Determination of the Effect of Temperature on the Concentration of keto form of D-Fructose by FT-IR Spectroscopy

Abstract

FT-IR spectroscopy has been employed in the detection of the carbonyl absorption band centered at 1728 cm^?1 and assigned to the open form of D-fructose in deuterium oxide or water. Changes in the intensity of the band at 1728 cm^?1 allowed the monitoring of the concentration of the open form of the keto sugar D-fructose at different temperatures and pHs. The concentration of the open form was observed to increase with increasing temperature and was an order of magnitude higher at 80 °C compared to 30 °C. The buildup of the open form was found to be extremely rapid. The new equilibrium can be reversed with decreasing temperature with a slight hysteresis. This work demonstrates the potential of applying FT-IR spectroscopy in studying the effect of environmental factors on the level of the open chain form of sugars.     

Bis(phenylthienyl)ethene-tethered beta-cyclodextrin dimers as photoswitchable hosts

Two beta-cyclodextrin dimers tethered by photoswitchable bis(phenylthienyl)ethene moieties were synthesized as potentially tunable receptor molecules. The cyclodextrin cavities of these dimers were linked via their secondary sides, with the photochromic bis(phenylthienyl)ethene unit either directly connected to the secondary rim (7) or via propyl spacers (10). By irradiation with light the dimers were reversibly switched between a relatively flexible (open) form and a rigid (closed) form. The photostationary states for both dimers consisted of 92% of the open and 8% of the closed form, enabling the nearly complete conversion between the two forms. The binding properties of the open and closed forms of dimers 7 and 10 were assessed by complexation studies with meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) using isothermal titration calorimetry. For the rigidly tethered dimer 7, a factor 8 difference in binding affinity between the open and closed form of the dimer was found. This difference in binding affinity reflects the difference in enthalpy of binding for the two dimers, indicating that the beta-cyclodextrin cavities of the closed dimer 7b are spaced too far apart from each other by the rigid closed bis(phenylthienyl)ethene tether to cooperatively bind TSPP. The difference in binding affinity was sufficient to enable the phototriggered release of TSPP from dimer. The thermodynamic parameters obtained for dimer 10 suggested that the closed tether substantially contributes to the binding of TSPP. The open and closed form of dimer 10 bound TSPP with similar association constants, although the enthalpy of binding for the complexation of TSPP by the closed form of dimer 10 was more favorable than that found for the open form of the dimer.     

Phenyl-bridging in the 2-phenylethyl radical. A molecular orbital study

Density functional theory at the UB3PW91/6-31G(d.p) level on the open and bridged forms of the 2-phenylethyl radical are reported here together with activation energies and calculated rate constants for rearrangement of the bridged to the open radical. In addition, the effect of substituents on the aryl ring upon the relative energies, activation energies, and rate constants for rearrangement from the bridged to open forms are presented. Para-substituents include CH3, CF3.CN, CHO.OH, SH, O-, S-, and NO. The parent bridged radical is found to be 10.1 kcal/mol higher in enthalpy than the open form. The activation energy for conversion of the bridged to open radical is 3.96 kcal/mol. Para-substitution by CN or CHO significantly lowers the difference in energy between the species, while substitution by NO renders the bridged form more stable. Para-substitution by CN or CHO coupled with substitution with CN in the ortho-positions makes the open and bridged radical approximately equivalent in energy.     

A novel photochromic fulgide based on porphyrin for nondestructive information processing

A fulgide connected to porphyrin (FUL-TPP) can transform its open isomer to closed isomer upon the irradiation with UV or visible light. Herein, they can be used to write binary data. Furthermore, the open form can emit luminescence but the closed cannot form while irradiated in another light that will not cause the optical chemical reaction. Therefore, the data can be read out without destruction.     

Thermally activated and light-induced metal ion complexation of 5′-(hydroxy)spiroindolinonaphthooxazines in polar solvents

Chelation with Al(III), Fe(II) or Cu(II) of the open photomerocyanine form obtained under steady irradiation of spiroindolinonaphthooxazines, with a hydroxyl group at the 5′ position in the naphthooxazine moiety, induces a slight hypsochromic shift of its visible absorption band and increases the lifetime of this form, slowing down its thermal bleaching in the dark (rate constant  10⁻³ s⁻¹). Complexation with Al(III), Fe(II) or Cu(II) allows the spiroindolinonaphthooxazines to isomerize to their open coloured form even under dark conditions giving a complex spectroscopically identical to the photoinduced product. The activation energy of thermal complexation is independent of the metal ion which implies the ring opening as the rate determining step.     

