BIOCHEMISTRY: Water Photolysis in Biology

Photochemistry of Photoinduced-Reaction Generated Bubbles

UV light can create and grow bubbles (herein referred to as PIRGBs for photoinduced-reaction generated bubbles) at liquid/solid interfaces through photoinduced reactions that produce gases. Unlike the simple experience of blowing water bubbles through a straw, in which the bubbles quickly move away from their nucleation sites, not only can a deep UV laser beam create PIRGBs in liquid acetone, but also can hold and grow them. Free bubbles could be attracted to the excitation region from millimeters away, indicating that the reactions cause radial inward flow on the liquid surface. The radial flow can be due to imbalanced surface tensions at the interfaces. Raman measurements reveal that the gases in the PIRGBs include C2H6, CO, and H2, and in liquid acetone, sp2-carbon species are detected upon the UV excitation. Time series Raman measurement discloses a photocarbonization process in which small acyclic carbon species gradually form small clusters with carbon rings and eventually produce a large piece of amorphous carbon at the top of a PIRGB in pure liquid acetone. The photocarbonization may open new avenues for development of carbonaceous materials. Using PIRGB, miniature or microscale gas production reactors can be developed for producing gases.

Do water and soil nutrient scarcities differentially impact the performance of diploid and tetraploid Solidago gigantea (Giant Goldenrod, Asteraceae)?

Plants require water and nutrients for survival, although the effects of their availabilities on plant fitness differ amongst species. Genome size variation, within and across species, is suspected to influence plant water and nutrient requirements, but little is known about how variations in these resources concurrently affect plant fitness based on genome size. We examined how genome size variation between autopolyploid cytotypes influences plant morphological and physiological traits, and whether cytotype-specific trait responses differ based on water and/or nutrient availability. Diploid and autotetraploid Solidago gigantea (Giant Goldenrod) were grown in a greenhouse under four soil water:N+P treatments (L:L, L:H, H:L, H:H), and stomata characteristics (size, density), growth (above- and belowground biomass, R/S), and physiological (A_net , E, WUE) responses were measured. Resource availabilities and cytotype identity influenced some plant responses but their effects were independent of each other. Plants grown in high-water and nutrient treatments were larger, plants grown in low-water or high-nutrient treatments had higher WUE but lower E, and A_net and E rates decreased as plants aged. Autotetraploids also had larger and fewer stomata, higher biomass and larger A_net than diploids. Nutrient and water availability could influence intra- and interspecific competitive outcomes. Although S. gigantea cytotypes were not differentially affected by resource treatments, genome size may influence cytogeographic range patterning and population establishment likelihood. For instance, the larger size of autotetraploid S. gigantea might render them more competitive for resources and niche space than diploids.

Exploring solvent dependent catecholase activity in transition metal complexes: an experimental and theoretical approach

Four coordination compounds are designed with pyridinemethanol ligands, characterized with spectral, magnetic and X-ray analyses, and assessed for catecholase activity in various solvents.

Co-existence of ferro- and antiferromagnetic interactions in a hexanuclear mixed-valence CoMnMn cluster sustained by a multidentate Schiff base ligand

The successful utilization of the "direct synthesis" approach yielded the unprecedented hexanuclear complex of formula [Co₂MnMn(L¹)₄Cl₂(μ₃-O)₂(dmf)₄]·2dmf (1) (H₃L is the Schiff base derived from the condensation of salicylaldehyde and 3-aminopropane-1,2-diol). Single crystal X-ray analysis revealed that 1 crystallizes in the monoclinic system P2₁/c and it contains a rare mixed-valence {CoMnMn(μ₂-O)₈(μ₃-O)₂} core where all metal ions are linked through the phenolato and alkoxo groups of the L^3- ligand. Besides the charge balance resulting from the X-ray structure, the oxidation state of the metal ions has been confirmed by XPS spectroscopy. Cryomagnetic studies indicate the coexistence of ferro- (Mn^IV-Mn^II, J₂ = +1.10(3) cm^-1, J₃ = +2.19(3) cm^-1; Mn^II-Mn^II, j = +0.283(3) cm^-1) and antiferromagnetic interactions (Mn^IV-Mn^IV, J₁ = -17.31(4) cm^-1), with the six-coordinate Co^III ions being diamagnetic. DFT type calculations were carried out to substantiate these values. The energy diagram for the different spin states using the best-fit parameters shows the occurrence of six low-lying spin states (S = 0-5) which are close in energy but clearly separated from the remaining ones, with the ground spin state being S = 5. Complex 1 is found to be the first example where weak ferromagnetic exchange between Mn^II ions through the long -O-Mn^IV-O- pathway takes place.

