Electron Transport In Photosynthesis Research Articles

A Review on the Toxicity Mechanisms and Potential Risks of Engineered Nanoparticles to Plants

Studies have reported that typical nanomaterials (NMs) have been applied on a large scale to various human consumer products due to their rapid development. Nanoparticles (NPs) flowing into the environment inevitably exhibit toxicological effects on organisms at the physiological and molecular levels. To identify the mechanisms of phytotoxicity regulation of typical metal-based engineered NPs, a review of ecotoxicological effects and environmental risks of particles is required. The biological transport of NPs in cells frequently affects the physiological state of plants, resulting in growth restriction of seedlings and structural damage to cellular tissues. Subsequently, excessive accumulation of reactive oxygen species (ROS) enhances oxidative stress, metabolic disorders, cell death, and DNA damage. The transcriptome and metabolome of the antioxidant system were also highly activated to resist ROS, which inevitably caused cytotoxicity and dysregulation of metabolic activity. At the biological process, protein folding, imbalance of calcium homeostasis, impaired mitosis, abnormal electron transport in photosynthesis, and dysregulation of TCA cycle in respiration have been extensively explored in the nanotoxicology field. Trophic transfer in the food chain indicates that ingested NPs are highly probable to accumulate in aquatic and terrestrial food chains and eventually pose potential threats to human health. Overall, this review aims to emphasize the toxicological effects of typical NMs on plants, reveal the toxicity mechanisms of plant cells with metal-based NPs in the environment, and discuss potential risks of particles in aquatic and terrestrial food chains, paving a bright path for environmental risk studies of NMs.

Unexpected Heme Redox Potential Values Implicate an Uphill Step in Cytochrome b6f.

Cytochromes bc, key enzymes of respiration and photosynthesis, contain a highly conserved two-heme motif supporting cross-membrane electron transport (ET) that connects the two catalytic quinone-binding sites (Qn and Qp). Typically, this ET occurs from the low- to high-potential heme b, but in photosynthetic cytochrome b6f, the redox midpoint potentials (Ems) of these hemes remain uncertain. Our systematic redox titration analysis based on three independent and comprehensive low-temperature spectroscopies (continuous wave and pulse electron paramagnetic resonance (EPR) and optical spectroscopies) allowed for unambiguous assignment of spectral components of hemes in cytochrome b6f and revealed that Em of heme bn is unexpectedly low. Consequently, the cross-membrane ET occurs from the high- to low-potential heme introducing an uphill step in the energy landscape for the catalytic reaction. This slows down the ET through a low-potential chain, which can influence the mechanisms of reactions taking place at both Qp and Qn sites and modulate the efficiency of cyclic and linear ET in photosynthesis.

The function of chloroplast ferredoxin-NADP+ oxidoreductase positively regulates the accumulation of bamboo mosaic virus in Nicotiana benthamiana.

A gene down‐regulated in Nicotiana benthamiana after bamboo mosaic virus (BaMV) infection had high identity to the nuclear‐encoded chloroplast ferredoxin NADP+ oxidoreductase gene (NbFNR). NbFNR is a flavoenzyme involved in the photosynthesis electron transport chain, catalysing the conversion of NADP+ into NADPH. To investigate whether NbFNR is involved in BaMV infection, we used virus‐induced gene silencing to reduce the expression of NbFNR in leaves and protoplasts. After BaMV inoculation, the accumulation of BaMV coat protein and RNA was significantly reduced. The transient expression of NbFNR fused with orange fluorescent protein (OFP) localized in the chloroplasts and elevated the level of BaMV coat protein. These results suggest that NbFNR could play a positive role in regulating BaMV accumulation. Expressing a mutant that failed to translocate to the chloroplast did not assist in BaMV accumulation. Another mutant with a catalytic site mutation could support BaMV accumulation to some extent, but accumulation was significantly lower than that of the wild type. In an in vitro replication assay, the replicase complex with FNR inhibitor, heparin, the RdRp activity was reduced. Furthermore, BaMV replicase was revealed to interact with NbFNR in yeast two‐hybrid and co‐immunoprecipitation experiments. Overall, these results suggest that NbFNR localized in the chloroplast with functional activity could efficiently assist BaMV accumulation.

