Cyclophane-Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18-Porphyrinyl Dicyanomethyl Diradicals

2,18-Bis(dicyanomethyl)-substituted Ni^II porphyrin 8 and Zn^II porphyrin 11 were prepared and subjected to oxidation with PbO₂ in CH₂Cl₂ at 298 K to give cyclophane-type chlorin dimers ( 9 )₂ and ( 12 )₂ as a consequence of double recombination of biradicals 9 and 12 , respectively. Dimer ( 9 )₂ takes a syn-conformation of two distorted Ni^II chlorins but ( 12 )₂ takes an anti-conformation of relatively planar Zn^II chlorins. At 298 K, dimer ( 9 )₂ is stable and its ¹H NMR spectrum is sharp but becomes broad at high temperature, while the ¹H NMR spectrum of ( 12 )₂ is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of ( 12 )₂ is much less than that of ( 9 )₂. The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer ( 16 )₂ to give homo dimers ( 9 )₂ and ( 15 )₂.     

Mechanism of protection of adenosine from sulphate radical anion and repair of adenosine radicals by caffeic acid in aqueous solution

The photooxidation of adenosine in presence of peroxydisulphate (PDS) has been studied by spectrophotometrically measuring the absorbance of adenosine at 260 nm. The rates of oxidation of adenosine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of adenosine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^• and protects adenosine from it. Sulphate radical anion competes for adenosine as well as for caffeic acid. The quantum yields of photooxidation of adenosine have been calculated from the rates of oxidation of adenosine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (Φ_expt1) and the quantum yields calculated (Φ_cal) assuming caffeic acid acting only as a scavenger of SO ₄ ^• show that Φ_expt1 values are lower than Φ_expt1 values. The ǵf values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for ₄ ^• scavenging by caffeic acid, are also found to be greater than Φ_expt1 values. These observations suggest that the transient adenosine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

Oxidation of caffeine by phosphate radical anion in aqueous solution under anoxic conditions

The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (∼7.5) has been carried out in a quantum yield reactor using a high-pressure mercury lamp. The reactions were followed spectrophotometrically by measuring the absorbance of caffeine at λ_max (272 nm). The rates of reaction were calculated under different experimental conditions. The quantum yields were calculated from the rates of oxidation of caffeine and the intensity of light at 254 nm which was measured by using peroxydisulphate solution as a standard chemical actinometer. The reaction rates of oxidation of caffeine by PDP increase with increase in [PDP] as well as with increase in light intensity, while they are independent of [caffeine]. The quantum yields of oxidation of caffeine by PDP are independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by PDP increase with increase in caffeine concentration. On the basis of these experimental results and product analysis, a probable mechanism has been suggested in which PDP is activated to phosphate radical anions (PO₄ ^·2−) by direct photolysis of PDP and also by the sensitizing effect of caffeine. The phosphate radical anions thus produced react with caffeine by electron transfer reaction, resulting in the formation of caffeine radical cation, which deprotonates in a fast step to produce C8-OH adduct radicals. These radicals might react with PDP to give final product 1,3,7-trimethyluric acid and PO₄ ^·2− radicals, the latter propagates the chain reaction.     

Spirooxindole-fused pyrazolo pyridine derivatives: NiO–SiO2 catalyzed one-pot synthesis and antimicrobial activities

We describe here a one-pot synthesis of 13 spirooxindole-fused pyrazolo pyridine derivatives using NiO–SiO₂ catalyst via three-component reaction of isatin, 5-amino-3-methylpyrazole, and malononitrile. This multicomponent one-pot protocol also features shorter reaction time, good yield, and simple work-up using a recoverable and reusable solid acid heterogeneous catalyst. The NiO–SiO₂ catalyst was characterized using different instrumental techniques such as X-ray diffraction study, surface area analysis, transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, UV-DRS (diffuse reflectance spectroscopy), and energy dispersive X-ray analysis (EDX). The new compounds were tested for in-vitro anti-microbial activity.     

