2,2,2-trifluoroethanol-induced molten globule state of concanavalin a and energetics of 8-anilinonaphthalene sulfonate binding: calorimetric and spectroscopic investigation

The interaction of 2,2,2-trifluoroethanol (TFE) with concanavalin A has been investigated by using a combination of differential scanning calorimetry, isothermal titration calorimetry (ITC), circular dichroism (CD), and fluorescence spectroscopy at pH 2.5 and 5.2. All of the calorimetric transitions at both the pH values were found to be irreversible. In the presence of 4 mol kg(-1) TFE at pH 2.5, concanavalin A is observed to be in a partially folded state with significant loss of native tertiary structure. The loss of specific side chain interactions in the transition from native to the TFE-induced partially folded state is demonstrated by the loss of cooperative thermal transition and reduction of the CD bands in the aromatic region. Acrylamide quenching, 8-anilinonaphthalene sulfonate (ANS) binding, and energy transfer also suggest that in the presence of 4 mol kg(-1) TFE at pH 2.5 concanavalin A is in a molten globule state. ITC has been used for the first time to characterize the energetics of ANS binding to the molten globule state. ITC results indicate that the binding of ANS to the molten globule state and acid-induced state at pH 2.5 displays heterogeneity with two classes of non-interacting binding sites. The results provide insights into the role of hydrophobic and electrostatic interactions in the binding of ANS to concanavalin A. The results also demonstrate that ITC can be used to characterize the partially folded states of the protein both qualitatively and quantitatively.     

Condensed cyclic and bridged-ring systems. VII. Acid-catalysed cyclisation of methyl substituted benzylcyclohexanols. Factors influencing the nature of cyclisation products

The gross structures of the cyclised products from the acid-catalysed cyclisations of 2-benzyl-1, 3-dimethylcyclohexanol (6) and 1-benzyl-3, 5-dimethylcyclohexanol (11) revealing the influence of the structure of the benzylcyclohexanol derivative, and of the cyclisation reagent, have been evaluated. Polyphosphoric acid and aluminium chloride catalysed cyclisations of (6) result in the formation of predominantly 1, 4a-dimethyl-1, 2, 3, 4, 4a, 9a-hexahydrofluorene (7) and 4, 9-dimethyl-7, 8-benzobicyclo [3.3.1] non-7-ene (9) respectively. Under the same conditions, (11) produced cyclised products consisting mostly the benzobicyclo [3.3.1] non-7-ene derivative (12), characterised through 1,3-dimethyl-7,8-benzobicyclo [3.3.1] non-6-oxo-7-ene (14) by oxidation with chromium trioxide. Phosphorus pentoxide induced cyclisation of (6), followed by oxidation gave a mixture of the bridged-ring ketone (10) and the 9-oxohydrofluorene (8) in a ratio ofca. 3 : 2, whereas 2-benzyl-5-methylcyclohexanol (19) resulted in mostly 2-methyl-7,8-benzobicyclo [3.3.1] non-6-oxo-7-ene (19).     

Kinetics and mechanism of oxidation of methanol by acid bromate

The kinetics of oxidation of methanol by bromate ion in hydrochloric acid medium has been investigated. A mechanism consistent with the experimental observations is suggested.

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Spectroscopic and computational studies on the adenosylcobalamin-dependent methylmalonyl-CoA mutase: evaluation of enzymatic contributions to Co-C bond activation in the Co3+ ground state

