Unraveling the Intrinsic Color of Chlorophyll

A theory for colors of strongly correlated electronic systems

Many strongly correlated transition metal insulators are colored, even though they have band gaps much larger than the highest energy photons from the visible light. An adequate explanation for the color requires a theoretical approach able to compute subgap excitons in periodic crystals, reliably and without free parameters-a formidable challenge. The literature often fails to disentangle two important factors: what makes excitons form and what makes them optically bright. We pick two archetypal cases as examples: NiO with green color and MnF2 with pink color, and employ two kinds of ab initio many body Green's function theories; the first, a perturbative theory based on low-order extensions of the GW approximation, is able to explain the color in NiO, while the same theory is unable to explain why MnF2 is pink. We show its color originates from higher order spin-flip transitions that modify the optical response, which is contained in dynamical mean-field theory (DMFT). We show that symmetry lowering mechanisms may determine how 'bright' these excitons are, but they are not fundamental to their existence.

Molecular and genetic perspectives of cold tolerance in wheat

Environmental variation is the most crucial problem as it is causing food insecurity and negatively impacts food availability, utilization, assessment, and stability. Wheat is the largest and extensively cultivated staple food crop for fulfilling global food requirements. Abiotic stresses including salinity, heavy metal toxicity, drought, extreme temperatures, and oxidative stresses being the primary cause of productivity loss are a serious threat to agronomy. Cold stress is a foremost ecological constraint that is extremely influencing plant development, and yield. It is extremely hampering the propagative development of plant life. The structure and function of plant cells depend on the cell's immune system. The stresses due to cold, affect fluid in the plasma membrane and change it into crystals or a solid gel phase. Plants being sessile in nature have evolved progressive systems that permit them to acclimatize the cold stress at the physiological as well as molecular levels. The phenomenon of acclimatisation of plants to cold stress has been investigated for the last 10 years. Studying cold tolerance is critical for extending the adaptability zones of perennial grasses. In the present review, we have elaborated the current improvement of cold tolerance in plants from molecular and physiological viewpoints, such as hormones, the role of the posttranscriptional gene, micro RNAs, ICE-CBF-COR signaling route in cold acclimatization and how they are stimulating the expression of underlying genes encoding osmoregulatory elements and strategies to improve cold tolerance in wheat.

Thermal site energy fluctuations in photosystem I: new insights from MD/QM/MM calculations

Cyanobacterial photosystem I (PSI) is one of the most efficient photosynthetic machineries found in nature. Due to the large scale and complexity of the system, the energy transfer mechanism from the antenna complex to the reaction center is still not fully understood. A central element is the accurate evaluation of the individual chlorophyll excitation energies (site energies). Such an evaluation must include a detailed treatment of site specific environmental influences on structural and electrostatic properties, but also their evolution in the temporal domain, because of the dynamic nature of the energy transfer process. In this work, we calculate the site energies of all 96 chlorophylls in a membrane-embedded model of PSI. The employed hybrid QM/MM approach using the multireference DFT/MRCI method in the QM region allows to obtain accurate site energies under explicit consideration of the natural environment. We identify energy traps and barriers in the antenna complex and discuss their implications for energy transfer to the reaction center. Going beyond previous studies, our model also accounts for the molecular dynamics of the full trimeric PSI complex. Via statistical analysis we show that the thermal fluctuations of single chlorophylls prevent the formation of a single prominent energy funnel within the antenna complex. These findings are also supported by a dipole exciton model. We conclude that energy transfer pathways may form only transiently at physiological temperatures, as thermal fluctuations overcome energy barriers. The set of site energies provided in this work sets the stage for theoretical and experimental studies on the highly efficient energy transfer mechanisms in PSI.

