High-potential Form Research Articles

К ВОПРОСУ О ТРАНСФОРМАЦИИ МЕДИЦИНСКОГО ОБРАЗОВАНИЯ

The transformation of medical education at the present stage has determined the problem of this study, the issues of high-quality training of medical personnel necessary for the society have not been fully resolved, pedagogical technologies and methodological approaches that contribute to the growth of educational results have not been fully disclosed. The aim of this study to analyze the existing high-potential forms of training that could be used in the transformation of the educational space of a medical university and contribute to better training of future specialists in the medical industry. The study of literature and other various sources of information was chosen as the main research method that allowed to describe the active and interactive forms of training under discussion as well as to reveal the possibilities of immersive learning and the use of artificial intelligence technologies. Our analysis shows that the use of nontraditional forms of learning gives the required effect of change and novelty, evokes an emotion of interest in students and is a powerful incentive to gain knowledge, skills and abilities. It was noted that all changes in the transformable educational process must take into account the motivational aspect in learning, since the most important factor affecting the effectiveness of the educational process, increasing the effectiveness of educational activities at all stages of acquiring professional medical knowledge is the motivation of students to learn. Those medical graduates gain more success in the future carrier, who not only have special knowledge and a set of professional skills, but they also have incentives for education related to the formation of professional competencies, know universal technologies of self-development, understand how to motivate themselves to receive, process and apply new knowledge, are able to critically evaluate the information received, adapt to the requirements of a particular activity, have a stable positive attitude towards the profession.

What conditions favor high-potential entrepreneurship? Unpacking the nexus between the industrial structure and startup typologies

In this paper, we question the implicit assumption that more entrepreneurship drives more innovation and growth, asserting that specific typologies of entrepreneurship are responsible for these phenomena. A growing number of studies has analyzed this relationship while focusing on the overall level of entrepreneurship. This paper exploits recent advancements in measuring the sophistication and connectedness of economic systems, brought by the notions of economic complexity and relatedness, to study the nexus of industrial structure and high-potential forms of entrepreneurship. The present study uses a panel dataset for Italy for the period 2015–2019. The results show a differentiated pattern among the high-potential startups considered, with relatedness and complexity having a positive effect for innovative startups, a negative one for high-growth startups, and no effect for pioneers. These results inform potential entrepreneurs of the importance of analyzing how external conditions can have distinctive effects on the process of opportunity identification among different typologies of high-potential startups.

A multi-tiered approach to map and assess the natural heritage potential to provide ecosystem services at a national level

Natural heritage (NH) possesses an outstanding universal value that can be described as “natural significance” at a national level. The ecosystems can be considered as the spatial units which represent the NH of the particular area in terms of their value to people. Recreation and tourism are amongst the important values which are strongly dependent on the NH and they have a certain impact on the ecosystems' condition and the quality of the services they provide. The efforts through the Mapping and Assessment of Ecosystems and thier Services (MAES) process led to the development of a multi-tiered approach that considers different methods at different levels of detail and complexity and can be applied according to specific needs, data and resource availability. In this paper, we propose the development of this methodology for the specific need for mapping and assessment of the NH as a source of ecosystem services (ES) for recreation and tourism. The conceptual scheme of the study demonstrates how the MAES framework can be adapted to the specific needs of the work and arrange the methods into three tiers according to the data availability and resources. The mapping and assessment procedure is based on an algorithm for spatial data analyses which enables the evaluation of the NH potential to provide 15 ecosystem services. The results show that the NH of Bulgaria is a valuable source of ES which are well presented in most parts of the country. The areas with very high potential form several clusters that correspond to the country's tourist regions. The proposed approach is applicable on the national scale and solves the problem of data availability limitations for various ES. The algorithm ensures the optimal quality of the results using the available data and resources. Instead of an expert-based assessment for all services which is easier, but less accurate, the proposed approach provides the means how to define more precise indicators, based on statistical data or models where possible. The study provides appropriate data for analyses of the methods’ performance at different tiers.

