ANALYSIS OF STUDENTS' ABILITY TO IDENTIFY SYMBOLIC REPRESENTATIONS IN CHEMISTRY

To describe the behavior of matter and energy, chemists classify them in three distinct domains: macroscopic, microscopic, and symbolic. The ability to use these three representations is the basis for understanding concepts in chemistry. This study aims to analyze students' ability to identify symbolic representations in chemistry. The research design uses a one shot case study. The subjects of this study were students of prospective science teachers as many as 85 students. Data collection techniques using tests and rubrics. The results showed that of the ten symbolic representation statements, only three statements achieved the highest percentage of correct answers, namely statements about writing ionization reactions and writing electron symbols. There are two statements where almost 90 percent of students answered incorrectly. The statement is about reversible or irreversible reaction equations and exothermic reaction equations. From this research, it can be concluded that students' ability to identify symbolic representations in chemistry still needs to be improved, because the average score is still low.

A Surface Network Based on Polytyramine/ Gold Nanoparticles: Characterization, Kinetics, Thermodynamics and Selective Determination of Norepinephrine

A solid-state sensor was fabricated by a spontaneous electrochemical deposition of polytyramine (Ptyr) film onto a glassy carbon electrode (GCE) which was further peripherally supported by gold nanoparticles (AuNPs). The surface materials of the developed sensor (AuNPs.Ptyr-GCE) were characterized by X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The rate constant of charge transfers (kct) of the as-prepared sensor (8.77 × 10-4 cm/s) was evaluated by fitting the charge transfer resistance (Rct) data in the presence of ferric-ferrous hexacyanide redox couple solution, [Fe(CN)6]3-/4-. The voltammetric behavior of norepinephrine (NOR) was confirmed to follow an irreversible reaction mechanism at which the estimated diffusion coefficient value was 7.39 × 10-5 cm2/s. The sensor showed a large enhancement on NOR oxidation and comparatively lowered its detection limit (DL3s) to 0.130 mM (22 ppb). It was also applied for selective determination of NOR in the presence of high concentrations of ascorbic acid (AA) and uric acid (UA). The interference study highlighted the great stability of the proposed sensor by generating a similar sensitivity as in the pure NOR solution. The analytical performance of the proposed system was validated successfully for pharmaceutical and biological samples with tolerable recovery percentages.

Parametric Control of Melting Gel Morphology and Chemistry via Electrospray Deposition

Melting gels are a class of hybrid organic-inorganic, silica-based sol-gels which are solid below their glass transition temperatures, near room temperature, but show thermoplastic behavior when heated. While this phase change can be repeated multiple times, heating the gel past its consolidation temperature, typically above 130 °C, initiates an irreversible reaction that produces highly crosslinked glassy organic/inorganic materials via hydrolysis and polycondensation. This ability makes melting gels uniquely compatible with processing techniques inaccessible to other sol-gels. By properly tuning their properties, it should be possible to create protective coatings for electronics and anti-corrosive coatings for metals that are highly hydrophobic and insulating. However, melting gel consolidation reactions are highly dependent on charge interactions, raising the question of how these materials will respond to a processing technique, like electrospray deposition (ESD), which is dependent on charge delivery. In this study, we focus on the role that substrate temperature and charge polarity play on film morphology, consolidation chemistry, and surface properties when processing via ESD. Optical images, film thickness measurements, and FTIR were used to characterize the sprayed melting gel with the goal of developing a robust processing space for producing highly cross linked, hydrophobic, dielectric coatings.

