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千葉大学教育学部研究紀要. II, 人文・社会科学編 4

Time-Dependent Photodimerization of α-<i>trans</i>-Cinnamic Acid Studied by Photocalorimetry and NMR Spectroscopy

The time course of photochemical solid-state reactions is routinely monitored by using spectroscopic methods such as NMR or IR spectroscopies, but is comparatively less investigated with thermal methods. In this work, a combination of thermal methods (thermogravimetric analysis and differential scanning calorimetry) was applied together with irradiation with UV light to quantify the conversion and monitor the progress of a well-known photochemical reaction, the [2 + 2] dimerization of <i>trans</i>-cinnamic acid, and the results are compared with the conversion determined by using ¹H NMR spectroscopy. The conversion was correlated with thermodynamic parameters for the reactant such as molar enthalpy, entropy, and melting temperature

Photoinduced Dynamics of Oxyluciferin Analogues: Unusual Enol “Super”photoacidity and Evidence for Keto–Enol Isomerization

The first systematic pico-nanosecond time-resolved spectroscopic study of the firefly emitter oxyluciferin and two of its chemically modified analogues revealed that in the excited state the enol group is more acidic than the phenol group. The 6′-dehydroxylated derivative, in which only the 4-enolic hydroxyl proton is acidic, has an experimentally determined p<i>K</i>_a* of 0.9 in dimethyl sulfoxide and an estimated p<i>K</i>_a* of −0.3 in water. Moreover, this compound provided direct evidence that in a nonpolar, basic environment the keto form in the excited state can tautomerize into the enol, which subsequently undergoes excited-state proton transfer (ESPT) to produce enolate ion. This observation presents the first experimental evidence of excited-state keto–enol tautomerization of a firefly fluorophore, and it could be important in resolving the enol–keto conundrum related to the color-tuning mechanism of firefly bioluminescence. The 6′-dehydroxylated form of oxyluciferin adds a very rare case of a stable enol to the family of “super”photoacids

Direct Observation of Asphaltene Nanoparticles on Model Mineral Substrates

The propensity for adherence to solid surfaces of asphaltenes, a complex solubility class of heteropolycyclic aromatic compounds from the heavy fraction of crude oil, has long been the root cause of scale deposition and remains an intractable problem in the petroleum industry. Although the adhesion is essential to understanding the process of asphaltene deposition, the relationship between the conformation of asphaltene molecules on mineral substrates and its impact on adhesion and mechanical properties of the deposits is not completely understood. To rationalize the primary processes in the process of organic scale deposition, here we use atomic force microscopy (AFM) to visualize the morphology of petroleum asphaltenes deposited on model mineral substrates. High imaging contrast was achieved by the differential adhesion of the tip between asphaltenes and the mineral substrate. While asphaltenes form smooth continuous films on all substrates at higher concentrations, they deposit as individual nanoparticles at lower concentrations. The size, shape, and spatial distribution of the nanoaggregates are strongly affected by the nature of the substrate; while uniformly distributed spherical particles are formed on highly polar and hydrophilic substrates (mica), irregular islands and thicker patches are observed with substrates of lower polarity (silica and calcite). Asphaltene nanoparticles flatten when adsorbed on highly oriented pyrolytic graphite due to π–π interactions with the polycyclic core. Force–distance profiles provide direct evidence of the conformational changes of asphaltene molecules on hydrophilic/hydrophobic substrates that result in dramatic changes in adhesion and mechanical properties of asphaltene deposits. Such an understanding of the nature of adhesion and mechanical properties tuned by surface properties, on the level of asphaltene nanoaggregates, would contribute to the design of efficient asphaltene inhibitors for preventing asphaltene fouling on targeted surfaces. Unlike flat surfaces, the AFM phase contrast images of defected calcite surfaces show that asphaltenes form continuous deposits to fill the recesses, and this process could trigger the onset for asphaltene deposition

Mechanistic Insight into Marine Bioluminescence: Photochemistry of the Chemiexcited Cypridina (Sea Firefly) Lumophore

