collective movement of all the PBSs to find the “balance” position. Based on the continuous fluorescence fluctuation during light state transition and also variety of solar spectra, it may be deduced that light state transition of oxygen-evolution organisms is a natural behavior that occurs daily rather than an artificial phenomenon at extreme light conditions in laboratory.

Monomerization and trimerization of photosystem I (PSI) in cyanobacteria are reversible to response to light switched off and on, which leads to “energy spillover” to regulate excitation of the two photosystems in balance. Considering that PSI is a trans-membrane protein embedded in thylakoid membranes, the monomerization or trimerization must involve a movement of PSI in the membranes. In this work, the mobility of PSI was demonstrated by dependence of the monomerization and trimerization on temperature for intact Spirulina platensis cells undergoing a light-to-dark or a dark-to-light transition. Based on the characteristic absorbance of monomers and trimmers, it confirms that both monomerization and trimerization are temperature-sensitive. The relative populations of the monomers and trimmers are invariable above the phase transition temperature (TPT) while directly proportional to temperature below TPT. On the other hand, the rate to reach the equilibrium population is proportional to temperature above TPT but invariable below TPT. The PSI mobility and the temperature-dependent population are contrary to those of plastoquinone (PQ) molecules because PSI is a trans-membrane protein while PQ molecules are small diffusive electron carriers in thylakoid membranes as well as their distinctive sizes and environments. The less monomerization of PSI but the invariable time constant at lower temperature below TPT may be due to that accumulation of the reduced PQ molecules results in decrease of the stromal-side H+ concentration which is a driving force of PSI monomerization.

Glycine betaine (GB) is a biologically important small molecule protecting cells, proteins and enzymes in vivo and in vitro under environmental stresses. Recently, it was found that GB could also relax the structure and inactivate the function of phycobiliproteins and phycobilisome (PBS), a kind of supra-molecular complexes, in cyanobacterial cells. The molecular mechanisms for the opposite phenomena are quite ambiguous. Taking PBS and a trimeric or monomeric C-phycocyanin (C-PC) as models, the molecular mechanism for the interaction of GB with supra-molecular complexes or nuclear proteins was investigated. The energetic decoupling of PBS components induced by GB suggests that the PBS core-membrane linking polypeptide was the most sensitive site while the rod-core linker was the next. Biochemistry analysis proves that PBS structure was loosened but not dissociated into the components. On the basis of the results and structure knowledge, it was proposed that GB screened the electrostatic attraction of the opposite charges on a linker and a protein leading to a much looser structure. It was observed that GB induced a spectral blue shift for trimeric C-PC but a red shift for a monomeric C-PC (a nuclear protein), which were ascribed to GB’s screening of the electrostatic attraction of a linker to a protein and strengthening of the hydrophobic interaction between C-PC monomers. The trimers and monomers’ forming of the same products under high concentration of GB was ascribed to a compromise of the opposite interaction forces.

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.

Previously, it was clarified that phycobilisome (PBS) mobility and energy spillover were both involved in light-to-dark induced state transitions of intact Spirulina platensis cells. In this work, by taking advantage of the characteristic fluorescence spectra of photosystem I (PSI) trimers and monomers as indicators, the relative contributions for the “mobile PBS” and “energy spillover” are quantitatively estimated by separating the fluorescence contribution of PBS mobility from that of PSI oligomeric change. Above the phase transition temperature (TPT) of the membrane lipids, the relative proportion of the contributions is invariable with 65% of “mobile PBS” and 35% of “energy spillover”. Below TPT, the proportion for the “mobile PBS” becomes larger under lowering temperature even reaching 95% with 5% “energy spillover” at 0°C. It is known that lower temperature leads to a further light state due to a more reduced or oxidized PQ pool. Based on the current result, it can be deduced that disequilibrium of the redox state of the PQ pool will trigger PBS movement instead of change in the PSI oligomeric state.

Temperature effects on state transitions have been studied in the cyanobacterium Spirulina platensis. At lower temperatures the time to reach completion took longer and the extent of the state transitions was larger. Effects were limited to the temperature range below the phase transition temperature of the membrane lipids. In the presence of the artificial electron acceptor phenyl-1,4-benzoquinone (PBQ) state transitions became completely temperature-independent. State transitions induced by a change in the light climate or in darkness by a switch from aerobic to anaerobic conditions responded similar to temperature; the occurrence of state transitions solely by a change of the temperature has been excluded. Our conclusion is that the temperature-dependent mobility of plastoquinone molecules in the thylakoid membranes is the intrinsic cause of temperature effects on state transitions.

A novel 2-taurine substituted hypocrellin B (THB) was designed and synthesized in this work. Both the light absorbance in the phototherapeutic window (600–900 nm) and the amphiphilicity (a compromise between the lipophilicity and hydrophilicity) were greatly improved. The semiquinone anion radical (THB˙−), superoxide anion (O2˙−), hydroxyl radical (˙OH) (Type I mechanism) and singlet oxygen (1O2) (Type II mechanism) could be generated via the photosensitization of THB, confirming its photosensitization activity. In this work, it was also found that the cytochrome c reduction method is not the exclusive one for detecting O2˙− in photosensitization systems.

Not only the photosensitivity properties but also the amphiphilicity (both hydrophilicity and lipophilicity) are important factors for hypocrellins to be clinically applicable to photodynamic therapy of vas capillary diseases. A chemically modified derivative, 14-carboxyl hypocrellin B (HBO2H), proved to possess far better amphiphilicity than its parent hypocrellin B (HB) and hypocrellin A (HA). In this paper, the photophysical and photochemical properties of HBO2H were investigated by spectrophotometric methods and electron paramagnetic resonance (EPR) and its photodynamic action on mouse endothelial cells was also confirmed. In oxygen-free DMSO solution, semiquinone anion radicals (HBO2H˙−) are photogenerated via electron transfer between the excited triplet HBO2H and electron donors or ground state HBO2H (Type I mechanism). When oxygen is present, superoxide anion radicals (O2˙−) are generated via electron transfer from HBO2H˙− to the ground state oxygen molecules. Singlet oxygen (1O2) can be produced via energy transfer from the triplet state HBO2H to the ground state oxygen molecules (Type II mechanism). The quantum yield of singlet oxygen was estimated to be 0.63 in CHCl3 with 0.76 for HB as a reference. Furthermore, investigations on the competition and transformation between 1O2 and HBO2H˙− suggested that the relative importance of Type I and Type II reactions would depend on the oxygen content in the target tissue.