Enhanced Reversible Zinc Ion Intercalation in Deficient Ammonium Vanadate for High-Performance Aqueous Zinc-Ion Battery

Ammonium vanadate with bronze structure (NH4V4O10) is a promising cathode material for zinc-ion batteries due to its high specific capacity and low cost. However, the extraction of $${\text{NH}}_{{4}}^{ + }$$ NH 4 + at a high voltage during charge/discharge processes leads to irreversible reaction and structure degradation. In this work, partial $${\text{NH}}_{{4}}^{ + }$$ NH 4 + ions were pre-removed from NH4V4O10 through heat treatment; NH4V4O10 nanosheets were directly grown on carbon cloth through hydrothermal method. Deficient NH4V4O10 (denoted as NVO), with enlarged interlayer spacing, facilitated fast zinc ions transport and high storage capacity and ensured the highly reversible electrochemical reaction and the good stability of layered structure. The NVO nanosheets delivered a high specific capacity of 457 mAh g−1 at a current density of 100 mA g−1 and a capacity retention of 81% over 1000 cycles at 2 A g−1. The initial Coulombic efficiency of NVO could reach up to 97% compared to 85% of NH4V4O10 and maintain almost 100% during cycling, indicating the high reaction reversibility in NVO electrode.

Metabolic Homeostasis in Life as We Know It: Its Origin and Thermodynamic Basis

Living organisms require continuous input of energy for their existence. As a result, life as we know it is based on metabolic processes that extract energy from the environment and make it available to support life (energy metabolism). This metabolism is based on, and regulated by, the underlying thermodynamics. This is important because thermodynamic parameters are stable whereas kinetic parameters are highly variable. Thermodynamic control of metabolism is exerted through near equilibrium reactions that determine. (1) the concentrations of metabolic substrates for enzymes that catalyze irreversible steps and (2) the concentrations of small molecules (AMP, ADP, etc.) that regulate the activity of irreversible reactions in metabolic pathways. The result is a robust homeostatic set point (−ΔGATP) with long term (virtually unlimited) stability. The rest of metabolism and its regulation is constrained to maintain this set point. Thermodynamic control is illustrated using the ATP producing part of glycolysis, glyceraldehyde-3-phosphate oxidation to pyruvate. Flux through the irreversible reaction, pyruvate kinase (PK), is primarily determined by the rate of ATP consumption. Change in the rate of ATP consumption causes mismatch between use and production of ATP. The resulting change in [ATP]/[ADP][Pi], through near equilibrium of the reactions preceding PK, alters the concentrations of ADP and phosphoenolpyruvate (PEP), the substrates for PK. The changes in ADP and PEP alter flux through PK appropriately for restoring equality of ATP production and consumption. These reactions appeared in the very earliest lifeforms and are hypothesized to have established the set point for energy metabolism. As evolution included more metabolic functions, additional layers of control were needed to integrate new functions into existing metabolism without changing the homeostatic set point. Addition of gluconeogenesis, for example, resulted in added regulation to PK activity to prevent futile cycling; PK needs to be turned off during gluconeogenesis because flux through the enzyme would waste energy (ATP), subtracting from net glucose synthesis and decreasing overall efficiency.

A REP-FAMSEC Method as a Tool in Explaining Reaction Mechanisms: A Nucleophilic Substitution of 2-Phenylquinoxaline as a DFT Case Study

In search for the cause leading to low reaction yields, each step along the reaction energy profile computed for the assumed oxidative nucleophilic substitution of hydrogen (ONSH) reaction between 2-phenylquinoxaline and lithium phenylacetylide was modelled computationally. Intermolecular and intramolecular interaction energies and their changes between consecutive steps of ONSH were quantified for molecular fragments playing leading roles in driving the reaction to completion. This revealed that the two reactants have a strong affinity for each other, driven by the strong attractive interactions between Li and two N-atoms, leading to four possible reaction pathways (RP-C2, RP-C3, RP-C5, and RP-C10). Four comparable in energy and stabilizing molecular system adducts were formed, each well prepared for the subsequent formation of a C–C bond at either one of the four identified sites. However, as the reaction proceeded through the TS to form the intermediates (5a–d), very high energy barriers were observed for RP-C5 and RP-C10. The data obtained at the nucleophilic addition stage indicated that RP-C3 was both kinetically and thermodynamically favored over RP-C2. However, the energy barriers observed at this stage were very comparable for both RPs, indicating that they both can progress to form intermediates 5a and 5b. Interestingly, the phenyl substituent (Ph1) on the quinoxaline guided the nucleophile towards both RP-C2 and RP-C3, indicating that the preferred RP cannot be attributed to the steric hindrance caused by Ph1. Upon the introduction of H2O to the system, both RPs were nearly spontaneous towards their respective hydrolysis products (8a and 8b), although only 8b can proceed to the final oxidation stage of the ONSH reaction mechanism. The results suggest that RP-C2 competes with RP-C3, which may lead to a possible mixture of their respective products. Furthermore, an alternative, viable, and irreversible reaction path was discovered for the RP-C2 that might lead to substantial waste. Finally, the modified experimental protocol is suggested to increase the yield of the desired product.