Rural Tourism Destination: The Ligurian Farmers’ Perspective

Rural tourism is considered a high potential form of tourism, enhanced by the demand for more sustainable and nature-based solutions, and able to contribute to territory resilience. A rural area is not necessarily a tourist destination, but it might become one, if agricultural enterprises are willing to diversify their economic activities by investing in rural tourism, and local actors provide active support and co-participation. This research focuses on the development of rural tourism in hinterland, mountainous, and hilly areas of the province of Savona in Liguria (North-West of Italy) in order to gather the farmers’ perspectives about local rural tourism destination development. Liguria is known above all as a seaside tourist destination. In recent years, policy makers have initiated a debate with local actors to relaunch Ligurian tourism by trying to develop alternative forms of tourism, such as rural tourism. A sample of 32 farmers already proposing rural tourism activities such as agritourism were involved in a mixed methodological approach aimed at validating local interest toward rural tourism and collecting information for designing future local development policies. At first, a questionnaire set up by a panel of experts was carried out, followed by one-to-one semi-structured interviews, and finally the Nominal Group Technique (NGT) was used to identify local priorities, strategies, and tools. The results highlight the willingness of farmers to invest in the rural tourism sector, the presence of heterogeneous interests, and the complexity of management of the relationship among the various stakeholders. Findings are partially explained by the early stage of development of rural tourism in the analyzed area, a phase in which starting a constructive dialogue on objective and project ideas among all stakeholders seems to be a crucial priority among the farmers taking part in the study.

Two-Component Model of Microalgae Growth in the Turbidostat

The paper focuses on the study of light influence mechanisms on microalgae culture growth in the turbidostat. The method of turbidostat culture provides the same light conditions for all cells, stabilization of their biochemical composition, as well as the constancy of all physicochemical factors of the environment. The main approaches and principles of modeling the microalgae culture growth are presented. Modern models are shown to be based on classical concepts of considering cell biomass as the sum of two or more compounds. The use of two-component models for microalgae is due to both photochemical and enzymatic processes of biosynthesis of cell structures from mineral substances due to the energy of high-potential forms of macroergs. The proposed mathematical model is represented by a system of two differential equations describing the synthesis of reserve biomass compounds at the expense of light and biosynthesis of structural components from reserve ones. The model takes into account that a part of the reserve compounds is spent on replenishing the pool of macroergs, and a part of the structural component can be converted into a reserve one. The rates of synthesis of structural and reserve forms of biomass are given by linear splines and expressed in terms of the reduced fluxes of the energy or plastic substrate. The model was verified on the experimental data of the chlorophyllostat culture Tetraselmis viridis. It is shown that the light curve of T. viridis is characterized by a double change in the limiting factor and can be divided into a region of metabolic, light limiting and a region of saturation. A decrease in the specific growth rate is related to a decrease in the efficiency of light energy conversion.

Hormone receptor, human epidermal growth factor receptor-2, and Ki-67 status in primary breast cancer and corresponding recurrences or synchronous axillary lymph node metastases.

The therapeutic strategy for breast cancer is determined by the surrogate subtype, which is defined by biomarkers, such as estrogen receptor (ER), progesterone receptor (PgR), human epidermal growth factor receptor-2 (HER2), and Ki-67. In previous reports, the rate of discordance in ER, PgR, and HER2 between primary breast cancer and recurrent lesions or synchronous axillary lymph node metastasis was 15-25, 25-40, and 5-25 or 7-50, 10-50, and 3-30%, respectively. Overall, hormone receptors tended to weaken during the metastatic process, while patterns of HER2 were not uniform. Regarding the Ki-67 labeling index, an increase in metastatic lesions compared with primary lesions was the dominant pattern, suggesting that aggressive subclones with high proliferative potential form metastases. The loss of expression of hormone receptor or an increase in the Ki-67 labeling index in metastasis seemed to be associated with a poor prognosis. However, most previous studies did not report the background characteristics of patients, or they included subjects with varied characteristics, including those on systemic therapy, and were based on relatively small populations; therefore, definitive conclusions could not be drawn. Future studies should explore how to select therapies according to the biomarkers in primary breast cancer and/or its metastasis.

Antimycin A inhibits cytochrome b559-mediated cyclic electron flow within photosystem II

