Spectroscopic studies of electron donor-acceptor interactions: 2,4-dinitrophenol as an electron acceptor

The electron donor-acceptor interactions between 2,4-dinitrophenol and a series of π electron donors have been investigated in hexane by electronic absorption spectroscopy. The interaction is moderately strong. Linear correlations exist between spectroscopic and thermodynamic properties of complexes and the ionization potentials of donors. The charge transfer forces seem to explain the stability of complexes.     

Spectrophotometric and spectroscopic studies of charge transfer complexes of p-toluidine as an electron donor with picric acid as an electron acceptor in different solvents

The charge transfer complexes of the donor p-toluidine with π-acceptor picric acid have been studied spectrophotometrically in various solvents such as carbon tetrachloride, chloroform, dichloromethane acetone, ethanol, and methanol at room temperature using absorption spectrophotometer. The results indicate that formation of CTC in non-polar solvent is high. The stoichiometry of the complex was found to be 1:1 ratio by straight-line method between donor and acceptor with maximum absorption bands. The data are discussed in terms of formation constant (K_CT), molar extinction coefficient (?_CT), standard free energy (ΔG^o), oscillator strength (f), transition dipole moment (μ_EN), resonance energy (R_N) and ionization potential (I_D). The results indicate that the formation constant (K_CT) for the complex was shown to be dependent upon the nature of electron acceptor, donor and polarity of solvents that were used.     

The effectivity of solvents as electron pair donors

The effectivity of solvents as electron pair donors, their donicity as expressed by their donor numbers DN, is reexamined. The linear dependence of the enthalpy and the Gibbs free energy for the reaction of donor solvents with antimony pentachloride is affirmed. Extension of the DN scale by other measured quantities, via their linear correlations for sets of solvents where both kinds of data are known is applied to Drago's E-C scale, Kamlet's scale, and Koppel's B scale. This extension, added to previous extensions employing Selbin's D_{I, II} and Popov's ²³Na, produces a DN scale for 170 solvents. The relation of DN measured for isolated solvent molecules in 1,2-dichloroethane to DN values measured for bulk solvents is examined by means of the scaled particle theory and solute-solvent interaction terms. A scale of normalized donor numbers DN ^N is presented. These values are practically the same as the scale, and for those solvents for which DN values are presented here and values are lacking, they can serve in their place for those preferring to use the scale for donor solvents.     

Spectroscopic studies of interactions involving horseradish peroxidase and Tb3+

The spectroscopic properties of interactions involving horseradish peroxidase (HRP) and Tb(3+) in the simulated physiological solution was investigated with some electrochemical and spectroscopic methods, such as cyclic voltammetry (CV), circular dichroism (CD), X-ray photoelectron spectroscopy (XPS) and synchronous fluorescence (SF). It was found that Tb(3+) can coordinate with oxygen atoms in carbonyl groups in the peptide chain of HRP, form the complex of Tb(3+) and HRP (Tb-HRP), and then lead to the conformation change of HRP. The increase in the random coil content of HRP can disturb the microstructure of the heme active center of HRP, in which the planarity of the porphyrin cycle in the heme group is increased and then the exposure extent of the electrochemical active center is decreased. Thus Tb(3+) can inhibit the electrochemical reaction of HRP and its electrocatalytic activity for the reduction of H(2)O(2) at the Au/Cys/GC electrode. The changes in the microstructure of HRP obstructed the electron transfer of Fe(III) in the porphyrin cycle of the heme group, thus HRP catalytic activity is inhibited. The inhibition effect of Tb(3+) on HRP catalytic activity is increased with the increasing of Tb(3+) concentration. This study would provide some references for better understanding the rare earth elements and heavy metals on peroxidase toxicity in living organisms.     

Weak interactions and molecular recognition in systems involving electron transfer proteins

Electrostatic interactions and other weak interactions between amino acid side chains on protein surfaces play important roles in molecular recognition, and the mechanism of their intermolecular interactions has gained much interest. We established that charged peptides are useful for investigating the molecular recognition character of proteins and their molecular interaction induced structural changes. Positively charged lysine peptides competitively inhibited electron transfer from reduced cytochrome f (cyt f or cytochrome c (cyt c) to oxidized plastocyanin (PC), due to neutralization of the negatively charged site of PC by formation of PC-lysine peptide complexes. Lysine peptides also inhibited electron transfer from cyt c to cytochrome c peroxidase. Likewise, negatively charged aspartic acid peptides interacted with the positively charged sites of cytfand cyt c, and competitively inhibited electron transfer from reduced cytfor cyt c to oxidized PC and from [Fe(CN)6]4- to oxidized cyt c. Changes in the geometry and a shift to a higher redox potential of the active site Cu of PC on oligolysine binding were detected by spectroscopic and electrochemical measurements, owing to the absence of absorption in the visible region for lysine peptides. Structural and redox potential changes were also observed for cyt f and cyt c by interaction with aspartic acid peptides.     