Review Paper on Effect of Micronutrients for Crop Production

Micronutrients are essentially as important as macronutrients to have better growth, yield and quality in plants. There requirement by plants is in trace amounts. Boron, iron, copper, zinc, manganese, magnesium and molybdenum constitute main micronutrients required by different crops in variable quantities. The requirement of micronutrients is partly met from the soil or through chemical fertilizer or through other sources. Various physical and metabolic functions are governed by these mineral nutrients. Boron is particularly essential in pollen germination, copper plays major role in photosynthesis and increases sugar content in fruits, chlorophyll synthesis and phosphorus availability is enhanced by manganese, iron acts as an oxygen carrier and promotes chlorophyll formation, while, zinc aids plant growth hormones and enzyme system. Yield and quality of agricultural products increased with micronutrients application, therefore human and animal health is protected with feed of enrichment plant materials. Each essential element only when can perform its role in plant nutrition properly that other necessary elements are available in balanced ratios for plant. therefore in the plant manganese plays an important role on oxidation and reduction processes, as electron transport in photosynthesis. Manganese deficiency has very serious effects on non-structural carbohydrates, and roots carbohydrates especially. Crops quality and quantity decreased due to manganese deficiency, and this is due to low fertility of pollen and low in carbohydrates during grain filling. In the xylem routes zinc is transmitted to divalent form or with organic acids bond. In the phloem sap zinc makes up complex with organic acids with low molecular weight, and increases its concentration. Zinc deficiency can be seen in eroded, calcareous and weathering acidic soils. Zinc deficiency is often accompanied with iron deficiency in calcareous soils. Iron in the soil is the fourth abundant element on earth, but its amount was low or not available for the plants and microorganisms needs, due to low solubility of minerals containing iron in many places the world, especially in arid region with alkaline soils.

Transgenic tobacco co-expressing flavodoxin and betaine aldehyde dehydrogenase confers cadmium tolerance through boosting antioxidant capacity.

Excessive heavy metal (HM) levels in soil have become a source of concern due to their adverse effects on human health and the agriculture industry. Soil contamination by HMs leads to an accumulation of reactive oxygen species (ROSs) within the plant cell and disruption of photosynthesis-related proteins. The response of tobacco lines overexpressing flavodoxin (Fld) and betaine aldehyde dehydrogenase (BADH) to cadmium (Cd) toxicity was investigated in this study. PCR results demonstrated the expected amplicon length of each gene in the transgenic lines. Absolute qRT-PCR demonstrates a single copy of T-DNA integration into each transgenic line. Relative qRT-PCR confirmed overexpression of Fld and BADH in transgenic lines. The maximum quantum yield of photosystem II (Fv/Fm) was measured under Cd toxicity stress and revealed that transgenic lines had a higher Fv/Fm than wild-type (WT) plants. Accumulation of proline, glycine betaine (GB), and higher activity of antioxidant enzymes alongside lower levels of malondialdehyde (MDA) and hydrogen peroxide (H2O2) was indicative of a robust antioxidant system in transgenic plants. Therefore, performing a loop in reducing the ROS produced in the photosynthesis electron transport chain and stimulating the ROS scavenger enzyme activity improved the plant tolerance to Cd stress.

Exogenous methyl jasmonate enhances lipid production in Isochrysis galbana under nitrogen deprivation and high light

Nitrogen deprivation under high light is the most employed cultivation condition for the lipid's accumulation in microalgae. However, nitrogen deprivation causes the over-reduction of the photosystem (PS) II acceptor side in the presence of high light, confirmed by an increased VJ (the relative variable fluorescence at J-step) during cultivation. This in turn results in photo-inhibition of microalgal cells and photodamage to cellular metabolism which hinders the lipid production. To explore modifications for lipid accumulation, the exogenous methyl jasmonate (MeJA) effect was investigated on antioxidant capacity and lipid accumulation in Isochrysis galbana. Treatment with MeJA under high light nitrogen deprivation, (100 μM concentration), significantly enhanced the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase, and facilitated the electron transport in photosynthesis as indicated by increased ΦPSII (effective quantum yield of PSII) after MeJA treatment. The exogenous MeJA increased the antioxidant potential of related enzymes (ascorbate peroxidase and superoxide dismutase) and significantly alleviated the accumulation of reactive oxygen species (ROS) under high light nitrogen deprivation for improved cell survival and cellular metabolism in I. galbana. The improved cellular metabolism further increased the pyruvate content, which can be converted into acetyl coenzyme A (an important precursor for fatty acid synthesis). MeJA addition increased the substrate pool required for fatty acid and lipid production. Overall, exogenous MeJA enhanced lipid production in I. galbana under high light nitrogen deprivation by regulating the antioxidant capability and biosynthesis of the substrate.