Вольтамперометрия кинетически необратимого электрохимического процесса на шероховатом электроде

Исследована роль эффекта морфологической неоднородности поверхности электрода в вольтамперометрическом отклике необратимого электрохимического процесса, протекающего в смешанно-кинетическом режиме. Разработан алгоритм численного моделирования электродной реакции, включающей последовательные стадии необратимого переноса заряда и диффузионного массопереноса, с применением метода конечных элементов в компьютерном пакете Comsol Multiphysics. Численным решением диффузионно-кинетической задачи получены поляризационныекривые необратимого электрохимического процесса на электродах с шероховатой поверхностью, формируемой неровностями различного геометрического типа (синусоидальная поверхность, поверхность с выступами, трапецеидальная поверхность, пилообразная поверхность и «случайная» поверхность). Установлены условия использования вольтамперометрического метода изучения кинетики электрохимических процессов, при которых необходимо учитывать шероховатость электрода. Найдено, что при относительно больших скоростях сканирования потенциала вольтамперометрический максимум на поляризационной кривой формируется в условиях весьмамалой толщины диффузионного слоя, который повторяет профиль шероховатой поверхности, поэтому сила тока пика пропорциональна фактору шероховатости. Если же скорость сканирования относительно мала, то к моменту достижения пика на вольтамперограмме фронт диффузии полностью сглаживается, и шероховатость поверхности электрода уже не влияет на ток максимума. При этом форма неровностей, ответственных за шероховатость, не оказывает заметного влияния на вольтамперометрическом отклике необратимого электрохимического процесса. ЛИТЕРАТУРА 1. Compton R. G., Banks Craig E. Understanding Voltammetry. World Scientific; 2007. 371 p. DOI:https://doi.org/10.1142/64302. Vvedenskii A. V., Kozaderov О. А. Linear voltammetry of anodic selective dissolution of homogeneousmetallic alloys. In: Saito Y., Kikuchi T. (eds.) Voltammetry: theory, types and applications. New York: NovaScience Publishers, Inc.; 2014. 349 p.3. Bard A. J., Faulkner L. R. Electrochemical methods. Fundamentals and applications. 2nd edition. New York:Wiley; 2000. 856 p.4. Галюс З. Теоретические основы электрохимического анализа. М.: Мир; 1974. 552 с.5. Matsuda H., Ayabe J. Z. Zur theorie der randlessevcikschen kathodenstrahl-polarographie. Z. Electrochem.1955;59( 6): 494–503. DOI: https://doi.org/10.1002/bbpc.195505906056. Menshykau D., Streeter I., Compton R. G. Influence of electrode roughness on cyclic voltammetry. J.Phys. Chem. C. 2008;112(37): 14428–14438. DOI: https://doi.org/10.1021/jp80474237. Menshykau D., Compton R. G. Infl uence of electrode roughness on stripping voltammetry: mathematicalmodeling and numerical simulation. J. Phys. Chem. C. 2009;113(35): 15602–15620. DOI: https://doi.org/10.1021/jp904187t8. Козадеров О. А., Введенский А. В. Вольтамперометрия селективного растворения бинарногогомогенного металлического сплава в условиях твердофазного массопереноса. Конденсированныесреды и межфазные границы. 2011;13(4): 452–459. Режим доступа: http://www.kcmf.vsu.ru/resources/t_13_4_2011_010.pdf9. Козадеров О. А., Лозовский В. В., Введенский А. В. Хроновольтамперометрия анодного растворения сплавов Ag-Au в нитратной среде. Физико-химия поверхности и защита материалов.2008;44(4): 359–368. Режим доступа: http://naukarus.com/hronovoltamperometriya-anodnogorastvoreniya-splavov-ag-au-v-nitratnoy-srede10. Козадеров О. А., Введенский А. В. Вольтамперометрия селективного растворения Ag,Au-сплавов в условиях твердофазно-жидкофазного массопереноса. Физикохимия поверхности и защитаматериалов. 