Irradiation-dependent equilibrium between open and closed form of UV absorbers of the 2-(2-hydroxyphenyl)-1,3,5-triazine type

2-(2-Hydroxyphenyl)-4,6-diaryl-1,3,5-triazines (HPTs) bearing one or more intramolecular hydrogen bonds (IMHBs) show phosphorescence in polar solvents at 77 K which increases in intensity with UV-irradiation time (λ_irr = 333 nm) until an equilibrium value is reached (phosphorescence evolution). Phosphorescence emission is produced from open conformers of the molecules with intermolecular rather than intramolecular hydrogen bonds, which are formed in polar solvents under the influence of UV radiation. All IMHBs of an individual molecule must be broken to enable triplet state population. Reformation of the closed form is observed for HPTs after dark periods resulting in a lower initial phosphorescence intensity upon renewed irradiation (relaxation). The methoxy derivatives (MPTs), where the OH groups are replaced by methoxy groups, can be employed as spectroscopic models for the open form; they emit phosphorescence independent of irradiation time. For the equilibrium between open and closed form of M-OH-P (2-(2-hydroxy-4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine) under constant irradiation (λ_irr = 333 nm, 100 W-HBO lamp) a proportion of 5% of open form was evaluated.     

Properties of stable organic bond-stretched non-Lewis molecules

The conditions required for the existence of a stable bond-stretched singlet isomer of hetero derivatives of bicyclo[2.1.0]pentane (which is a cyclopentane-1,3-diyl derivative) are discussed. Such species are non-Lewis systems with a ruptured C-C bond (formally diradicals), in which two electrons occupy the nonbonding orbital. A high-level calculation shows that in contrast with the carbon substituted compounds, in which the open form is a transition state between two classical-bonded closed bicyclic forms, in the heterosubstituted molecules, the open form is calculated to be a stable minimum. The ionization potentials of the open forms are considerably lower than those of their bicyclic isomers and also of regular organic radicals/diradicals. Nitrogen atoms are found to be more effective than oxygen or sulfur in stabilizing the open isomer. In this case, the open isomer is calculated to be a little more stable than the bicyclic compound, and a barrier of approximately 40 kcal/mol is computed for the ring closing reaction. Thus, the open isomer is both thermodynamically and kinetically stable. This result rationalizes some experimental observations that indicated the existence of non-Lewis singlet species.     

Photochromic Properties of β-Cyclodextrin Dimer Linked by Dithienylethene

A fi-cyclodextrin dimer tethered by photoswitchable dithienylethene moieties was synthesized as a potentially tunable receptor. The dimer exhibits pronounced photochromic properties. Irradiation of the dimer in open form with UV light at 254 nm resulted in immediate photocyclization to the pink closed form; the colorless open form could be regenerated by irradiation with visible light of wavelength >460 nm. The reaction kinetics of the forward photoprocess were also studied. To investigate the binding ability of the dimer in open and closed forms, a fluorescence titration was performed. It was found that the stability constant for the binding of TSPP (meso-tetrakis (4-sulfonatophenyl)porphyrin) by the closed form of the dimer is a factor of 5 higher than that of the open form.     

7-trifluoromethylquinoline-functionalized luminescent photochromic spiropyran with the stable merocyanine species both in solution and in the solid state

A new spiropyran (SP2) with the stable merocyanine form (MC2) both in solution and in the solid state at room temperature was designed and synthesized. The stability of MC2 is believed to be due to the electron-withdrawing effect of both the quinoline and the trifluoromethyl groups. (1)H NMR spectra indicate that the ratio of the open form vs the closed form of SP2 is dependent on the polarity of solvents. Single crystals composed of only the open form (MC2) were successfully obtained. X-ray structural analysis indicates that except trifluoromethyl and two methyl groups MC2 is completely planar with an s-trans,s-cis conformation. It should be noted that this is the first report of the X-ray crystal structure of the pure open form of spiropyran. MC2 can be slowly transformed into SP2 at -30 degrees C or lower temperature, and the process is accelerated by visible light irradiation. This special photochromic behavior can be explained by the calculated thermodynamic data. The spectral properties of SP2/MC2 in the presence of different metal ions are also studied, and the results show the potential application of SP2/MC2 in sensing metal ions.     

Solid-phase synthesis of thalidomide and its analogues

A novel solid-phase synthesis of thalidomide and its metabolites and analogues is described. The synthetic strategy involves the coupling of hydroxymethyl polystyrene with phthalic anhydride to form the resin-linked acid. The acid is then reacted with primary amines followed by acid or base treatment to form thalidomide and its analogues with either open or closed phthalimide rings. Most of the analogues are synthesized with high yields (40.3-98.1% in three steps) and purities (92.3-98.9%).     