The light reactions of photosynthesis are known to comprise both linear and cyclic electron flow in order to convert light energy into chemical energy in the form of NADPH and ATP. Antimycin A (AA) has been proposed as an inhibitor of ferredoxin-dependent cyclic electron flow around photosystem I (CEF-PSI) in photosynthesis research. However, its precise inhibitory mechanism and target site had not been elucidated yet. Here we show that AA inhibits the cyclic (alternative) electron flow via cytochrome b559 (Cyt b559) within photosystem II (CEF-PSII). When AA was applied to thylakoid membranes isolated from spinach leaves, the high potential form of Cyt b559, which was reduced in the dark, was transformed into the lower potential forms and readily oxidized by molecular oxygen. In the absence of AA, the reduced Cyt b559 was oxidized by P680+ upon light illumination and re-reduced in the dark, mainly by the electron from the QB site on the acceptor side of PSII. In contrast, AA suppressed the oxidation of Cyt b559 and induced its reduction under the illumination. This inhibition of Cyt b559 oxidation by AA enhanced photoinhibition of PSII. Based on the above results, we propose caution regarding the use of AA for evaluating CEF-PSI per se and concurrently propose that AA provides for new insights into, and interpretations of, the physiological importance of Cyt b559, rather than that of CEF-PSI in photosynthetic organisms.

Фундаментальные принципы моделирования фотобиосинтеза микроводорослей

The paper lists the classic and modern views on the microalgae's culture growth, which constitute a new paradigm of modeling photobiosynthesis. Microalgae are considered as lower obligate photoautotrophs capable of oxygenic photosynthesis, characterized by the separation of photochemical and enzymatic processes. A model scheme of the light and dark reactions conjugation in microalgae is proposed. The process of biosynthesis of cell biochemical structures from minerals is carried out due to the energy of high-potential forms of high-energy compounds (NADP·H, ATP). It is shown that the microalgae's growth can be considered as a set of energyexchange reactions. The concept of the absence of limiting microalgae's growth by environmental factors is proposed. The rate of biomass synthesis is determined by the reduced flow density of the energy or plastic substrate to the key enzyme. Also, for convenience in practical terms, productivity can be expressed through the biochemical measured ratio of structural and reserve forms of biomass. In any case, the dependence of the rate of biomass synthesis on the value of the reduced flux density is described by linear splines, which allows you to clearly set the switching point of the limiting factor and obtain simple mathematical solutions.

Junction formation and current transport mechanisms in hybrid n-Si/PEDOT:PSS solar cells.

We investigated hybrid inorganic-organic solar cells combining monocrystalline n-type silicon (n-Si) and a highly conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS). The build-in potential, photo- and dark saturation current at this hybrid interface are monitored for varying n-Si doping concentrations. We corroborate that a high build-in potential forms at the hybrid junction leading to strong inversion of the n-Si surface. By extracting work function and valence band edge of the polymer from ultraviolet photoelectron spectroscopy, a band diagram of the hybrid n-Si/PEDOT:PSS heterojunction is presented. The current-voltage characteristics were analyzed using Schottky and abrupt pn-junction models. The magnitude as well as the dependence of dark saturation current on n-Si doping concentration proves that the transport is governed by diffusion of minority charge carriers in the n-Si and not by thermionic emission of majorities over a Schottky barrier. This leads to a comprehensive explanation of the high observed open-circuit voltages of up to 634 mV connected to high conversion efficiency of almost 14%, even for simple planar device structures without antireflection coating or optimized contacts. The presented work clearly shows that PEDOT:PSS forms a hybrid heterojunction with n-Si behaving similar to a conventional pn-junction and not, like commonly assumed, a Schottky junction.

Evidence for the involvement of loosely bound plastosemiquinones in superoxide anion radical production in photosystem II.

Recent evidence has indicated the presence of novel plastoquinone-binding sites, QC and QD, in photosystem II (PSII). Here, we investigated the potential involvement of loosely bound plastosemiquinones in superoxide anion radical (O2 •−) formation in spinach PSII membranes using electron paramagnetic resonance (EPR) spin-trapping spectroscopy. Illumination of PSII membranes in the presence of the spin trap EMPO (5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide) resulted in the formation of O2 •−, which was monitored by the appearance of EMPO-OOH adduct EPR signal. Addition of exogenous short-chain plastoquinone to PSII membranes markedly enhanced the EMPO-OOH adduct EPR signal. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, the EMPO-OOH adduct EPR signal was suppressed by 50% when the urea-type herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) was bound at the QB site. However, the EMPO-OOH adduct EPR signal was enhanced by binding of the phenolic-type herbicide dinoseb (2,4-dinitro-6-sec-butylphenol) at the QD site. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, DCMU and dinoseb inhibited photoreduction of the high-potential form of cytochrome b 559 (cyt b 559). Based on these results, we propose that O2 •− is formed via the reduction of molecular oxygen by plastosemiquinones formed through one-electron reduction of plastoquinone at the QB site and one-electron oxidation of plastoquinol by cyt b 559 at the QC site. On the contrary, the involvement of a plastosemiquinone formed via the one-electron oxidation of plastoquinol by cyt b 559 at the QD site seems to be ambiguous. In spite of the fact that the existence of QC and QD sites is not generally accepted yet, the present study provided more spectroscopic data on the potential functional role of these new plastoquinone-binding sites.