Importance of C–H⋯π interactions in stabilizing the syn/anti arrangement of pendant alkoxy side arms in two manganese(iv) Schiff base complexes: exploration of catechol oxidase and phenoxazinone synthase like activities

Two manganese(iv) complexes have been prepared and characterized. Both complexes exhibit catechol oxidase and phenoxazinone synthase like activities.

Hexagonal nanoplates of NiO/CoO/Fe2O3 composite acting as an efficient photocatalytic and electrocatalytic water oxidation catalyst

A unique hexagonal sheet-shaped NiO/CoO/Fe2O3 composite with irregularly shaped nanoparticles was fabricated for the first time through a simple co-precipitation and hydrothermal method. The NiO/CoO/Fe2O3 composite was characterized by numerous techniques (TEM, HRTEM, PXRD, EDX, ICP-AES, BET, and XPS) to confirm its structure and composition. This structure of the NiO/CoO/Fe2O3 composite may enhance the photocatalytic and electrocatalytic performance for water oxidation. Compared with NiO, CoO and Fe2O3, the NiO/CoO/Fe2O3 composite exhibits a lower overpotential and a much smaller Tafel slope of 49 mV dec(-1) for water oxidation. At the same time, the composite possesses beneficial ferromagnetic properties and superior stability; thus, it can be used repeatedly without any loss in activity.

Synthesis of mixed-valence hexanuclear Mn(II/III) clusters from its Mn(II) precursor: variations of catecholase-like activity and magnetic coupling

One Mn(II) coordination polymer, [Mn(o-(NO2)C6H4COO)2(pyz)(H2O)]n (1), has been synthesized and oxidized with n-Bu4NMnO4 in non-aqueous media to two mixed-valence hexanuclear Mn(II/III) complexes [MnIII2MnII4O2(pyz)0.61/(MeOH)0.39(o-(NO2)C6H4COO)10·(H2O)·{(CH3)2CO}2]·(CH3)2CO (2) and [MnIII2MnII4O2(pyz)0.28/(MeCN)3.72(o-(NO2)C6H4COO)10·(H2O)] (3) (where pyz = pyrazine). All three complexes were characterized by elemental analyses, IR spectroscopy, single-crystal X-ray diffraction analyses, and variable-temperature magnetic measurements. The structural analyses reveal that complex 1 is comprised of linear chains of pyz bridged Mn(II), which are further linked to one another by syn–anti carboxylate bridges, giving rise to a two-dimensional (2D) net. Complexes 2 and 3 feature mixed valence [MnIII2MnII4] units in which each of the six manganese centres reside in an octahedral environment. Apart from the variations in terminal ligands (acetone for 2 and acetonitrile for 3), the complexes are very similar. Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, the catecholase-like activity of the complexes has been studied and it is found that the mixed valent Mn6 complexes (2 and 3) are much more active towards aerial oxidation of catechol compared to the Mn(II) complex (1). Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed the presence of antiferromagnetic coupling in all three complexes. The magnetic data have been fitted with a 2D quadratic model derived by Lines, giving the exchange constant J/kB = −0.0788(5) K for 1. For 2 and 3, antiferromagnetic interactions within the Mn6 cluster have been fitted with models containing three exchange constants: JA/kB = −70 K, JB/kB = −0.5 K, JC/kB = −2.9 K for 2 and JA/kB = −60 K, JB/kB = −0.3 K, JC/kB = −2.8 K for 3.