Chlorophyll- and anthocyanin-rich cell organelles affect light scattering in apple skin

Apple skin contains several groups of strongly absorbing cell organelles with pigments that change dynamically in type and concentration during fruit maturation. Chlorophylls and carotenoids, both primarily involved in photosynthesis, are found in the grana of chloroplasts, while anthocyanin vacuolar inclusions (AVIs) accumulate for light protection in red-skinned cultivars. A Mie model describing light scattering by absorbing spherical particles in a non-absorbing medium allowed to theoretically investigate the explicit influence of grana and AVIs on the effective scattering coefficient [Formula: see text] and the absorption coefficient [Formula: see text]. The reconstruction of the complex refractive indices of the organelles predicted anomalous dispersion, i.e., a local increase in the real part of the refractive index in the spectral regions with high chlorophyll and anthocyanin absorption, in agreement with the Kramers-Kronig relations. As a result, peaks in [Formula: see text] were predicted to be shifted to longer wavelengths compared to the corresponding [Formula: see text] bands. This selective scattering effect was confirmed experimentally with integrating sphere measurements for red- or green-skinned apple samples of the cultivars 'Elstar', 'Gala' or 'Jonagold'. Comparison between simulations and measurements indicated that the Soret bands of chlorophyll a and chlorophyll b are at 435 nm and 469 nm, respectively, and overlap with the absorption of carotenoids, whose red-most edge is at 488 nm. For anthocyanin absorption, a pronounced blue shift from 550 to 520 nm was observed, indicating structural or chemical changes of AVIs.

Monochromatic light filters to enhance biomass and carotenoid productivities of Dunaliella salina in raceway ponds

Monochromatic blue and red wavelengths are more efficient for light to algal biomass conversion than full-spectrum sunlight. In this study, monochromatic light filters were used to down-regulate natural sunlight to blue (400-520 nm) and red (600-700 nm) wavelengths to enhance biomass productivity of Dunaliella salina in outdoor raceway ponds. Growth indices such as cell size, pigment concentrations, biomass yield, photosynthetic efficiency, and major nutritional compositions were determined and compared against a control receiving unfiltered sunlight. Results showed that red light increased biomass productivity, lipid, and carotenoid contents but decreased cell volume, chlorophyll production, and cell weight. Conversely, blue light increased cell volume by 200%, cell weight by 68%, and enhanced chlorophyll a and protein contents by 35% and 51%, respectively, over red light. Compared to the control treatment, photoinhibition of D. salina cells at noon sunshine was decreased 60% by utilizing optical filters on the pond's surface.

Visible Light-Induced Antibacterial Activity of Pigments Extracted from Dregs of Green and Black Teas

Chlorophyll and its derivatives are potential natural sensitizers frequently applied in antimicrobial photodynamic therapy. Chlorophyll derivatives are formed naturally during tea processing, but they do not contribute to the color of tea infusions and thus are presumably left in the tea dregs. The present study aimed to investigate (i) the chlorophyll remnants in the pigments recovered from dregs of green and black teas and (ii) the antibacterial activity of pigments extracted from the tea dregs upon illumination using a light-emitting diode (LED) as the light source. Pigment analysis using high-performance liquid chromatography (HPLC) revealed the presence of main degradation products of chlorophylls, such as pheophytin and its epimers, pyropheophytin, and pheophorbides. In vitro assays demonstrated significant reductions in the number of viable bacteria in the presence of the pigments after 30 min of incubation with LED light irradiation. The descending order of bacterial susceptibility was Listeria monocytogenes > Staphylococcus aureus > Escherichia coli > Salmonella typhi. At an equivalent irradiation intensity, the blue and red LEDs could stimulate a comparable inactivation effect through photodynamic reactions. These findings demonstrated the valorization potential of tea dregs as a source of chlorophyll derivatives with visible light-induced antibacterial activity.

Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone

A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.

Intrinsic Photophysics of Light-harvesting Charge-tagged Chlorophyll a and b Pigments

Chlorophylls a and b (Chla/b) are responsible for light-harvesting by photosynthetic proteins in plants. They display broad absorption in the visible region with multiple bands, due to the asymmetry of the macrocycle and strong vibronic coupling. Their photophysics relies on the microenvironment, with regard to transition energies as well as quenching of triplet states. Here, we firmly establish the splitting of the Q and Soret bands into x- and y- polarized bands for the isolated molecules in vacuo, and resolve vibronic features. Storage-ring experiments reveal that dissociation of photoexcited charge-tagged complexes occurs over several milliseconds, but with two different time constants. A fast decay is ascribed to dissociation after internal conversion and a slow decay to the population of a triplet state that acts as a bottleneck. Support for the latter is provided by pump-probe experiments, where a second laser pulse probes the long-lived triplet state.