Cypridina hilgendorfii (sea firefly) is a bioluminescent crustacean whose bioluminescence (BL) reaction is archetypal for a number of marine organisms, notably other bioluminescent crustaceans and coelenterates. Unraveling the mechanism of its BL is paramount for future applications of its strongly emissive lumophore. Cypridina produces light in a three-step reaction: First, the cypridinid luciferin is activated by an enzyme to produce a peroxide intermediate, cypridinid dioxetanone (CDO), which then decomposes to generate excited oxyluciferin (OxyCLnH*). Finally, OxyCLnH* deexcites to its ground state along with emission of bright blue light. Unfortunately, the detailed mechanism of the critical step, the thermolysis of CDO, remains unknown, and it is unclear whether the light emitter is generated from a neutral form (CDOH) or anionic form (CDO^–) of the CDO precursor. In this work, we investigated the key step in the process by modeling the thermal decompositions of both CDOH and CDO^–. The calculated results indicate that the decomposition of CDO^– occurs via the gradually reversible charge transfer (CT)-initiated luminescence (GRCTIL) mechanism, whereas CDOH decomposes through an entropic trapping mechanism without an obvious CT process. The thermolysis of CDO^– is sensitive to solvent effects and is energetically favorable in polar environments compared with the thermolysis of CDOH. The thermolysis of CDO^– produces the excited oxyluciferin anion (OxyCLn^–*), which combines with a proton from the environment to form OxyCLnH*, the actual light emitter for the natural system

Probing Structural Perturbation in a Bent Molecular Crystal with Synchrotron Infrared Microspectroscopy and Periodic Density Functional Theory Calculations

The range of unit cell orientations generated at the kink of a bent single crystal poses unsurmountable challenges with diffraction analysis and limits the insight into the molecular-scale mechanism of bending. On a plastically bent crystal of hexachlorobenzene, it is demonstrated here that spatially resolved microfocus infrared spectroscopy using synchrotron radiation can be applied in conjunction with periodic density functional theory calculations to predict spectral changes or to extract information on structural changes that occur as a consequence of bending. The approach reproduces well the observed trends, such as the wall effects, and provides estimations of the vibrational shifts, unit cell deformations, and intramolecular parameters. Generally, expansion of the lattice induces red-shift while compression induces larger blue-shift of the characteristic ν­(C–C) and ν­(C–Cl) modes. Uniform or non-uniform expansion or contraction of the unit cell of 0.1 Å results in shifts of several cm^–1, whereas deformation of the cell of 0.5° at the unique angle causes shifts of <0.5 cm^–1. Since this approach does not include parameters related to the actual stimulus by which the deformation has been induced, it can be generalized and applied to other mechanically, photochemically, or thermally bent crystals

Glucosamine Salts: Resolving Ambiguities over the Market-Based Compositions

The neutral form of glucosamine, C₆H₁₃NO₅, one the most effective and most widely used over-the-counter health supplements for the relief of osteoarthritis, is very unstable in air. It is marketed as chloride and sulfate salts. Unlike the stable glucosamine chloride, direct use in pharmaceutical formulations of the sulfate, ostensibly the physiologically more active form, is hindered by its strong hygroscopicity. Copious patent literature exists describing methods for stabilization of the sulfate by converting it into double and/or mixed salts, usually with alkaline or earth alkaline sulfates and chlorides. Aiming to unravel the structures of the alleged double/mixed salts, we attempted synthesis of the stabilized forms of the sulfate following literature procedures. Our repeated attempts did <i>not</i> yield true glucosamine sulfate or any real (in the chemical sense) double or mixed salts. Instead, Fourier transform infrared spectroscopy, powder X-ray diffraction, thermogravimetric analysis, and elemental analyses consistently showed that physical mixtures of the stable glucosamine chloride, which has a strong propensity to crystallize out from solutions, and the respective alkaline salts are obtained in all cases. Expectedly, these mixtures were non-hygroscopic. The analysis of the commercially available sample of “glucosamine sulfate” showed that it is a mixture of glucosamine chloride and K₂SO₄, in accordance with the above conclusions. By using a simple ion exchange in glucosamine chloride, we devised a simple method to generate glucosamine sulfate. As anticipated, the latter is a very hygroscopic powder in the solid state and is chemically moderately unstable in solution. Along with the conclusions based on the products obtained following published procedures, reaction of this compound with alkali chlorides readily affords the (non-hygroscopic) glucosamine chloride in a mixture with the respective alkali sulfates. We are tempted to conclude that the alleged “stabilization” of glucosamine sulfate by formation of double/mixed salts is (in the chemical sense) misleading. We believe that these compounds have probably never been obtained, and the related published synthetic procedures should be reinvestigated. The conclusions of this study could have important implications on the effective amount of the active ingredient required to achieve physiological activity, because such “stabilized” mixtures contain less than the optimal amount of the physiologically active ingredient, which could also have some commercial implications