The Significance of Chloroplast NAD(P)H Dehydrogenase Complex and Its Dependent Cyclic Electron Transport in Photosynthesis.

Chloroplast NAD(P)H dehydrogenase (NDH) complex, a multiple-subunit complex in the thylakoid membranes mediating cyclic electron transport, is one of the most important alternative electron transport pathways. It was identified to be essential for plant growth and development during stress periods in recent years. The NDH-mediated cyclic electron transport can restore the over-reduction in stroma, maintaining the balance of the redox system in the electron transfer chain and providing the extra ATP needed for the other biochemical reactions. In this review, we discuss the research history and the subunit composition of NDH. Specifically, the formation and significance of NDH-mediated cyclic electron transport are discussed from the perspective of plant evolution and physiological functionality of NDH facilitating plants’ adaptation to environmental stress. A better understanding of the NDH-mediated cyclic electron transport during photosynthesis may offer new approaches to improving crop yield.

Shmuel Malkin (1934-2017) : Listening to photosynthesis and making music.

We present here the life and work of Shmuel Malkin (1934-2017), an accomplished scientist and a gifted musician who touched the lives of many around the world. His early scientific work addressed the dynamics of light harvesting and electron transport in photosynthesis. Later, he used photoacoustic and photothermal methodologies to explore all aspects of photosynthesis. As a musician, Shmuel played the piano often for family and friends but after his formal retirement, he produced a body of original musical compositions, many of which were performed publicly. Throughout his life, Shmuel was a caring and deeply thoughtful man, respected and loved by colleagues, family, and friends. This tribute presents a summary of Shmuel's work as well as remembrances written by his wife, Nava Malkin, their son, Eyal Malkinson, and many of his colleagues: Michael Havaux from France; Sandra and Marcel Jansen from Ireland; David Cahen, Marvin Edelmann, Joop and Onnie de Graaf, Jonathan Gressel, Uri Pick, Yona Siderer, and Elisha Tel-Or from Israel; Ulrich Schreiber from Germany; James Barber and Alison Telfer from the UK; Govindjee, Stephen Herbert and Thomas Sharkey from the USA. Minnie Ho and Iris Malkin of the USA wrote contributions about Shmuel's music.

Effect of hydrogen peroxide on Microcystic aeruginosa: Role of cytochromes P450

Cyanobacterial bloom has been rising as a worldwide issue owing to its adverse effects to water quality and ecological health. To solve this problem, hydrogen peroxide (H2O2) has been considered as a potential algaecide because no by-products are generated after treatment and because it kills cyanobacteria selectively. In addition, cytochromes P450 (CYPs) was reported to be related with H2O2, but the roles of CYPs in the regulation of H2O2 in cyanobacteria have yet to be investigated. In this study, the CYPs suicide inhibitor 1-aminobenzotriazole (ABT) was added to the representative cyanobacteria Microcystis aeruginosa (M. aeruginosa) exposed to H2O2. The results showed that CYPs mediates the effects of H2O2 on M. aeruginosa. To be exact, the addition of ABT induced greater inhibitory effects on the growth and higher reactive oxygen species levels in M. aeruginosa comparing to those treated with H2O2 alone. At the same time, photosynthetic parameters significantly decreased, and the content of extracellular microcystins (MCs) increased but the total MCs decreased due to the combined effect of H2O2 and ABT. ABT also intensified the aggregation of Fe, which might explain the effects on photosynthesis and synthesis of MCs. Furthermore, the transcriptional levels of MCs-synthesis genes (mcyA and mcyD) decreased but MCs-release gene (mcyH) increased, and photosynthetic genes (psaB, psbD1 and rbcL) decreased, which confirmed the effects on the MC production/release and electron transport of photosynthesis, respectively. In summary, this study illuminated the mediation role of CYPs in the adverse effects on M. aeruginosa induced by H2O2, thus providing new theoretical basis for the explanation of H2O2 as potential algaecide.