Synthesis of New Reagent 2,6‐Diacetylpyridine Bis‐4‐Phenyl‐3‐Thiosemicarbazone (2,6‐DAPBPTSC): Selective,Sensitive and Extractive Spectrophotometric Determination of Co(II) in Vegetable,Soil, Pharmaceutical and Alloy Samples

New synthesized reagent 2,6‐diacetylpyridine bis‐4‐phenyl‐3‐thiosemicarbazone (2,6‐DAPBPTSC) is proposed as a sensitive and selective analytical reagent for the extractive spectrophotometric determination of cobalt(II). Cobalt(II) forms a reddish brown colored complex with 2,6‐DAPBPTSC, which is extracted into isoamylalcohol, under optimum conditions. The maximum absorption of the isoamylalcohol extract is measured at 400 nm. Beer's law is applied in the range 0.6‐6.0 ppm of cobalt(II). The molar absorptivity and Sandell's sensitivity of the complex is calculated as 2.2 × 10⁴ L mol^?1 cm^?1 and 2.68 × 10^?3 μg cm^?2, respectively. An adequate linearity with a correlation coefficient value of 0.969 is obtained for the Co(II)‐2,6‐DAPBPTSC complex. The instability constant of the complex, calculated from Asmus' method is 3.75 × 10^?4 The precision and accuracy of the method is checked with calculation of relative standard deviation (n = 5), 0.388 and the detection limit a value is 0.0028 μg mL^?1. The method is successfully employed for the determination of cobalt(II) in real samples, such as vegetables, soil, water samples, standard alloy samples, and the performance of the present method was evaluated in terms of Student ‘t’ test and Variance ‘f’ test, which indicates the significance of the present method is an inter comparison of the experimental values, using atomic absorption spectrometer (AAS).     

Homocarbaporphyrinoids: The m-o-m and p-o-p Terphenyl Embedded Expanded Porphyrin Analogues and Their RhI Complexes

Stable homocarbaporphyrinoids were successfully synthesized by incorporating the m-o-m and p-o-p terphenyl units into the porphyrin core. The distinct bonding modes of terphenyl in the macrocycle generated two structural isomers with two and four carbon atoms in the macrocyclic environment. The core was utilized to stabilize the Rh^I ion. The spectral and structural analyses revealed that the restricted (m-o-m) and allowed (p-o-p) conjugation in the macrocyclic core provide overall non-aromatic characteristics both to the free bases and their complexes.     

An LC–MS/MS method for determination of O6-benzylguanine and its metabolite O6-benzyl-8-oxoguanine in human plasma

O⁶-benzylguanine (O⁶BG) is an inhibitor of O⁶-alkylguanine-DNA alkyltransferase (AGT). It binds to AGT by transferring its benzyl moiety to the cysteine residue at the active site of the enzyme. O⁶BG synergizes the cytotoxic effects of alkylating agents by halting AGT-mediated DNA repair. O⁶-benzyl-8-oxoguanine (8-oxo-O⁶BG) is a metabolite of O⁶BG, which is an equally potent inhibitor of AGT. In this work, we report the development and validation of an LC–MS/MS method for simultaneous determination of O⁶BG and 8-oxo-O⁶BG in human plasma. O⁶BG and 8-oxo-O⁶BG along with the analog internal standard, pCl-O⁶BG, were extracted from alkalinized human plasma by liquid–liquid extraction using ethyl acetate, dried under nitrogen and reconstituted in the mobile phase. Reverse-phase chromatographic separation was achieved using isocratic elution with a mobile phase containing 80% acetonitrile and 0.05% formic acid in water at a flow rate of 0.600 ml/min. Quantification was performed using multiple-reaction-monitoring mode with positive ion-spray ionization. The linear calibration ranges of the method for O⁶BG and 8-oxo-O⁶BG were 1.25–250 ng/ml and 5.00–1.00 × 10³ ng/ml, respectively, with acceptable assay accuracy, precision, recovery and matrix factor. This method was applied to the measurement of O⁶BG and 8-oxo-O⁶BG in patient plasma samples from a prior phase I clinical trial.     

Synthesis,antibacterial activity,and docking studies of some novel N′-benzylidene-2-(2,4,5-trifluorophenyl)acetohydrazides

Research on Chemical Intermediates - An improved method was developed for synthesis of 2,4,5-trifluorobenzohydrazide derivatives through condensation of trifluorophenylacetohydrazide with different...