The role of the high potential form of the cytochrome b559: Study of Thermosynechococcus elongatus mutants

Cytochrome b559 is an essential component of the photosystem II reaction center in photosynthetic oxygen-evolving organisms, but its function still remains unclear. The use of photosystem II preparations from Thermosynechococcus elongatus of high integrity and activity allowed us to measure for the first time the influence of cytochrome b559 mutations on its midpoint redox potential and on the reduction of the cytochrome b559 by the plastoquinone pool (or QB). In this work, five mutants having a mutation in the α-subunit (I14A, I14S, R18S, I27A and I27T) and one in the β-subunit (F32Y) of cytochrome b559 have been investigated. All the mutations led to a destabilization of the high potential form of the cytochrome b559. The midpoint redox potential of the high potential form was significantly altered in the αR18S and αI27T mutant strains. The αR18S strain also showed a high sensitivity to photoinhibitory illumination and an altered oxidase activity. This was suggested by measurements of light induced oxidation and dark re-reduction of the cytochrome b559 showing that under conditions of a non-functional water oxidation system, once the cytochrome is oxidized by P680+, the yield of its reduction by QB or the PQ pool was smaller and the kinetic slower in the αR18S mutant than in the wild-type strain. Thus, the extremely positive redox potential of the high potential form of cytochrome b559 could be necessary to ensure efficient oxidation of the PQ pool and to function as an electron reservoir replacing the water oxidation system when it is not operating.

X-ray crystallographic and computational studies of the O 2 -tolerant [NiFe]-hydrogenase 1 from Escherichia coli

The crystal structure of the membrane-bound O(2)-tolerant [NiFe]-hydrogenase 1 from Escherichia coli (EcHyd-1) has been solved in three different states: as-isolated, H(2)-reduced, and chemically oxidized. As very recently reported for similar enzymes from Ralstonia eutropha and Hydrogenovibrio marinus, two supernumerary Cys residues coordinate the proximal [FeS] cluster in EcHyd-1, which lacks one of the inorganic sulfide ligands. We find that the as-isolated, aerobically purified species contains a mixture of at least two conformations for one of the cluster iron ions and Glu76. In one of them, Glu76 and the iron occupy positions that are similar to those found in O(2)-sensitive [NiFe]-hydrogenases. In the other conformation, this iron binds, besides three sulfur ligands, the amide N from Cys20 and one Oε of Glu76. Our calculations show that oxidation of this unique iron generates the high-potential form of the proximal cluster. The structural rearrangement caused by oxidation is confirmed by our H(2)-reduced and oxidized EcHyd-1 structures. Thus, thanks to the peculiar coordination of the unique iron, the proximal cluster can contribute two successive electrons to secure complete reduction of O(2) to H(2)O at the active site. The two observed conformations of Glu76 are consistent with this residue playing the role of a base to deprotonate the amide moiety of Cys20 upon iron binding and transfer the resulting proton away, thus allowing the second oxidation to be electroneutral. The comparison of our structures also shows the existence of a dynamic chain of water molecules, resulting from O(2) reduction, located near the active site.

Characterization of a unique [FeS] cluster in the electron transfer chain of the oxygen tolerant [NiFe] hydrogenase from Aquifex aeolicus

Iron-sulfur clusters are versatile electron transfer cofactors, ubiquitous in metalloenzymes such as hydrogenases. In the oxygen-tolerant Hydrogenase I from Aquifex aeolicus such electron "wires" form a relay to a diheme cytb, an integral part of a respiration pathway for the reduction of O(2) to water. Amino acid sequence comparison with oxygen-sensitive hydrogenases showed conserved binding motifs for three iron-sulfur clusters, the nature and properties of which were unknown so far. Electron paramagnetic resonance spectra exhibited complex signals that disclose interesting features and spin-coupling patterns; by redox titrations three iron-sulfur clusters were identified in their usual redox states, a [3Fe4S] and two [4Fe4S], but also a unique high-potential (HP) state was found. On the basis of (57)Fe Mössbauer spectroscopy we attribute this HP form to a superoxidized state of the [4Fe4S] center proximal to the [NiFe] site. The unique environment of this cluster, characterized by a surplus cysteine coordination, is able to tune the redox potentials and make it compliant with the [4Fe4S](3+) state. It is actually the first example of a biological [4Fe4S] center that physiologically switches between 3+, 2+, and 1+ oxidation states within a very small potential range. We suggest that the (1+/2+) redox couple serves the classical electron transfer reaction, whereas the superoxidation step is associated with a redox switch against oxidative stress.