Spectroscopic studies on the molecular interactions of curcumin and piperine

Monatshefte für Chemie - Chemical Monthly - The diarylheptanoid curcumin is the yellow coloring agent accumulated in the rhizome of the common spice turmeric (Curcuma longa L.). It has...     

Kinetic and thermodynamic studies on molecular interaction of antipyrine donor and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone as an electron acceptor in different solvents

The charge–transfer (CT) complex of donor antipyrine with Π‐acceptor 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) has been investigated spectrophotometrically in different halocarbon and acetonitrile solvents. The results indicated immediate formation of an electron donor–acceptor complex (DA), which is followed by two relatively slow consecutive reactions. The pseudo–first‐order rate constants for the formation of the ionic intermediate and the final product at various temperatures were evaluated from the absorbance–time data. The activation parameters, viz. activation energy, enthalpy, entropy, and free energy of activation, were computed from temperature dependence of rate constants. The stoichiometry of the complex was found to be 1:1 by Job's method of continuous variation. The formation constants of the resulting DA complexes were determined by the Benesi–Hildebrand equation at four different temperatures. The enthalpies and entropies of the complex formation reactions have been obtained by temperature dependence of the formation constants using Van't Hoff equation. The results indicate that DDQ complexes of antipyrine in all solvents are enthalpy stabilized but entropy destabilized. Both the kinetics of the interaction and the formation constants of the complexes are dependent upon the polarity of the solvents. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 81–91, 2013     

Spectroscopic studies of the molecular interactions in n-ethylamines and 2-nitropropane/n-ethylamine mixtures

New experimental results are reported on molecular interactions in the n-ethylamines and 2-nitropropane (2-NP)/n-ethylamine mixtures studied by Raman spectroscopy under pressure in a diamond anvil cell (0-50 GPa) and, at ambient pressure, by infrared spectroscopy. Modifications of the infrared spectra in 2-NP in presence of triethylamine (TEA) or diethylamine (DEA) have been observed at ambient pressure and interpreted as a specific molecular interaction. High-pressure fluorescence in the vicinity of the liquid-solid phase transition of the 2-NP/DEA and 2-NP/monoethylamine mixtures, is highlighted and discussed.     

Resorufin as an electron acceptor in glucose oxidase-catalyzed oxidation of glucose

Resorufin (1) has been found to act as an electron acceptor in glucose oxidase (GOD)-catalyzed oxidation of glucose. When a 1: 1: 1 mixture of solutions of 1 (5.0 microM), glucose, and GOD (4.0 mg/ml) in phosphate buffer (pH 7.4, 0.1 M) was incubated at 36 degrees C under aerobic conditions and the reaction was followed by a measurement of changes in fluorescence intensity due to 1, only two types of fluorometric traces were observed: (1) when a glucose solution of less than 0.7 mM was subjected to the enzymatic reaction, no consumption of 1 was observed; (2) the reaction with glucose at more than 1.0 mM always consumed 1, affording a regression fluorometric curve, and yet the obtained fluorometric traces could be almost superimposed on one another with no dependence on the glucose concentration. The reasons for the observed phenomena are discussed.     

Resazurin as an electron acceptor in glucose oxidase-catalyzed oxidation of glucose

The behavior of resazurin (1) as an electron acceptor in glucose oxidase (GOD)-catalyzed oxidation of glucose under anaerobic conditions is described. When a mixture of 1, glucose, and GOD in phosphate buffer (pH 7.4, 0.1 M) was incubated at 25 degrees C, the resulting solution turned purple to fluorescent pink due to the deoxygenated product, resorufin (2). On incubation of 1 with GOD alone or with H2O2 under essentially the same conditions, no color change was seen, indicating that generation of 2 in the enzymatic reaction is brought about through reduction of 1 by the reduced form (GODred) of GOD, which was also supported by the voltammetric behavior of 1. However, it was found that the enzymatic transformation of 1 to 2 is of no practical use as an indicator reaction for glucose determination using only GOD due to a slow reaction of 1 with GODred. Based on a ping-pong type mechanism with a steady-state approximation, KM and kcat for 1 as an electron acceptor from GODred were estimated to be 15+/-1.3 microM and (5.0+/-0.5) x 10(-2) s(-1), respectively.     

