Stereodynamics of tetramezine

The antidepressant drug tetramezine [1,2‐bis‐(3,3‐dimethyldiaziridin‐1‐yl)ethane] consists of two bridged diaziridine moieties with four stereogenic nitrogen centers, which are stereolabile and, therefore, are prone to interconversion. The adjacent substituents at the nitrogen atoms of the diaziridines moieties exist only in an antiperiplanar conformation, which results in a coupled interconversion. Therefore, three stereoisomers exist (meso form and two enantiomeric forms), which epimerize when the diaziridine moieties are regarded as stereogenic units due to the coupled interconversion. Here, we have investigated the epimerization between the meso and enantiomeric forms by dynamic gas chromatography. Temperature‐dependent measurements were performed, and reaction rate constants were determined using the unified equation of chromatography implemented in the software DCXplorer. The activation barriers of the epimerization were found to be ΔG^≠ = 100.7 kJ mol^?1 at 25°C and ΔG^≠ = 104.5 kJ mol^?1 at 37°C, respectively. The activation enthalpy and entropy were determined to be ΔH^≠ = 70.3 ± 0.4 kJ mol^?1 and ΔS^≠ = ?102 ± 2 J mol^?1 K^?1. Chirality, 2011. © 2010 Wiley‐Liss, Inc.     

Determination of the rates and barriers to conformational isomerization in the dipeptide L-pro-L-4hyp by direct 13C nmr thermal equilibration

The ¹³C nmr equilibration method lends itself as a tool for study of conformational rate processes involving aqueous media in conjunction with high activation barriers. This method is applied for measurement of kinetic and thermodynamic parameters of isomerism in the dipeptide L -Pro-L -4Hyp. The activation barrier for cis ? trans interconversion (ω 0° → 180°) is determined, ΔG^≠ = 22.3 kcal/mol. From low-temperature study, an upper limit ΔG^≠ < 9.7 kcal/mol is evaluated for cis′ ? trans′ rotation (ψ ?40° → 160°). These data are compared with computed values found in literature. The results are discussed in connection with the helix–coil transition of collagen involving Gly-L -Pro-L -4Hyp as nucleation sites.     

Biopolymer–surfactant interaction. I. Kinetics of binding of cetyltrimethyl ammonium bromide with carboxymethyl cellulose (Na Salt)

The kinetics of binding of the cationic surfactant cetyltrimethyl ammonium bromide with the Na salt of carboxymethyl cellulose was studied by the electrometric method using cetyltrimetlyl ammonium⁺ (CTA⁺) ion-selective polyvinyl chloride membrane electrode. The binding process followed the first-order kinetics and occurred in three stages. Its affinity increased with increasing CTA bromide concentration and decreased with ionic strength. The activation process comprised moderate E and ΔH^≠ and negative ΔS^≠ for all three stages with a ΔH^≠ < TδS^≠ trend proving it to be entropy controlled. The ΔG^≠ values followed the trend ΔG < ΔG < ΔG (in accordance with k₁ > k₂ > k₃). The enthalpies (ΔH^≠) and entropies (ΔS^≠) of activation followed a systematic and interdependent trend. The multiple-stage binding kinetics is grossly comparable with the kinetics of binding of proteins to solid surfaces. © 1995 John Wiley & Sons, Inc.     

Cis-Trans equilibrium and kinetic studies of acetyl-L-proline and glycyl-L-proline

By means of carbon-13 nmr (at 25 MHz) the trans/cis conformer ratio in glycyl-L -proline has been measured in aqueous (D₂O) solution over the temperature range 33–96°C. It is found that ΔH⁰ = ?4.2 kJ/mole and ΔS⁰ = ?9.7 J/mole/K. Measurements of the T₁ values for the proline ring carbons yielded values consistent with a fast puckering process involving both the β- and γ-carbons. Measurements of the rate of cis-trans conformational interconversion in glycyl-L -proline, using complete line-shape analysis for the glycyl α-carbon resonance, gave values for the trans → cis isomerization as follows: ΔH^≠ = 83.5 ± 0.2 kJ/mole; ΔS^≠ = 0.0 ± 10 J/mole/K. A more approximate determination from coalescence temperature observations gave a value of ΔG^≠ of 82.0 ± 0.4 kJ/mole for this process in acetyl-L -proline in aqueous solution. The presence of 12M NaSCN lowered this barrier by ca. 2.6 kJ/mole. Such measurements are relevant to present theoretical models of the denaturation-renaturation processes in proteins, in which proline residues may play a key role.     