Methylmalonyl-CoA mutase (MMCM) is an enzyme that utilizes the adenosylcobalamin (AdoCbl) cofactor to catalyze the rearrangement of methylmalonyl-CoA to succinyl-CoA. Despite many years of dedicated research, the mechanism by which MMCM and related AdoCbl-dependent enzymes accelerate the rate for homolytic cleavage of the cofactor's Co-C bond by approximately 12 orders of magnitude while avoiding potentially harmful side reactions remains one of the greatest subjects of debate among B(12) researchers. In this study, we have employed electronic absorption (Abs) and magnetic circular dichroism (MCD) spectroscopic techniques to probe cofactor/enzyme active site interactions in the Co(3+)Cbl "ground" state for MMCM reconstituted with both the native cofactor AdoCbl and its derivative methylcobalamin (MeCbl). In both cases, Abs and MCD spectra of the free and enzyme-bound cofactor are very similar, indicating that replacement of the intramolecular base 5,6-dimethylbenzimidazole (DMB) by a histidine residue from the enzyme active site has insignificant effects on the cofactor's electronic properties. Likewise, spectral perturbations associated with substrate (analogue) binding to holo-MMCM are minor, arguing against substrate-induced enzymatic Co-C bond activation. As compared to the AdoCbl data, however, Abs and MCD spectral changes for the sterically less constrained MeCbl cofactor upon binding to MMCM and treatment of holoenzyme with substrate (analogues) are much more substantial. Analysis of these changes within the framework of time-dependent density functional theory calculations provides uniquely detailed insight into the structural distortions imposed on the cofactor as the enzyme progresses through the reaction cycle. Together, our results indicate that, although the enzyme may serve to activate the cofactor in its Co(3+)Cbl ground state to a small degree, the dominant contribution to the enzymatic Co-C bond activation presumably comes through stabilization of the Co(2+)Cbl/Ado. post-homolysis products.     

Study of formation of nano-quasicrystals and crystallization kinetics of Zr-Al-Ni-Cu metallic glass

The formation of nano-quasicrystals on isothermal annealing of melt-spun ribbons of Zr_69.5Al_7.5Ni₁₁Cu₁₂ metallic glass has been investigated using transmission electron microscopy (TEM). The crystallization study of this metallic glass has been carried out using differential scanning calorimetry (DSC) in non-isothermal (linear heating) mode. It exhibits two-stage crystallization where the first stage corresponds to the precipitation of icosohedral nano-quasicrystalline phase. This has been confirmed with the help of TEM investigations. The crystallization parameters like the activation energy (E _c) and frequency factor (k ₀) have been derived using the Kissinger peak shift analysis. The activation energies for the first and second crystallization peak are found to be 278 and 295 kJ mol^–1, respectively. The frequency factors obtained for the two peaks are respectively 7.16·10¹⁹ and 1.42·10²⁰ s^–1. E _c, k ₀ and the Avrami exponent (n) have also been derived by fitting the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for the transformed volume fraction (x) to the crystallization data. JMAK results of E _c for the first and second crystallization peak turn out to be 270 and 290 kJ mol^–1 respectively. However, k ₀ and n are found to be heating rate dependent as reported in similar studies. The values of n for the first crystallization stage ranges between 1.66 and 2.57 indicating diffusion-controlled transformation in agreement with earlier reports.     

Kinetics and mechanism of formation and dissociation of copper(II) and nickel(II) complexes of the Schiff base bis(acetylacetone)ethylenediimine in aqueous-organic solvent media

Summary In NH₄NO₃+NH₄OH buffered 10% (v/v) dioxan-water media (pH 7.0–8.5), thePseudo-first-order rate constant for the formation of the title complexes M(baen),i.e. ML, conforms to the equation 1/k_obs=1/k+1/(kK_o.s · T_L), where T_L stands for the total ligand concentration in the solution, K_o.s is the equilibrium constant for the formation of an intermediate outer sphere complex and k is the rate constant for the formation of the complex ML from the intermediate. Under the experimental conditions the free ligand (pK_a>14) exists virtually exclusively in the undissociated form (baenH₂ or LH₂) which is present mostly as a keto-amine in the internally hydrogen-bonded state. Although the observed formation-rate ratio k^Cu/k^Ni is of the order of 10⁵, as expected for systems having normal behaviour, the individual rate constants are very low (at 25°C, k^Cu=50 s^–1 and k^Ni=4.7×10^–4s^–1) due to the highly negative S values (–84.2±3.3 JK^–1M^–1 for CuL and –105.8±4.1 JK^–1M^–1 for NiL); the much slower rate of formation of the nickel(II) complex is due to higher H value (41.2±1.0 kJM^–1 for CuL and 78.2±1.2 kJM^–1 for NiL) and more negative S value compared to that of CuL. The K_o.s values are much higher than expected for simple outer-sphere association between [M(H₂O)₆] and LH₂ and may be due to hydrogen bonding interaction.In acid media ([H⁺], 0.01–0.04 M) these complexes M(baen) dissociate very rapidly into the [M(H₂O)₆]²⁺ species and baenH₂, followed by a much slower hydrolytic cleavage of the ligand into its components,viz. acetylacetone and ethylenediamine (protonated). For the dissociation of the complexes k_obs=k₁[H⁺]+k₂[H⁺]². The reactions have been studied in 10% (v/v) dioxan-water media and also ethanolwater media of varying ethanol content (10–25% v/v) and the results are in conformity with a solvent-assisted dissociativeinterchange mechanism involving the protonated complexes.     

