Light-Quality-Adapted Carotenoid Photoprotection in the Photosystem of Roseiflexus castenholzii.

The photosystem of filamentous anoxygenic phototroph Roseiflexus (Rfl.) castenholzii comprises a light-harvesting (LH) complex encircling a reaction center (RC), which intensely absorbs blue-green light by carotenoid (Car) and near-infrared light by bacteriochlorophyll (BChl). To explore the influence of light quality (color) on the photosynthetic activity, we compared the pigment compositions and triplet excitation dynamics of the LH-RCs from Rfl. castenholzii was adapted to blue-green light (bg-LH-RC) and to near-infrared light (nir-LH-RC). Both LH-RCs bind γ-carotene derivatives; however, compared to that of nir-LH-RC (12%), bg-LH-RC contains substantially higher keto-γ-carotene content (43%) and shows considerably faster BChl-to-Car triplet excitation transfer (10.9 ns vs 15.0 ns). For bg-LH-RC, but not nir-LH-RC, selective photoexcitation of Car and the 800 nm-absorbing BChl led to Car-to-Car triplet transfer and BChl-Car singlet fission reactions, respectively. The unique excitation dynamics of bg-LH-RC enhances its photoprotection, which is crucial for the survival of aquatic anoxygenic phototrophs from photooxidative stress.

Daphnia magna uptake and excretion of luminescence-labelled polystyrene nanoparticle as visualized by high sensitivity real-time optical imaging.

The environmental and ecological consequences of nanoplastics (NPs) draw increasing research interests and social concerns. However, the in situ and real-time detection of NPs from living organisms and transferring media remains as a major technical obstacle for scientific investigation. Herein we report a novel time-gated imaging (TGI) strategy capable of real-time visualizing the intake of NPs by an individual living organism, which is based on the polystyrene NPs labelled with lanthanide up-conversion luminescence. The limit of detection (LOD) of the TGI apparatus was 600 pg (SNR = 3) in a field of view of 2.4 × 3.8 mm. Taking Daphnia magna as the aquatic model, we investigated the dynamics of uptake and accumulation of NPs (500 µg/L) for 24 h, and the subsequent excretion process (in clean medium) for 48 h, and quantitively analyzed the distribution and the overall mass of NPs deposited in D. magna. The uptake of NPs via filter-feeding occurred in a few minutes, whereas a longer accumulation was found, in a timescale of several hours. And similar behaviors (bi-phase elimination) were also seen in the excretion, indicating the migration of NPs into the circulatory system. The average mass of NPs accumulated in an individual D. magna was ∼12 ng after 24 h exposure, indicating that D. magna as a filter feeder tends to retain NPs. The observed NPs accumulation in D. magna exemplifies the potential risk of aquatic ecosystem on exposure to NP contamination.

Rhodobacter sphaeroides as a model to study the ecotoxicity of 1-alkyl-3-methylimidazolium bromide.

The purple non-sulfur bacterium Rhodobacter sphaeroides was selected as a biological model to investigate its response to the toxicity of 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br), a type of ionic liquid (IL), with different alkyl chain lengths (n describes the number of carbon atoms in the alkyl chain). The inhibition of bacterial growth by [Cnmim]Br was positively correlated with n. Morphological characterization revealed that [Cnmim]Br caused cell membrane perforation. The signal amplitude of the electrochromic absorption band shift of endogenous carotenoids showed a negatively linear correlation with n, and the amplitude of the blue-shift of the B850 band in light-harvesting complex 2 showed a positively linear correlation with n. Furthermore, an increase in blocked ATP synthesis and increase in antioxidant enzyme activity were observed in chromatophores treated with ILs containing longer alkyl chains. In summary, the purple bacterium can be developed as a model to monitor ecotoxicity and examine the mechanism of IL toxicity.

Photoinduced chlorophyll charge transfer state identified in the light-harvesting complex II from a marine green alga Bryopsis corticulans.

The light-harvesting complex II of Bryopsis corticulans (B-LHCII), a green alga, differs from that of spinach (S-LHCII) in chlorophyll (Chl) and carotenoid (Car) compositions. We investigated ultrafast excitation dynamics of B-LHCII with visible-to-near infrared time-resolved absorption spectroscopy. Absolute fluorescence quantum yield (Φ FL) of LHCII and spectroelectrochemical (SEC) spectra of Chl a and b were measured to assist the spectral analysis. Red-light excitation at Chl Qy-band, but not Car-band, induced transient features resembling the characteristic SEC spectra of Chl a ⋅+ and Chl b ⋅-, indicating ultrafast photogeneration of Chl-Chl charge transfer (CT) species; Φ FL and 3Car∗ declined whereas CT species increased upon prolonging excitation wavelength, showing positive correlation of 1Chl∗ deactivation with Chl-Chl CT formation. Moreover, ultrafast Chl b-to-Chl a and Car-to-Chl singlet excitation transfer were illustrated. The red-light induction of Chl-Chl CT species, as also observed for S-LHCII, is considered a general occurrence for LHCIIs in light-harvesting form.

A Possible Mechanism for Aggregation-Induced Chlorophyll Fluorescence Quenching in Light-Harvesting Complex II from the Marine Green Alga Bryopsis corticulans.