The stereodynamics of 1,2‐dipropyldiaziridines

N‐alkylated trans‐diaziridines are an intriguing class of compounds with two stereogenic nitrogen atoms which easily interconvert. In the course of our investigations of the nature of the interconversion process via nitrogen inversion or electrocyclic ring opening ring closure, we synthesized and characterized the three constitutionally isomeric diaziridines 1,2‐di‐n‐propyldiaziridine 1 , 1‐isopropyl‐2‐n‐propyldiaziridine 2 , and 1,2‐diisopropyldiaziridine 3 to study the influence of the substituents on the interconversion barriers. Enantiomer separation was achieved by enantioselective gas chromatography on the chiral stationary phase Chirasil‐β‐Dex with high separation factors α (1‐isopropyl‐2‐n‐propyldiaziridine: 1.18; 1, 2‐diisopropyldiaziridine: 1.24; 100°C 50 kPa He) for the isopropyl substituted diaziridines. These compounds showed pronounced plateau formation between 100 and 150°C, and peak coalescence at elevated temperatures. The enantiomerization barriers ΔG? and activation parameters ΔH? and ΔS? were determined by enantioselective dynamic gas chromatography (DGC) and direct evaluation of the elution profiles using the unified equation implemented in the software DCXplorer. Interestingly, 1‐isopropyl‐2‐n‐propyldiaziridine and 1,2‐diisopropyldiaziridine exhibit similar high interconversion barriers ΔG? (100°C) of 128.3 ± 0.4 kJ mol^?1 and 129.8 ± 0.4 kJ mol^?1, respectively, which indicates that two sterically demanding substituents do not substantially increase the barrier as expected for a distinct nitrogen inversion process. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Kinetic and thermodynamic properties of purified alkaline protease from Bacillus pumilus Y7 and non‐covalent immobilization to poly(vinylimidazole)/clay hydrogel

The characterization of the hydrogel was performed using Fourier‐transform infrared spectroscopy, X‐ray diffraction, and scanning electron microscopy. Purified Bacillus pumilus Y7‐derived alkaline protease was immobilized in Poly (vinylimidazole)/clay (PVI/SEP) hydrogel with 95% yield of immobilization. Immobilization decreased the pH optimum from 9 to 6 for free and immobilized enzyme, respectively. Temperature optimum 3°C decreased for immobilized enzyme. The K_m, V_m, and k_cat of immobilized enzyme were 4.4, 1.7, and 7.5‐fold increased over its free counterpart. Immobilized protease retained about 65% residual activity for 16^th reuse. The immobilized protease endured its 35% residual activity in the material after six cycle's batch applications. The results of thermodynamic analysis for casein hydrolysis showed that the ΔG^≠ (activation free energy) and ΔG^≠_E‐T (activation free energy of transition state formation) obtained for the immobilized enzyme decreased in comparison to those obtained for the free enzyme. On the other hand, the value of ΔG^≠_ES (free energy of substrate binding) was observed to have increased. These results indicate an increase in the spontaneity of the biochemical reaction post immobilization. Enthalpy value of immobilized enzyme that was 2.2‐fold increased over the free enzyme indicated lower energy for the formation of the transition state, and increased ΔS^≠ value implied that the immobilized form of the enzyme was more ordered than its free form.     

Synthesis and biological activity of pyrazolo[3,4-d]thiazolo[3,2-a]pyrimidin-4-one derivatives: in silico approach

Xanthine oxidase (XO) is responsible for the pathological condition called gout. Inhibition of XO activity by various pyrazolo[3,4-d]thiazolo[3,2-a]pyrimidine-4-one derivatives was assessed and compared with the standard inhibitor allopurinol. Out of 10 synthesized compounds, two compounds, viz. 3-amino-6-(2-hydroxyphenyl)-1H-pyrazolo[3,4-d]thiazolo[3,2-a]pyrimidin-4-one (3b) and 3-amino-6-(4-chloro-2-hydroxy-5-methylphenyl)-1H-pyrazolo[3,4-d]thiazolo[3,2-a]pyrimidin-4-one (3g) were found to have promising XO inhibitory activity of the same order as allopurinol. Both compounds and allopurinol inhibited competitively with comparable Ki (3b: 3.56?µg, 3g: 2.337?µg, allopurinol: 1.816?µg) and IC₅₀ (3b: 4.228?µg, 3g: 3.1?µg, allopurinol: 2.9?µg) values. The enzyme–ligand interaction was studied by molecular docking using Autodock in BioMed Cache V. 6.1 software. The results revealed a significant dock score for 3b (?84.976?kcal/mol) and 3g (?90.921?kcal/mol) compared with allopurinol (?55.01?kcal/mol). The physiochemical properties and toxicity of the compounds were determined in silico using online computational tools. Overall, in vitro and in silico study revealed 3-amino-6-(4-chloro-2-hydroxy-5-methylphenyl)-1H-pyrazolo[3,4-d]thiazolo[3,2–a]pyrimidin-4-one (3g) as a potential lead compound for the design and development of XO inhibitors.     