2013;49(6): 661–670. DOI: https://doi.org/10.7868/S004418561306009011. Prasad M. Arun, Sangaranarayanan M. V. Formulation of a simple analytical expression for irreversibleelectron transfer processes in linear sweep voltammetry and its experimental verifi cation. ElectrochimicaActa. 2004;49(16): 2569–2579. DOI: https://doi.org/10.1016/j.electacta.2004.01.02812. Singh T., Dutt J. Linear sweep voltammetry at the tubular graphite electrode: Part II. Totally irreversibleprocesses. J. of Electroanalytical Chem. and Interfacial Electrochemistr. 1985;196(1): 35–42. DOI:https://doi.org/10.1016/0022-0728(85)85078-613. Jin W., Cui H., Zhu L., Wang Sh. On the theory of the integer and half-integer integral and derivativelinear potential sweep voltammetry for a totally irreversible interfacial reaction. J. of ElectroanalyticalChem. and Interfacial Electrochemistr. 1991;309(1–2): 37–47. DOI: https://doi.org/10.1016/0022-0728(91)87002-L14. Aoki K., Tokuda K., Matsuda H. Theory of linear sweep voltammetry with fi nite diffusion space: Part II.Totally irreversible and quasi-reversible cases. J. of Electroanalytical Chem. and Interfacial Electrochemistr.1984;160(1–2): 33–45. DOI: https://doi.org/10.1016/S0022-0728(84)80113-815. Andricacos P. C., Cheh H. Y. The application of linear sweep voltammetry to a rotating disk electrodefor a fi rst-order irreversible reaction. J. of Electroanalytical Chem. and Interfacial Electrochemistr.1981;124(1–2): 95–101. DOI: https://doi.org/10.1016/S0022-0728(81)80287-216. Kohler H., Piron D. L., Bйlanger G. A Linear Sweep Voltammetry theory for irreversible electrodereactions with an order of one or higher: I. Mathematical formulation. J. of the Electrochemical Society.1987;134(1): 120–126. DOI: https://doi.org/10.1149/1.210038817. Nahir T. M., Clark R. A. Bowden E. F. Linearsweep voltammetry of irreversible electron transfer insurface-confi ned species using the marcus theory. Anal. Chem. 1994;66(15): 2595–2598. DOI: https://doi.org/10.1021/ac00087a02718. Трухан С. Н., Деревщиков В. С. Компьютерное моделирование процессов и явлений физическойхимии. Новосибирск: ННИГУ; 2012. 75 с.19. Красников Г. Е., Нагорнов О. В., Старостин Н. В. Моделирование физических процессов сиспользованием пакета Comsol Multiphysics. М.: НИЯУ МИФИ; 2012. 184 с.20. Дамаскин Б. Б., Петрий О. А., Цирлина Г. А. Электрохимия: Учебник для вузов. М.: Химия; 2001.624 c.

Protein-Templated Hit Identification via an Ugi Four-Component Reaction

Kinetic target-guided synthesis represents an efficient hit-identification strategy, in which the protein assembles its own inhibitors from a pool of building blocks via an irreversible reaction. Herein, we pioneered an in situ Ugi reaction for the identification of novel inhibitors of a model enzyme and binders for an important drug target, namely, the aspartic protease endothiapepsin and the bacterial ß-sliding clamp DnaN, respectively. Highly sensitive mass-spectrometry methods enabled monitoring of the protein-templated reaction of four reaction partners, which occurred in a background-free manner for endothiapepsin or with a clear amplification of two binders in the presence of DnaN. The Ugi products show low micromolar activity on endothiapepsin or moderate affinity for DnaN. We succeeded in expanding the portfolio of chemical reactions and biological targets and demonstrated the efficiency and sensitivity of this approach, which can find application on any drug target.<br>