The light-harvesting complex II of a green alga Bryopsis corticulans (B-LHCII) is peculiar in that it contains siphonein and siphonaxathin as carotenoid (Car). Since the S1 state of siphonein and siphonaxathin lies substantially higher than the Qy state of chlorophyll a (Chl a), the Chl a(Qy)-to-Car(S1) excitation energy transfer is unfeasible. To understand the photoprotective mechanism of algal photosynthesis, we investigated the influence of temperature on the excitation dynamics of B-LHCII in trimeric and aggregated forms. At room temperature, the aggregated form showed a 10-fold decrease in fluorescence intensity and lifetime than the trimeric form. Upon lowering the temperature, the characteristic 680 nm fluorescence (F-680) of B-LHCII in both forms exhibited systematic intensity enhancement and spectral narrowing; however, only the aggregated form showed a red emission extending over 690-780 nm (F-RE) with pronounced blueshift, lifetime prolongation, and intensity boost. The remarkable T-dependence of F-RE is ascribed to the Chl-Chl charge transfer (CT) species involved directly in the aggregation-induced Chl deactivation. The CT-quenching mechanism, which is considered to be crucial for B. corticulans photoprotection, draws strong support from the positive correlation of the Chl deactivation rate with the CT state population, as revealed by comparing the fluorescence dynamics of B-LHCII with that of the plant LHCII.

Spatial effects of photosensitization on morphology of giant unilamellar vesicles.

Singlet oxygen (1O2) formed through photosensitization may initiate oxidative destruction of biomembranes, however, the influence from the spatial organization of photosensitizers (PS) relative to membranes remains unclear. To clarify this issue, we loaded riboflavin 5'-(dihydrogen phosphate) monosodium (FMN-Na) as a hydrophilic PS into the lumen of halloysite nanotubes (HNTs), and attached the nanoassemblies (FMN-Na@HNTs), via Pickering effects, to the outer surfaces of giant unilamellar vesicles (GUVs) of phospholipids. We also prepared GUVs dopped with lumiflavin (LF) as a lipophilic PS having a 1O2 quantum yield comparable to FMN-Na. FMN-Na capsulated in HNT was characterized by a longer triplet excited state lifetime (12.1 µs) compared to FMN-Na free in solution (7.5 µs), and FMN-Na in both forms efficiently generated 1O2 upon illumination. The spatio-effects of PS on the photosensitized morphological changes of membranes were studied using conventional optical microscopy by monitoring GUV morphological changes. Upon light exposure (400-440 nm), the GUVs attached with FMN-Na@HNT merely experienced membrane deformation starting from the original spherical shape, ascribed to Type II photosensitization with 1O2 as oxidant. In contrast, photooxidation of LF dopped GUVs mainly led to membrane coarsening and budding assigned to Type I photosensitization. The spatial effects of PS on photosensitized morphological changes were related to the different lipid oxidation products generated through Type I and Type II photosensitized lipid oxidation.

Promotion effects of flavonoids on browning induced by enzymatic oxidation of tyrosinase: structure-activity relationship.

Tyrosinase, widely distributed in nature, is a copper-containing polyphenol oxidase involved in the formation of melanin. Flavonoids are most often considered as tyrosinase inhibitors but have also been confirmed to be tyrosinase substrates. Four structure-related flavonoids including flavones (apigenin and luteolin) and flavonols (kaempferol and quercetin) are found to promote not inhibit browning induced by tyrosinase catalyzed oxidation both in model systems and in mushrooms under aerobic conditions. A comparison with enzymatic oxidation and autooxidation of flavonoids alone has helped to clarify why flavonoids function as a substrate rather than an inhibitor. Flavonoids almost do not affect the kinetics of melanin formation from enzymatic oxidation of l-dopa in excess. In addition, a new brown complex formed during the reaction of flavonoid quinone and dopaquinone is suggested to enhance the browning effects by competing with isomerization and autooxidation. Structure-activity relationships of the four flavonoids in melanin formation leading to browning induced by autooxidation and enzymatic oxidation confirm the enzymatic nature of the browning.

Naturally occurring nanotube with surface modification as biocompatible, target-specific nanocarrier for cancer phototherapy.

Phototherapy has drawn increasing attention including the use of nanocarriers with high drug loading capacity and delivery efficacy for target-specific therapy. We have made use of naturally-occurring halloysite nanotubes (HNTs) to build a biomimetic nanocarrier platform for target-specific delivery of phototherapeutic agents. The HNTs were decorated with poly(sodium-p-styrenesulfonate) (PSS) to enhance the biocompatibility, and were further functionalized by lumen loading the type-II photosensitizer indocyanine green (ICG). The HNT-PSS-ICG nanocarrier, without further tethering targeting groups, was shown to associate with the membrane of giant unilamellar vesicles (GUVs) via Pickering effects. Application of HNT-PSS-ICG nanocarrier to human breast cancer cells gave rise to a cell mortality as high as 95%. The HNT-PSS-ICG nanocarrier was further coated with MDA-MB-436 cell membranes to endow it with targeting therapy performance against breast cancer, which was confirmed by in vivo experiments using breast cancer tumors in mice. The membrane-coated and biocompatible nanocarrier preferentially concentrated in the tumor tissue, and efficiently decreased the tumor volume by a combination of photodynamic and photothermal effects upon near-infrared light exposure. Our results demonstrate that the HNT-based nanocarrier by virtue of facial preparation and high loading capacity can be a promising candidate for membrane-targeting nanocarriers.