Excess manganese differentially inhibits photosystem I versus II in Arabidopsis thaliana

The effects of exposure to increasing manganese concentrations (50-1500 µM) from the start of the experiment on the functional performance of photosystem II (PSII) and photosystem I (PSI) and photosynthetic apparatus composition of Arabidopsis thaliana were compared. In agreement with earlier studies, excess Mn caused minimal changes in the PSII photochemical efficiency measured as F(v)/F(m), although the characteristic peak temperature of the S(2/3)Q(B) (-) charge recombinations was shifted to lower temperatures at the highest Mn concentration. SDS-PAGE and immunoblot analyses also did not exhibit any significant change in the relative abundance of PSII-associated polypeptides: PSII reaction centre protein D1, Lhcb1 (major light-harvesting protein of LHCII complex), and PsbO (OEC33, a 33 kDa protein of the oxygen-evolving complex). In addition, the abundance of Rubisco also did not change with Mn treatments. However, plants grown under excess Mn exhibited increased susceptibility to PSII photoinhibition. In contrast, in vivo measurements of the redox transients of PSI reaction centre (P700) showed a considerable gradual decrease in the extent of P700 photooxidation (P700(+)) under increased Mn concentrations compared to control. This was accompanied by a slower rate of P700(+) re-reduction indicating a downregulation of the PSI-dependent cyclic electron flow. The abundance of PSI reaction centre polypeptides (PsaA and PsaB) in plants under the highest Mn concentration was also significantly lower compared to the control. The results demonstrate for the first time that PSI is the major target of Mn toxicity within the photosynthetic apparatus of Arabidopsis plants. The possible involvement mechanisms of Mn toxicity targeting specifically PSI are discussed.

Psb28 is involved in recovery of photosystem II at high temperature in Synechocystis sp. PCC 6803

Psb28 is an extrinsic protein of photosystem II (PSII), which is conserved among photosynthetic organisms from cyanobacteria to higher plants. A unicellular cyanobacterium, Synechocystis sp. PCC 6803, has two homologs of Psb28, Psb28-1 and Psb28-2. However, the role of these proteins remains poorly understood. In this study, we disrupted the psb28-1 (sll1398) and psb28-2 (slr1739) genes in wild-type Synechocystis sp. PCC 6803 and examined their photosynthetic properties to elucidate the physiological role of Psb28 in photosynthesis. We also disrupted the psb28-1 gene in a dgdA mutant defective in the biosynthesis of digalactosyldiacylglycerol, in which Psb28-1 significantly accumulates in PSII. The disruption of the psb28-1 gene in the wild-type resulted in growth retardation under high-light conditions at high temperatures with a low rate of restoration of photodamaged photosynthetic machinery. Similar phenomena were observed at normal growth temperatures in the psb28-1/dgdA double mutant. In contrast, disruption of psb28-2 in the wild-type and dgdA mutant did not affect host strain phenotype, suggesting that Psb28-2 does not contribute to the recovery of PSII. In addition, protein analysis using strains expressing His-tagged Psb28-1 revealed that Psb28-1 is mainly associated with the CP43-less PSII monomer. In the dgdA mutant, the CP43-less PSII monomer accumulated to a greater extent than in the wild-type, and its accumulation caused greater accumulation of Psb28-1 in PSII. These results demonstrate that Psb28-1 plays an important role in PSII repair through association with the CP43-less monomer, particularly at high temperatures.

Biological water-oxidizing complex: a nano-sized manganese-calcium oxide in a protein environment

The resolution of Photosystem II (PS II) crystals has been improved using isolated PS II from the thermophilic cyanobacterium Thermosynechococcus vulcanus. The new 1.9 Å resolution data have provided detailed information on the structure of the water-oxidizing complex (Umena et al. Nature 473: 55-61, 2011). The atomic level structure of the manganese-calcium cluster is important for understanding the mechanism of water oxidation and to design an efficient catalyst for water oxidation in artificial photosynthetic systems. Here, we have briefly reviewed our knowledge of the structure and function of the cluster.