Differential profiles of membrane proteins, fatty acids, and sterols associated with genetic variations in heat tolerance for a perennial grass species, hard fescue (Festuca Trachyphylla)

Cellular membrane composition controls membrane stability, affecting plant tolerance to abiotic stress. The objectives of this study were to examine differential changes in membrane constituents (sterol, fatty acid and membrane protein) in response to heat stress for a cool-season perennial grass species, hard fescue (Festuca trachyphylla) and identify major membrane constituents associated with the genetic variations in heat tolerance for two varieties contrasting in heat tolerance. Plants of heat-tolerant ‘Reliant IV’ and heat-sensitive ‘Predator’ were exposed to heat stress at 38/33°C (day/night) in growth chambers. Under heat stress, ‘Reliant IV’ showed better turf quality, greater membrane stability and higher photochemical efficiency compared to ‘Predator’. Leaf membranes of ‘Reliant IV’ showed greater increase of ethyl sterols (fucosterol, stigmasterol, sitosterol and avenasterol), and two fatty acids [oleic acid (18:1) and linoleic acid (18:2)] under heat stress compared to those for ‘Predator’. Several membrane proteins were less down-regulated or more up-regulated by heat stress in ‘Reliant IV’ than in ‘Predator’, including down-regulated proteins involved in electron transport of photosynthesis (ATP synthase subunits and cytochrome b6-f complex iron-sulfur subunit), signaling (lectin-domain containing receptor kinase A4.2), and protein modification (S-acyltransferase), and up-regulated proteins involved in carboxylation in photosynthesis (Rubisco activase beta form) and stress defense (disease resistance protein 1). These differentially accumulated sterols, fatty acids, and proteins in cellular membranes could contribute to the differential level of heat tolerance between the two varieties of hard fescues. These compounds could be used as biomarkers to select for heat-tolerant germplasm in hard fescues.

Effects of Pseudomonas syringae pv. tabaci infection on tobacco photosynthetic apparatus under light or dark conditions.

Pseudomonas syringae pv. tabaci (Pst) is a hemi-biotrophic bacterial pathogen that causes the formation of brown spots named wildfire disease. Pst has received considerable attention in recent years. However, most of the studies focused on the tolerance and defense mechanisms of the host and non-host plants against Pst infection and a toxin originally described as being from Pst named tabtoxin, little information is available on the photosynthetic performance of tobacco leaves after Pst infection. Exploring the effects of Pst on the photosystem Ⅱ (PSⅡ) will not only help in clarifying tobacco-Pst interaction mechanisms, but also deepen the understanding of bacterial pathogen disease from a physiological perspective. By analyzing chlorophyll a fluorescence transient, performing western blot of thylakoid membrane and measuring the content of reactive oxygen species (ROS) and total chlorophyll, the effects of Pst on PS2 in tobacco were studied under light (200 μmol·m-2·s-1) or dark conditions. The results showed that chlorophyll content significantly decreased and significant chlorosis of the infiltrated zone was observed compared to the untreated ones, and tobacco leaves exhibited a visible and overt wildfire symptom at 3 days post Pst infection (dpi) under light and dark conditions. The H2O2 content increased at 3 dpi compared to untreated ones in tobacco leaves under light and dark conditions, and was much higher under light than dark condition. Besides, markedly increase of the normalized relative variable fluorescence at the K step (WK) and the relative variable fluorescence at the J step (VJ), significant decrease of maximal quantum yield of PS2 (Fv/Fm) and density of QA- reducing PS2 reaction centers per cross section (RC/CSm) were observed in tobacco leaves after Pst infection at 3 dpi under light and dark conditions. Moreover, inhibition of the K and J steps was more pronounced in the dark, as indicated by the greater increase of WK and VJ under darkness compared with the light conditions during Pst inoculation. Dramatic (net) degradation of D1 protein and PsaO, the core protein of PS2 reaction center and oxygen evolving complex (OEC) respectively, at 3 dpi after Pst infection was observed in tobacco leaves under both light or dark conditions, and the decline was more exacerbated under dark than light condition. The results indicated that the electron transport from QA to QB of photosynthesis electron transport chain was severely blocked, OEC was damaged on both the donor and acceptor sides, and the reaction center of PS2 was severely damaged by Pst infection in tobacco lea-ves under either light or dark condition. Photoinhibition and photoinhibition-like damage of PSⅡ was observed after Pst infection, and the damage to PS2 under dark condition was much more severe than under light condition in tobacco leaves.