Biomimetic Crystalline Actuators: Structure–Kinematic Aspects of the Self-Actuation and Motility of Thermosalient Crystals

While self-actuation and motility are habitual for humans and nonsessile animals, they are hardly intuitive for simple, lifeless, homogeneous objects. Among mechanically responsive materials, the few accidentally discovered examples of crystals that when heated suddenly jump, propelling themselves to distances that can reach thousands of times their own size in less than 1 ms, provide the most impressive display of the conversion of heat into mechanical work. Such <i>thermosalient crystals</i> are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to the exclusivity and incongruity of the phenomenon, as well as because of the unavailability of ready analytical methodology for its characterization, the reasons behind this colossal self-actuation remain unexplained. Aimed at unraveling the mechanistic aspects of the related processes, herein we establish the first systematic assessment of the interplay among the thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The collective results are consistent with a latent but very rapid anisotropic unit cell deformation in a two-stage process that ultimately results in crystal explosion, separation of debris, or crystal reshaping. The structural perturbations point to a mechanism similar to phase transitions of the martensitic family

Biomimetic Crystalline Actuators: Structure–Kinematic Aspects of the Self-Actuation and Motility of Thermosalient Crystals

While self-actuation and motility are habitual for humans and nonsessile animals, they are hardly intuitive for simple, lifeless, homogeneous objects. Among mechanically responsive materials, the few accidentally discovered examples of crystals that when heated suddenly jump, propelling themselves to distances that can reach thousands of times their own size in less than 1 ms, provide the most impressive display of the conversion of heat into mechanical work. Such <i>thermosalient crystals</i> are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to the exclusivity and incongruity of the phenomenon, as well as because of the unavailability of ready analytical methodology for its characterization, the reasons behind this colossal self-actuation remain unexplained. Aimed at unraveling the mechanistic aspects of the related processes, herein we establish the first systematic assessment of the interplay among the thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The collective results are consistent with a latent but very rapid anisotropic unit cell deformation in a two-stage process that ultimately results in crystal explosion, separation of debris, or crystal reshaping. The structural perturbations point to a mechanism similar to phase transitions of the martensitic family

Biomimetic Crystalline Actuators: Structure–Kinematic Aspects of the Self-Actuation and Motility of Thermosalient Crystals

While self-actuation and motility are habitual for humans and nonsessile animals, they are hardly intuitive for simple, lifeless, homogeneous objects. Among mechanically responsive materials, the few accidentally discovered examples of crystals that when heated suddenly jump, propelling themselves to distances that can reach thousands of times their own size in less than 1 ms, provide the most impressive display of the conversion of heat into mechanical work. Such <i>thermosalient crystals</i> are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to the exclusivity and incongruity of the phenomenon, as well as because of the unavailability of ready analytical methodology for its characterization, the reasons behind this colossal self-actuation remain unexplained. Aimed at unraveling the mechanistic aspects of the related processes, herein we establish the first systematic assessment of the interplay among the thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The collective results are consistent with a latent but very rapid anisotropic unit cell deformation in a two-stage process that ultimately results in crystal explosion, separation of debris, or crystal reshaping. The structural perturbations point to a mechanism similar to phase transitions of the martensitic family