High and low potential forms of the Q A quinone electron acceptor in Photosystem II of Thermosynechococcus elongatus and spinach

The redox potential of Q A in Photosystem II (PSII) from Thermosynechococcus elongatus was titrated monitoring chlorophyll fluorescence. A high potential form ( E m = +60 ± 25 mV) was found in the absence of Mn 4Ca, the active site for water oxidation. The low potential form ( E m = −60 ± 48 mV), which is difficult to measure in conventional titration experiments, could be “locked in” by cross-linking the active enzyme. This indicates that the presence of Mn 4Ca is relayed to the quinone site by significant structural changes in the protein. The presence of high and low potential forms agrees with what has been seen in plants, algae from our lab and in T. elongatus (Shibamoto et al., Biochemistry 48 (2009) 10682–10684). In the latter work, the potentials of Q A were shifted to lower potentials compared to other measurements. The redox potential of Q A in Mn-depleted PSII from spinach was titrated in the presence of redox mediators and the midpoint potential was shifted by 80 mV towards a more negative value compared to titrations without mediators. The lower values of the midpoint potential of the ( Q A / Q A - ) redox couple in the literature could be due to a perturbation due to a specific mediator.

A High Redox Potential Form of Cytochrome c550 in Photosystem II from Thermosynechococcus elongatus

Cytochrome c(550) (cyt c(550)) is a component of photosystem II (PSII) from cyanobacteria, red algae, and some other eukaryotic algae. Its physiological role remains unclear. In the present work, measurements of the midpoint redox potential (E(m)) were performed using intact PSII core complexes preparations from a histidine-tagged PSII mutant strain of the thermophilic cyanobacterium Thermosynechococcus (T.) elongatus. When redox titrations were done in the absence of redox mediators, an E(m) value of +200 mV was obtained for cyt c(550). This value is ∼300 mV more positive than that previously measured in the presence of mediators (E(m) = -80 mV). The shift from the high potential form (E(m) = +200 mV) to the low potential form (E(m) = -80 mV) of cyt c(550) is attributed to conformational changes, triggered by the reduction of a component of PSII that is sequestered and out of equilibrium with the medium, most likely the Mn(4)Ca cluster. This reduction can occur when reduced low potential redox mediators are present or under highly reducing conditions even in the absence of mediators. Based on these observations, it is suggested that the E(m) of +200 mV obtained without mediators could be the physiological redox potential of the cyt c(550) in PSII. This value opens the possibility of a redox function for cyt c(550) in PSII.

Spectroscopic and Functional Characterizations of Cyanobacterium Synechocystis PCC 6803 Mutants on and near the Heme Axial Ligand of Cytochrome b559 in Photosystem II

The functional role of cytochrome (cyt) b(559) in photosystem II (PSII) was investigated in H22K alpha and Y18S alpha cyt b(559) mutants of the cyanobacterium Synechocystis sp. PCC6803. H22K alpha and Y18S alpha cyt b(559) mutant carries one amino acid substitution on and near one of heme axial ligands of cyt b(559) in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of cyt b(559) in H22K alpha mutant reaction centers, at least in isolated reaction centers. The maximum absorption of cyt b(559) in Y18S alpha mutant PSII core complexes was shifted to 561 nm. Y18S alpha and H22K alpha mutant PSII core complexes contained predominately the low potential form of cyt b(559). The findings lend support to the concept that the redox properties of cyt b(559) are strongly influenced by the hydrophobicity and ligation environment of the heme. When the cyt b(559) mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of cyt b(559) in protection of PSII under photoinhibition conditions in vivo.