Evidence on the formation of singlet oxygen in the donor side photoinhibition of photosystem II: EPR spin-trapping study

When photosystem II (PSII) is exposed to excess light, singlet oxygen (¹O₂) formed by the interaction of molecular oxygen with triplet chlorophyll. Triplet chlorophyll is formed by the charge recombination of triplet radical pair ³[P680^•+Pheo^•−] in the acceptor-side photoinhibition of PSII. Here, we provide evidence on the formation of ¹O₂ in the donor side photoinhibition of PSII. Light-induced ¹O₂ production in Tris-treated PSII membranes was studied by electron paramagnetic resonance (EPR) spin-trapping spectroscopy, as monitored by TEMPONE EPR signal. Light-induced formation of carbon-centered radicals (R^•) was observed by POBN-R adduct EPR signal. Increased oxidation of organic molecules at high pH enhanced the formation of TEMPONE and POBN-R adduct EPR signals in Tris-treated PSII membranes. Interestingly, the scavenging of R^• by propyl gallate significantly suppressed ¹O₂. Based on our results, it is concluded that ¹O₂ formation correlates with R^• formation on the donor side of PSII due to oxidation of organic molecules (lipids and proteins) by long-lived P680^•+/TyrZ^•. It is proposed here that the Russell mechanism for the recombination of two peroxyl radicals formed by the interaction of R^• with molecular oxygen is a plausible mechanism for ¹O₂ formation in the donor side photoinhibition of PSII.

OsNRAMP5, a major player for constitutive iron and manganese uptake in rice

Manganese (Mn) and iron (Fe) are essential mineral micronutrients for plants and their deficiency and or toxicity represents a serious agricultural problem. In rice the information about genes involved in Mn uptake from soil is scarce. Recently, we showed that OsNRAMP5 is a plasma membrane protein involved in Mn and Fe transport. The concentration of Mn in roots, shoots and xylem sap of OsNRAMP5 RNAi (OsNRAMP5i) plants was significantly reduced compared with WT plants. The expression of OsNRAMP5 is not controlled by Fe deficiency in root and was also observed in pistil, ovary, lemma and palea. These data show that rice would utilize OsNRAMP5 for constitutive Fe and Mn uptake, while OsNRAMP5 would also play a role in Fe and Mn transport during flowering and seed development.

Characterizing the role of rice NRAMP5 in Manganese, Iron and Cadmium Transport

Metals like manganese (Mn) and iron (Fe) are essential for metabolism, while cadmium (Cd) is toxic for virtually all living organisms. Understanding the transport of these metals is important for breeding better crops. We have identified that OsNRAMP5 contributes to Mn, Fe and Cd transport in rice. OsNRAMP5 expression was restricted to roots epidermis, exodermis, and outer layers of the cortex as well as in tissues around the xylem. OsNRAMP5 localized to the plasma membrane, and complemented the growth of yeast strains defective in Mn, Fe, and Cd transport. OsNRAMP5 RNAi (OsNRAMP5i) plants accumulated less Mn in the roots, and less Mn and Fe in shoots, and xylem sap. The suppression of OsNRAMP5 promoted Cd translocation to shoots, highlighting the importance of this gene for Cd phytoremediation. These data reveal that OsNRAMP5 contributes to Mn, Cd, and Fe transport in rice and is important for plant growth and development.