Selective five- and six-membered cyclic amine syntheses via capture of episulfonium ions

Amide nitrogens open episulfonium ions to form pyrrolidines or piperidines selectively, depending on the nitrogen substituent, in either reversible or irreversible reactions.     

Photochromism of 6′-cyanosubstituted spirooxazines in frozen alcohol matrices

For three 6′-cyanosubstituted spironaphthooxazines, spectral characteristics of an open form and quantum yields of photoisomerization were determined both at room temperature and in frozen alcohol matrices. Spironaphthooxazines have demonstrated fairly high (0.01–0.02) quantum yields of open form appearance at 77 K. The observed peculiarities of the open form UV spectra were explained by the temperature dependence of the open form isomers distribution. Partial stabilization of the nonequilibrium isomers of an open form in the low-temperature matrices was revealed.     

Conformational Changes and Redox Properties of Bimetallic Single Helicates of Hexapyrrole-α,ω-dicarbaldehydes

The hexapyrrole-α,ω-dicarbaldehydes 1 a and 1 b were metallated with Cu^II, Ni^II, and Pd^II to give bimetallic complexes where a pair of 3 N+O four-coordinate metal planes are helically distorted and the central 2,2′-bipyrrole subunit adopts a cis or trans conformation. X-ray crystallographic analysis of the bisCu complex revealed a closed form with a cis-2,2′-bipyrrole subunit and an open form with a trans-2,2′-bipyrrole subunit. The bisPd complexes took a closed form both in the solid state and in solution. They are regarded as single helicates of two turns and the energy barrier for the interchange between an M helix and a P helix was remarkably influenced by the bulky 3,3′-substituent of the central 2,2′-bipyrrole subunit. Although the bisNi complexes adopt a closed form in the solid state, they exist as a homohelical open C₂-symmetric form or a heterohelical open C_i-symmetric form in solution. A theoretical study suggested that the closed form of 1 a Pd was stabilized by the Pd–Pd interaction. Compound 1 a Pd was reversibly oxidized by one electron at 0.14 V versus ferrocene/ferrocenium (Fc/Fc⁺) and this oxidized species showed Vis/NIR absorption bands at λ=767 and 1408 nm.     

A photochromic,electrochromic, thermochromic Ru complexed benzannulene: an organometallic example of the dimethyldihydropyrene-metacyclophanediene valence isomerization

The preparation of the first photochromic, organometallic derivative of the diarylethene class, the CpRu-complexed benzodimethyldihydropyrene 3, in which the organometallic is directly attached to the photochromic core, is described. The negative dark purple photochrome 3 readily bleaches to form the almost colorless cyclophanediene 3' on irradiation with visible light. The latter switches back to 3 either photochromically with UV light, electrochromically on reduction, or thermochromically on heating. Essentially quantitative conversion between the two states is possible. The open complex 3' thermally closes 2.6 times faster than the uncomplexed parent 2', but the closed form 3 opens with visible light at about 30% of the rate of uncomplexed 2. Both open forms, complexed 3' and uncomplexed 2' close equally fast with UV light.     

1H and 13C NMR characterization of hemiamidal isoniazid-NAD(H) adducts as possible inhibitors of InhA reductase of Mycobacterium tuberculosis

Isoniazid (INH) is easily oxidized with manganese(III) pyrophosphate, a chemical model of the KatG protein involved in activation of INH inside the bacteria Mycobacterium tuberculosis. Performed in the presence of NAD(+), this oxidation generates a family of isomeric INH-NAD(H) adducts, which have been shown to be effective inhibitors of InhA, an enzyme essential in mycolic acid biosynthesis. In this work, we fully characterized by (1)H and (13)C NMR spectroscopy four main species of INH-NAD(H) adducts that coexist in solution. Two of them are open diastereoisomers consisting of the covalent attachment of the isonicotinoyl radical at position four of the nicotinamide coenzyme. The other two result from a cyclization involving the amide group from the nicotinamide and the carbonyl group from the isonicotinoyl radical to give diastereoisomeric hemiamidals. Although an INH-NAD(H) adduct with a 4S configuration has been characterized within the active site of InhA from Xray crystallography and this bound adduct interpreted as an open form (Rozwarski et al., Science 1998, 279, 98-102), it is legitimate to raise the question about the effective active form(s), open or cyclic, of INH-NAD(H) adduct(s). Is there a single active form or are several forms able to inhibit the InhA activity with different levels of inhibitory potency?     