Differential mechanism of light-induced and oxygen-dependent restoration of the high-potential form of cytochrome b559 in Tris-treated Photosystem II membranes

The effect of illumination and molecular oxygen on the redox and the redox potential changes of cytochrome b 559 (cyt b 559) has been studied in Tris-treated spinach photosystem II (PSII) membranes. It has been demonstrated that the illumination of Tris-treated PSII membranes induced the conversion of the intermediate-potential (IP) to the reduced high-potential (HP Fe2+) form of cyt b 559, whereas the removal of molecular oxygen resulted in the conversion of the IP form to the oxidized high-potential (HP Fe3+) form of cyt b 559. Light-induced conversion of cyt b 559 from the IP to the HP form was completely inhibited above pH 8 or by the modification of histidine ligand that prevents its protonation. Interestingly, no effect of high pH or histidine modification was observed during the conversion of the IP to the HP form of cyt b 559 after the removal of molecular oxygen. These results indicate that conversion from the IP to the HP form of cyt b 559 proceeds via different mechanisms. Under illumination, conversion of the IP to the HP form of cyt b 559 depends primarily on the protonation of the histidine residue, whereas under anaerobic conditions, the conversion of the IP to the HP form of cyt b 559 is driven by higher hydrophobicity of the environment around the heme iron resulting from the absence of molecular oxygen.

Superoxide oxidase and reductase activity of cytochrome b559 in photosystem II

This study provides evidence for the superoxide oxidase and the superoxide reductase activity of cytochrome b 559 (cyt b 559) in PSII. It is reported that in Tris-treated PSII membranes upon illumination, both the intermediate potential (IP) and the reduced high potential (HP red) forms of cyt b 559 exhibit superoxide scavenging activity and interconversion between IP and HP red form. When Tris-treated PSII membranes were illuminated in the presence of spin trap EMPO, the formation of superoxide anion radical (O 2 − ) was observed, as confirmed by EPR spin-trapping spectroscopy. The observations that the addition of enzymatic (superoxide dismutase) and non-enzymatic (cytochrome c, α-tocopherol and Trolox) O 2 − scavengers prevented the light-induced conversion of IP ↔ HP red cyt b 559 confirmed that IP and HP red cyt b 559 are reduced and oxidized by O 2 − , respectively. Redox changes in cyt b 559 by an exogenous source of O 2 − reconfirmed the superoxide oxidase and reductase activity of cyt b 559. Furthermore, the light-induced conversion of IP to HP red form of cyt b 559 was completely inhibited at pH > 8 and by chemical modification of the imidazole ring of histidine residues using diethyl pyrocarbonate. We proposed that a change in the environment around the heme iron, induced by the protonation and deprotonation of His 22 residue generates a favorable condition for the oxidation and reduction of O 2 − , respectively.

Mobility and speciation of heavy metals in soils impacted by hazardous waste

This study describes the mobility and chemical fractionation of heavy metals (HMs) from a site impacted by auto battery slag that was generated from secondary lead smelting operations. Samples were collected from the waste pile and from the immediate surrounding soil at four depths to assess the migration and potential bioavailability of Pb, Cd, Cr, Ni and Zn. Total levels of the HMs and their fractionation were determined. The results indicate that highest levels of HMs are present in the uppermost layer with significant migration down the depth, thereby posing a threat to groundwater quality. In the fractions, the concentrations of the metals follow this sequence: Pb>Zn>Cd>Cr>Ni. The chemical fractions of Pb, Cd, Cr, Ni and Zn in the samples, expressed as mean concentrations of the sum of the individual chemical fractions, demonstrate that the HMs exist mainly in the non-residual fractions. For instance, the percentage of non-residual fractions of lead in the waste pile and the surrounding soil ranged from 48.9 to 95.6% and 69.4 to 98.3% respectively. The mobility factors of the heavy metals are significantly high indicating high potential mobility and bioavailable forms of these HMs. The high concentrations of the HMs particularly Pb in the non-residual fractions, as observed in this study, shows the impact of anthropogenic activities on enrichment of natural soil with bioavailable HMs. Consequently, there is a need to be cautious in the way waste that is generated from heavy metals projects is added to natural soil.

Comparison of Electron-Transfer Rates for Metal- versus Ring-Centered Redox Processes of Porphyrins in Monolayers on Au(111)

The standard electron-transfer rate constants ( k ( 0 )) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between Fe (III) and Fe (II) oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the pi-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu 4NCl) conditions, the Zn (II) center has a coordinated Cl (-) ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed a low-potential form ( E l (0) approximately -0.6 V) wherein the metal center has a coordinated Cl (-) ion when it is in either the Fe (II) or Fe (III) state and a high-potential form ( E h (0) approximately +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the Fe (III) to Fe (II) states. The k ( 0 ) values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k ( 0 ) values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k ( 0 ) values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl (-) exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a pi molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.