Enhanced sensitivity of oceanic CO2 uptake to dust deposition by iron‐light colimitation

AbstractThe iron hypothesis suggests that in large areas of the ocean phytoplankton growth and thus photosynthetic CO2 uptake is limited by the micronutrient iron. Phytoplankton requires iron in particular for nitrate uptake, light harvesting, and electron transport in photosynthesis, suggesting a tight coupling of iron and light limitation. One important source of iron to the open ocean is dust deposition. Previous global biogeochemical modeling studies have suggested a low sensitivity of oceanic CO2 uptake to changes in dust deposition. Here we show that this sensitivity is increased significantly when iron‐light colimitation, i.e., the impact of iron bioavailability on light‐harvesting capabilities, is explicitly considered. Accounting for iron‐light colimitation increases the shift of export production from tropical and subtropical regions to the higher latitudes of subpolar regions at high dust deposition and amplifies iron limitation at low dust deposition. Our results reemphasize the role of iron as a key limiting nutrient for phytoplankton.

Teaching the Z-Scheme of electron transport in photosynthesis: a perspective.

This paper deals with how Govindjee taught the Z-Scheme of electron transport in oxygenic photosynthesis at Ravenshaw University, Cuttack, Odisha, India, in 2014, in a unique and highly effective fashion-using students to act as molecules, representing the entire electron transport chain from water to nicotinamide adenine dinucleotide phosphate (NADP(+)). It culminated in a show by B.Sc. students in the garden of the Department of Botany, Ravenshaw University. The first author (PKM) personally acted as Ferredoxin NADP Reductase (FNR) catalyzing the reduction of NADP(+) to NADPH, taking electrons from reduced ferredoxin at the end of Photosystem I. On the other hand, the Q-cycle was played by M.Sc. students, who acted as molecules running this ingenious cycle that produces extra protons. An interesting event was when a student, acting as a herbicide, who was dressed like a devil (fierce looking, in black clothes with a sword; "Yamaraj: The God of Death", as he called himself), stopped all reactions by throwing out QB, the second plastoquinone molecule of Photosystem II, and that too aggressively, taking its position instead. The second author was the major organizer of the Z-scheme show. We provide here a basic background on the process, a bit on Govindjee's teaching, and some selected pictures from the drama played in March, 2014 at Ravenshaw University. Here, we also recognize the teacher Govindjee for his ingenious and fun-filled teaching methods that touched the hearts and the souls of the students as well as the teachers of Ravenshaw University. He was rated as one of the most-admired teachers of plant biology at our university.

Comparative analysis of the chloroplast proteomes of a wheat (Triticum aestivum L.) single seed descent line and its parents

To understand the photosynthetic basis in a single seed descent line 10 (SSDL10) of wheat contained high ATP in leaves, the chloroplast proteome was compared to SSDL10 and its parents using a combination of 2-DE and MALDI-TOF MS and MS/MS. More than 300 protein spots could be reproducibly detected in the 2D gel. 18 spots were differentially expressed between SSDL10 and the parents, 16 of which were identified by MS with the localization in chloroplasts. These proteins are grouped into diverse functional categories, including Calvin cycle and electron transport in photosynthesis, redox homeostasis, metabolism, and regulation. In addition to Rubisco large subunit, the content of photosynthetic electron transfers such as chlorophyll a-b binding protein, ATP synthase δ subunit, ferredoxin-NADP+ oxidoreductase (FNR) was higher in SSDL10 than in its parents. Furthermore, cyclic electron transfer around photosystem I (CET) was faster in SSDL10 than in the parents. Analysis of NADPH-NBT oxidoreductase activity combined with immuno-detection further revealed that, the activity of two high molecular mass protein complexes containing FNR probably involved, the CET appeared higher in SSDL10 than in the parents. The possible mechanism for the regulative role of CET in photosynthesis in SSDL10 is discussed.