Theoretical studies of sulbactam: Reactions after acylation

DFT methods were used on the model molecular to determine the mechanism of β‐lactamase inhibitor. The results show that the thiazole ring can be opened as well as β‐lactam ring. The thiazole ring‐open product can be formed via β‐lactam ring‐open product or tetrahedral intermediate directly. Those products, in imine or enamine form, can tautomerize via hydrogen migration. The sulforious form products may be transformed into sulfonyl ones. Water or hydroxyl group in residues on active site will help lower the activation energy of above reactions, and if structure permitted, they also effect the route selection of whole reaction system. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Refining the active site structure of iron-iron hydrogenase using computational infrared spectroscopy

Iron-iron hydrogenases ([FeFe]H2ases) are exceptional natural catalysts for the reduction of protons to dihydrogen. Future biotechnological applications based on these enzymes require a precise understanding of their structures and properties. Although the [FeFe]H2ases have been characterized by single-crystal X-ray crystallography and a range of spectroscopic techniques, ambiguities remain regarding the details of the molecular structures of the spectroscopically observed forms. We use density functional theory (DFT) computations on small-molecule computational models of the [FeFe]H2ase active site to address this problem. Specifically, a series of structural candidates are geometry optimized and their infrared (IR) spectra are simulated using the computed C-O and C-N stretching frequencies and infrared intensities. Structural assignments are made by comparing these spectra to the experimentally determined IR spectra for each form. The H red form is assigned as a mixture of an Fe(I)Fe(I) form with an open site on the distal iron center and either a Fe(I)Fe(I) form in which the distal cyanide has been protonated or a Fe(II)Fe(II) form with a bridging hydride ligand. The Hox form is assigned as a valence-localized Fe(I)Fe(II) redox level with an open site at the distal iron. The Hox(air)(ox) form is assigned as an Fe(II)Fe(II) redox level with OH(-) or OOH(-) bound to the distal iron center that may or may not have an oxygen atom bound to one of the sulfur atoms of the dithiolate linker. Comparisons of the computed IR spectra of the (12)CO and (13)CO inhibited form with the experimental IR spectra show that exogenous CO binds terminally to the distal iron center.     

Hinged molecular capsules: synthesis and conformational control via temperature, pH, or solvent composition

Three new covalently linked molecular capsules were synthesized from their resorcinarene cavitand precursors in good yields. The capsules undergo reversible conformational switching between the closed "vase" form and the open "kite" form upon temperature or pH variation. The kite conformation obtained via either method in CDCl(3) switches to vase conformation upon addition of polar solvents such as acetone-d(6) or THF-d(8).     

Effect of complexation on spectral and kinetic properties of crown-substituted photochromic spironaphthoxazines

The insertion of benzo-15-crown-5 or benzo-18-crown-6 as a substituent at the 9'-position of a spironaphthoxazine molecule increases the lifetime of the open form of spironaphthoxazine by a factor ofca. 2. The complex formation between the crown groups and Mg²⁺ or Ba²⁺ ions changes the lifetime slightly. The addition of benzo-18-crown-6 at the 5'-position leads to an increase in the lifetime of the open form by an order of magnitude and the appearance of absorption with a maximum at 16420 cm^–1 under dark conditions, which testifies to the stabilization of the open form. The addition of Ba²⁺ ions to this compound results in a further increase in the lifetime byca. 35% due to complex formation. The luminescence of the benzo-15-crown-5-substituted compound is probably caused by both the complexation with Mg²⁺ and the photochemical reaction induced by photoactive light and is accompanied by the irreversible loss of photochromism.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2220–2224, September. 1996.     

Donor–Acceptor Stenhouse Adducts: Exploring the Effects of Ionic Character

The effects of solution-state dielectric and intermolecular interactions on the degree of charge separation provide a route to understanding the switching properties and concentration dependence of donor–acceptor Stenhouse adducts (DASAs). Through solvatochromic analysis of the open-form DASA in conjunction with X-ray diffraction and computational theory, we have analyzed the ionic character of a series of DASAs. First- and third-generation architectures lead to a higher zwitterionic resonance contribution of the open form and a zwitterionic closed form, whereas the second-generation architecture possesses a less charge-separated open form and neutral closed form. This can be correlated with equilibrium control and photoswitching solvent compatibility. As a result of the high contribution of the zwitterionic resonance forms of first- and third-generation DASAs, we were able to control their switching kinetics by means of ion concentration, whereas second-generation DASAs were less affected. Importantly, these results show how the previously reported concentration dependence of DASAs is not universal, and that DASAs with a more hybrid structure in the open form can achieve photoswitching at high concentrations.