Role of novel dimeric Photosystem II (PSII)-Psb27 protein complex in PSII repair

The multisubunit membrane protein complex Photosystem II (PSII) catalyzes one of the key reactions in photosynthesis: the light-driven oxidation of water. Here, we focus on the role of the Psb27 assembly factor, which is involved in biogenesis and repair after light-induced damage of the complex. We show that Psb27 is essential for the survival of cyanobacterial cells grown under stress conditions. The combination of cold stress (30 °C) and high light stress (1000 μmol of photons × m(-2) × s(-1)) led to complete inhibition of growth in a Δpsb27 mutant strain of the thermophilic cyanobacterium Thermosynechococcus elongatus, whereas wild-type cells continued to grow. Moreover, Psb27-containing PSII complexes became the predominant PSII species in preparations from wild-type cells grown under cold stress. Two different PSII-Psb27 complexes were isolated and characterized in this study. The first complex represents the known monomeric PSII-Psb27 species, which is involved in the assembly of PSII. Additionally, a novel dimeric PSII-Psb27 complex could be allocated in the repair cycle, i.e. in processes after inactivation of PSII, by (15)N pulse-label experiments followed by mass spectrometry analysis. Comparison with the corresponding PSII species from Δpsb27 mutant cells showed that Psb27 prevented the release of manganese from the previously inactivated complex. These results indicate a more complex role of the Psb27 protein within the life cycle of PSII, especially under stress conditions.

Water (in water) as an intrinsically efficient proton acceptor in concerted proton electron transfers

The oxidation of PhOH in water by photochemically generated Ru(III)(bpy)(3) is taken as prototypal example disclosing the special character of water, in the solvent water, as proton acceptor in concerted proton-electron transfer reactions. The variation of the rate constant with temperature and driving force, as well as the variation of the H/D kinetic isotope effect with temperature, allowed the determination of the reaction mechanism characterized by three intrinsic parameters, the reorganization energy, a pre-exponential factor measuring the vibronic coupling of electronic states at equilibrium distance, and a distance-sensitivity parameter. Analysis of these characteristics and comparison with a standard base, hydrogen phosphate, revealed that electron transfer is concerted with a Grotthus-type proton translocation, leading to a charge delocalized over a cluster involving several water molecules. A mechanism is thus uncovered that may help in understanding how protons could be transported along water chains over large distances in concert with electron transfer in biological systems.

H-bond relays in proton-coupled electron transfers. Oxidation of a phenol concerted with proton transport to a distal base through an OH relay

Four molecules comprising a phenol moiety and a distal pyridine base connected by an intermediary H-bonding and an H-bonded alcohol group have been synthesized and their electrochemistry has been investigated by means of cyclic voltammetry. The molecules differ by the substituent at the alcohol functional carbon and by methyl groups on the pyridine. The reaction follows a concerted proton-electron transfer pathway as confirmed by the observation of a significant H/D kinetic isotope effect in all four cases. The standard rate constants characterizing each of the four compounds are analyzed in terms of reorganization energy and pre-exponential factor. Intramolecular and solvent reorganization energies appear as practically constant in the series, in which a previously investigated aminophenol is included, whereas significantly different pre-exponential factors are observed. That the latter, which is a measure of the efficiency of proton tunneling concerted with electron transfer, be substantially smaller with the H-bond relay molecules than with the aminophenol is related to the fact that two protons are moved in the first case instead of one in the second. Within the H-bond relay molecules, the pre-exponential factor varies with the substituent present at the alcohol functional carbon in the order CF(3) > H > CH(3), presumably as the result of a fine tuning of the balance between the H-bond accepting and H-bond donating properties of the central OH group. The kinetic H/D kinetic isotope effect increases accordingly in the same order.

Structural and mechanistic aspects of Mn-oxo and co-based compounds in water oxidation catalysis and potential applications in solar fuel production

To address the issues of energy crisis and global warming, novel renewable carbon-free or carbon-neutral energy sources must be identified and developed. A deeper understanding of photosynthesis is the key to provide a solid foundation to facilitate this transformation. To mimic the water oxidation of photosystem II oxygen evolving complex, Mn-oxo complexes and Co-phosphate catalytic material were discovered in solar energy storage. Building on these discoveries, recent advances in solar energy conversion showed a compelling working principle by combing the active Mn-oxo and Co-based catalysts in water splitting with semiconductor hetero-nanostructures for effective solar energy harnessing. In this review the appealing systems including Mn-oxo tetramer/Nafion, Mn-oxo dimer/TiO(2), Mn-oxo oligomer/WO(3), Co-Pi/Fe(2)O(3), and Co-Pi/ZnO are summarized and discussed. These accomplishments offer a promising framework and have a profound impact in the field of solar fuel production.