Chemoselective reduction of aldehydes with zinc borohydride in tetrahydrofuran

A variety of structurally different aldehydes undergo chemoselectire reduction over ketones with zinc borohydride in tetrahydrofuran at −10°C to the corresponding alcohols.     

Toward the development of new photolabile protecting groups that can rapidly release bioactive compounds upon photolysis with visible light

The synthesis and characterization of a new photolabile protecting group (caging group) for carboxylic acids, the 2-(dimethylamino)-5-nitrophenyl (DANP) group, is described. This compound has a major absorption band in the visible wavelength region with a maximum near 400 nm (epsilon400 = 9077 M(-1) cm(-1) at pH 7.4 and 21 degrees C). The caging group is attached through an ester linkage to the carboxyl functionality of beta-alanine, which activates the inhibitory glycine receptor in the mammalian central nervous system. Such caged compounds play an important role in transient kinetic investigations of fast cellular processes. Upon photolysis of DANP-caged beta-alanine, the caging group is released within 5 micros. Quantum yields of 0.03 and 0.002 were obtained in the UV region (308 and 360 nm) and the visible region (450 nm), respectively. Laser-pulse photolysis experiments, using 337 or 360 nm light, were performed with the caged compound equilibrated with HEK 293 cells transiently transfected with cDNA encoding the alpha1 homomeric, wild-type glycine receptor. The experiments demonstrated that neither DANP-caged beta-alanine nor its byproducts inhibit or activate the glycine receptors on the cell surface. Under physiological conditions, the DANP-caged beta-alanine is water-soluble and stable and can be used for transient kinetic measurements.     

Polyether azomethines. I. Synthesis and characterization

Six new polyether azomethines were synthesized by melt and solution polycondensation of six different diamines with 4,4′-[1,4-phenylene bis(oxy)] bisbenzaldehyde. The polymers synthesized by solution method are yellow to white in color and had inherent viscosities up to 0.59 dL/g in concentrated H₂SO₄. The polymers obtained by melt condensation show higher viscosity. Except polymer IV , others are insoluble in common organic solvents. The polymers were characterized by IR, x-ray, elemental analysis, and DSC study. The thermal stability of the polymers was evaluated by TGA and IGA study. Polymers I-III are highly thermally and thermooxidatively stable and exhibit no appreciable decomposition up to 420°C both in air and nitrogen atmosphere. It was shown that the curing of the polyazo-methines takes place by opening up of the ? CH?N? linkages at higher temperature. The electrical conductivities of the virgin and iodine doped polymers were as high as 10^?11?10^?16 and 10^?6?10^?8S cm^?1, respectively, at 30°C. Electronic spectra of the undoped polymers ( I-III ) indicated a large bathochromic shift of the ? – ?^* absorptions band (376 nm) due to ? C?N? bonds of the model compound. This can be attributed to extensive delocalization of the electrons along the polymer chain. © 1995 John Wiley & Sons, Inc.     

Isolation of sesquiterpenoids from sponge Dysidea avara and chemical modification of avarol as potential antitumor agents

The marine sponge Dysidea avara contained avarol (1) and avarone (2). Avarol on acylation yielded 2',5'-O-dibenzoylavarol (3); 2,5'-O-(4-chlorobenzoyl)avarol (4); 2,5'-O-dicinnamoylavarol (5); 2,5'-O-(4-bromobenzoyl)avarol(6); 2',5'-O-dioctanoylavarol (7); 2',5'-O-(4-fluorobenzoyl)avarol (8) and diacetylavarol (9). The structural elucidation of all the compounds 1-9 have been done by spectral analysis. The cytotoxicity of these compounds were also determined and evaluated. Compounds 6 and 9 showed selective cytotoxicity against Hepa (human hepatoma) and KB cell lines respectively.