Spectroscopic studies of the electron donor-acceptor interactions of aromatic hydrocarbons with tetrachlorophthalic anhydride

Electron donor-acceptor (EDA) interactions of tetrachlorophthalic anhydride (TCPA) with benzene,p-xylene, mesitylene, fluorene,t-stilbene and pyrene have been investigated by spectroscopic technique. The spectroscopic and thermodynamic parameters of the complexes formed are reported. The enthalpies of formation range between 0.2 to 5.0 kcal mole⁻¹. Among all the donors studied, pyrene appears to be the strongest electron donor towards tetrachlorophthalic anhydride.     

Spectroscopic,thermal studies and molecular modelling of charge transfer complexation between 5-amino-2-methoxypyridine with chloranilic acid in different polar solvents

Charge transfer (CT) interaction between 5-amino-2-methoxypyridine (5AMPy), as electron donor (proton acceptor), with 3,6-dichloro-2,5-dihydroxy-p-benzoquinone (chloranilic acid, H₂CA), as electron acceptor (proton donor), has been investigated spectrophotometrically in the polar protic solvents ethanol (EtOH) and methanol (MeOH) and the aprotic one acetonitrile (AN). Pink-coloured solution is formed instantaneously upon mixing 5AMPy with H₂CA solutions in all solvents, which is the hallmark evidence of CT complex formation. Based on Job’s method of continuous variations, as well as spectrophotometric titrations, the stoichiometry of the complex was found to be 1:1 [(5AMPy) (H₂CA)] in all solvents. Benesi–Hildebrand equation has been applied to estimate the formation constant of the produced CT complex (K_CT) and its molar absorptivity (ε), they reached high values, confirming the complex high stability. Solid CT complex has been synthesised and analysed by elemental analyses and FTIR, ¹H NMR spectroscopies, where 2:1 [(5AMPy)₂ (H₂CA)] CT complex was obtained.     

On the desorption kinetics of chemisorbed iodine in electron donor solvents

It was shown that the desorption kinetics of chemisorbed iodine in electron donor solvents obeys an equation obtained by solving a set of first order rate laws expressing the energetic non-uniformity of the solid surface. Reference desorption activation energies were calculated and compared with the CT complex formation free energies.     

碘和某些吡啶系列配合物的光谱和热力学研究

测定了碘与3-氯吡啶,4-氰基吡啶和S-氰基吡啶形成的授体配合物的可见光谱,并求得了一些相应的热力学参数△G,△H和△S.发现△G与π~*-σ~*跃迁吸收波数之间存在线性关系.证实了这一系列配合物的平衡常数符合Hammett方程,并且初步将它们引用于E-C方程.     

Photophysical behaviour of 2,6-diaminoanthraquinone in different solvents and at various pH

The absorption and fluorescence spectral properties of 2,6-diaminoanthraquinone (DAAQ) have been investigated in a series of organic solvents of different polarity and in aqueous solutions with H o/pH/H_ in the range -10 to 17. The Stokes shift of DAAQ is correlated with various polarity scales. The study reveals the fluorescence of DAAQ dication is red shifted on protonation. The abnormal fluorescence of DAAQ dication is found to be due to large solvent relaxation in polar medium. The acidity constants for various prototropic reactions in S o and S1 states are determined and discussed.     

Spectroscopic studies of ionic solvation. XX. Cesium-133 NMR study of cesium salts in different solvents

Cesium-133 chemical shifts were measured in a number of solvents as a function of salt concentration and of the counterion. Infinite-dilution chemical shifts (vs. aqueous Cs⁺ ion at infinite dilution) ranged from +59.8 ppm for nitromethane solutions to –29.4 ppm for pyridine. In general, the magnitude of the downfield chemical shift reflected the donor ability of the solvents. Ion-pair formation constants were calculated from the concentration dependences of¹³³Cs chemical shifts in several nonaqueous solvents.     

Fluorescence spectra of some xylenols in non-polar and hydrogen-bond acceptor solvents at different temperatures

The red-shift of the fluorescence bands from the excitation bands of 2,6-, 2,5-and 3,5-xylenol, measured in binary and ternary solutions at 300 K, is mainly due to dipole orientation, the hydrogen bond making only a small contribution In viscous media at 90 K, the fluorescence transition occurs from a partially relaxed equilibrium excited state.