Concerted proton-electron transfers: electrochemical and related approaches

Proton-coupled electron transfers (PCETs) are omnipresent in natural and artificial chemical processes. Given the contemporary challenges associated with energy conversion, pollution abatement, and the development of high-performance sensors, a greater understanding of the mechanisms that underlie the practical efficiency of PCETs is a timely research topic. In contrast to hydrogen-atom transfers, proton and electron transfers involve different centers in PCET reactions. The reaction may go through an electron- or proton-transfer intermediate, giving rise to the electron-proton transfer (EPT) and the proton-electron transfer (PET) pathways. When the proton and electron transfers are concerted (the CPET pathway), the high-energy intermediates of the stepwise pathways are bypassed, although this thermodynamic benefit may have a kinetic cost. The primary task of kinetics-based mechanism analysis is therefore to distinguish the three pathways, quantifying the factors that govern the competition between them, which requires modeling of CPET reactivity. CPET models of varying sophistication have appeared, but the large number of parameters involved and the uncertainty of the quantum chemical calculations they may have to resort to make experimental confrontation and inspiration a necessary component of model testing and refinement. Electrochemical PCETs are worthy of particular attention, if only because most applications in which PCET mechanisms are operative involve collection or injection of electricity through electrodes. More fundamentally, changing the electrode potential is an easy and continuous means of varying the driving force of the reaction, whereas the current flowing through the electrode is a straightforward measure of its rate. Consequently, the current-potential response in nondestructive techniques (such as cyclic voltammetry) can be read as an activation-driving force relationship, provided the contribution of diffusion has been taken into account. Intrinsic properties (properties at zero driving force) are consequently a natural outcome of the electrochemical approach. In this Account, we begin by examining the modeling of CPET reactions and then describe illustrating experimental examples inspired by two biological systems, photosystem II and superoxide dismutase. One series of studies examined the oxidation of phenols with, as proton acceptor, either an attached nitrogen base or water (in water as solvent). Another addressed interconversion of aquo-hydroxo-oxo couples of transition metal complexes, using osmium complexes as prototypes. Finally, the reduction of superoxide ion, which is closely related to its dismutation, allowed the observation and rationalization of the remarkable properties of water as a proton donor. Water is also an exceptional proton acceptor in the oxidation of phenols, requiring very small reorganization energies, both in the electrochemical and homogeneous cases. These varied examples reveal general features of PCET reactions that may serve as guidelines for future studies, suggesting that research emphasis might be profitably directed toward new biological systems on the one hand and on the role of hydrogen bonding and hydrogen-bonded environments (such as water or proteins) on the other.

Concerted proton-electron transfers in the oxidation of phenols

The oxidation of phenols is an emblematic example where the mechanisms of proton-coupled electron transfers could be investigated in depth thanks to non-destructive electrochemical techniques such as cyclic voltammetry. A concerted proton-electron transfer could then be shown to be the prevailing pathway in the oxidation of amino-phenols mimicking the tyrosine-histidine couple in Photosystem II. The theoretical model developed on this occasion leads to the introduction of two main parameters characterizing reorganization of heavy atoms in the reactant and in the solvent on the one hand and proton tunneling on the other. When water used as the solvent is at the same time the proton acceptor, the concerted pathway also predominates. It is characterized by a remarkably large standard rate constant both in electrochemistry and in the oxidation by homogenous reactants. Another aspect of the importance of H-bonding in concerted proton-electron transfer is provided by H-bond relays that efficiently mediate the electron transfer-triggered transport of protons between two sites over large distances thanks to the displacement of two protons concerted with electron transfer. Intermediary protonation of the relay is avoided by fine tuning of its H-bond acceptor and donor properties.