Photosynthetic traits of Siebold's beech and oak saplings grown under free air ozone exposure in northern Japan

We set up a free-air ozone (O3) exposure system for determining the photosynthetic responses of Siebold's beech (Fagus crenata) and oak (Quercus mongolica var. crispula) to O3 under field conditions. Ten-year-old saplings of beech and oak were exposed to an elevated O3 concentration (60 nmol mol−1) during daytime from 6 August to 11 November 2011. Ozone significantly reduced the net photosynthetic rate in leaves of both species in October, by 46% for beech and 15% for oak. In beech there were significant decreases in maximum rate of carboxylation, maximum rate of electron transport in photosynthesis, nitrogen content and photosynthetic nitrogen use efficiency, but not in oak. Stomatal limitation of photosynthesis was unaffected by O3. We therefore concluded photosynthesis in beech is more sensitive to O3 than that in oak, and the O3-induced reduction of photosynthetic activity in beech was due not to stomatal closure, but to biochemical limitation.

紅色光合成細菌Rubrivivax gelatinosus のイソ型高電位チトクロムc8の光合成電子伝達への寄与及び同遺伝子周辺領域のDNA解析

紅色光合成細菌Rubrivivax gelatinosus のイソ型高電位チトクロムc8の光合成電子伝達への寄与及び同遺伝子周辺領域のDNA解析

Mechanism study on the origin of delayed fluorescence by an analytic modeling of the electronic reflux for photosynthetic electron transport chain

A mathematical–physical analysis model, which describes individually the electronic reflux of several significant components in the photosynthesis electron transport chain, was firstly developed. The process of electrons flowing back to the oxidized reaction center P 680 + was simulated by a series of photochemical reaction equations, resulting in getting the linked differential equations of delayed fluorescence (DF) intensity. MATLAB provided a computationally efficient method to solve these linked equations. Simulations based on this model showed that the decay kinetics of DF accord with double exponential. DF components decaying in the millisecond range (fast phase) are related to the charge recombination of P 680 + and Q A - . The components decaying in the seconds range are associated with the recombination of P 680 + with Q B 2 - . The developed model was tested in maize leaves treated with different electron blockers to induce changes in photosynthesis electron transport chain. The experimental results demonstrated that the developed model can accurately determine the regulatory effects of electron blockers on photosynthesis electron transport chain. Therefore, the model presented here could be potentially useful for studying the electron transfer in plant. It also provides an experimental workbench for testing hypotheses as to the underlying mechanism controlling the change for different phases of DF.

Nicotianamine synthase: Gene isolation, gene transfer and application for the manipulation of plant iron assimilation

Basic cellular processes such as electron transport in photosynthesis and respiration require the precise control of iron homeostasis. To mobilise iron, plants have evolved at least two different strategies. The non-proteinogenic amino acid nicotianamine is an essential component of both pathways.

Isolation, characterization and cDNA cloning of nicotianamine synthase from barley. A key enzyme for iron homeostasis in plants.

Basic cellular processes such as electron transport in photosynthesis and respiration require the precise control of iron homeostasis. To mobilize iron, plants have evolved at least two different strategies. The nonproteinogenous amino acid nicotianamine which is synthesized from three molecules of S-adenosyl-L-methionine, is an essential component of both pathways. This compound is missing in the tomato mutant chloronerva, which exhibits severe defects in the regulation of iron metabolism. We report the purification and partial characterization of the nicotianamine synthase from barley roots as well as the cloning of two corresponding gene sequences. The function of the gene sequence has been verified by overexpression in Escherichia coli. Further confirmation comes from reduction of the nicotianamine content and the exhibition of a chloronerva-like phenotype due to the expression of heterologous antisense constructs in transgenic tobacco plants. The native enzyme with an apparent Mr of approximately 105 000 probably represents a trimer of S-adenosyl-L-methionine-binding subunits. A comparison with the recently cloned chloronerva gene of tomato reveals striking sequence homology, providing support for the suggestion that the destruction of the nicotianamine synthase encoding gene is the molecular basis of the tomato mutation.

FIQ and FIQ2, New Q-site inhibitors for photosynthetic electron transport: synthesis and the relationship between stereochemistry and biological activity

The synthetic hexahydrofuroisoquinoline 4 has, as a mixture of stereoisomers, been identified as a potent and specific inhibitor of electron transport in photosynthesis. We now report that the biological activity of 4 is, surprisingly, independent of configuration at C-1″ and C-2. A new inhibitor 26 has been synthesised, with similar inhibitory activity; the S-enantiomer displays ca. twice the activity of the R-form.