The conformational properties of α,β‐dehydroamino acids with a C‐terminal ester group

α,β‐Dehydroamino acid esters occur in nature. To investigate their conformational properties, a systematic theoretical analysis was performed on the model molecules Ac‐ΔXaa‐OMe [ΔXaa = ΔAla, (E)‐ΔAbu, (Z)‐ΔAbu, ΔVal] at the B3LYP/6‐311+ + G(d,p) level in the gas phase as well as in chloroform and water solutions with the self‐consistent reaction field‐polarisable continuum model method. The Fourier transform IR spectra in CCl₄ and CHCl₃ have been analysed as well as the analogous solid state conformations drawn from The Cambridge Structural Database. The ΔAla residue has a considerable tendency to adopt planar conformations C5 (?, ψ ≈ ? 180°, 180°) and β2 (?, ψ ≈ ? 180°, 0°), regardless of the environment. The ΔVal residue prefers the conformation β2 (?, ψ ≈ ? 120°, 0°) in a low polar environment, but the conformations α (?, ψ ≈ ? 55°, 35°) and β (?, ψ ≈ ? 55°, 145°) when the polarity increases. The ΔAbu residues reveal intermediate properties, but their conformational dispositions depend on configuration of the side chain of residue: (E)‐ΔAbu is similar to ΔAla, whereas (Z)‐ΔAbu to ΔVal. Results indicate that the low‐energy conformation β2 is the characteristic feature of dehydroamino acid esters. The studied molecules constitute conformational patterns for dehydroamino acid esters with various side chain substituents in either or both Z and E positions. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.     

Discovery of pyrrolo[2,3-d]pyrimidin-4-ones as corticotropin-releasing factor 1 receptor antagonists with a carbonyl-based hydrogen bonding acceptor

A new class of pyrrolo[2,3-d]pyrimidin-4-one corticotropin-releasing factor 1 (CRF₁) receptor antagonists has been designed and synthesized. In general, reported CRF₁ receptor antagonists possess a sp²-nitrogen atom as hydrogen bonding acceptor (HBA) on their core scaffolds. We proposed to use a carbonyl group of pyrrolo[2,3-d]pyrimidin-4-one derivatives as a replacement for the sp²-nitrogen atom as HBA in classical CRF₁ receptor antagonists. As a result, several pyrrolo[2,3-d]pyrimidin-4-one derivatives showed CRF₁ receptor binding affinity with IC₅₀ values in the submicromolar range. Ex vivo ¹²⁵I-sauvagine binding studies showed that 2-(dipropylamino)-3,7-dimethyl-5-(2,4,6-trimethylphenyl)-3,7-dihydro-4H-pyrrolo[2,3-d]pyrimidin-4-one (16b) (30 mg/kg, po) was able to penetrate into the brain and inhibit radioligand binding to CRF₁ receptors (frontal cortex, olfactory bulb, and pituitary) in mice. We identified pyrrolo[2,3-d]pyrimidin-4-one derivatives as the first CRF₁ antagonists with a carbonyl-based HBA.     

Discovery of substituted 3H-pyrido[2,3-d]pyrimidin-4-ones as potent,biased, and orally bioavailable sst2 agonist

The discovery of a novel 3H-pyrido[2,3-d]pyrimidin-4-one series as potent and biased sst2 agonists is described. This class of molecules exhibits excellent sst2 potency and selectivity against sst1, sst3, and sst5 receptors, and they are significantly more potent at inhibiting cAMP production than inducing internalization. The orally bioavailable 6-(3-chloro-5-methylphenyl)-3-(3-fluoro-5-hydroxyphenyl)-5-({methyl[(2S)-pyrrolidin-2-ylmethyl]amino}methyl)-3H,4H-pyrido[2,3-d]pyrimidin-4-one (36) also suppresses GH secretion in GHRH-challenged rats in a dose-dependent manner.     

A study of the growth of normal and iodinated collagen fibrils in vitro using electron microscope autoradiography

Electron microscopic autoradiographic observations on collagen fibrils grown in vitro allow growth rates in the N- and C-terminal directions to be measured on individual fibrils. Such observations, made on normal and iodinated collagen, show that normal fibrils grow at both ends (although rather more rapidly at the N-terminal end), whereas fully-iodinated collagen fibrils grow only at the N-terminal end. Measurements of growth rates at different temperatures provide estimates of the activation enthalpy (ΔH^≠) and entropy (ΔS^≠) of precipitation for the two types of collagen. Solubility measurements have also yielded values for the thermodynamic enthalpy (ΔH) and entropy (ΔS) of precipitation. Results show that the activated (rate-limiting) state is characterized by a large positive ΔH^≠ and ΔS^≠ similar in magnitude to the ΔH and ΔS of transition from solution to fibril. It is also concluded that the different rates of precipitation of normal and iodinated collagen cannot be explained in terms of fibril formation requiring ionization of the tyrosine residues.     

Study of the interaction between sodium salts of (2E)‐3‐(4'‐halophenyl)prop‐2‐enoyl sulfachloropyrazine and bovine serum albumin by fluorescence spectroscopy

Three sodium salts of (2E)‐3‐(4'‐halophenyl)prop‐2‐enoyl sulfachloropyrazine (CCSCP) were synthesized and their structures were determined by ¹H and ¹³C NMR, LC‐MS and IR. The binding properties between CCSCPs and bovine serum albumin (BSA) were studied using fluorescence spectroscopy in combination with UV–vis absorbance spectroscopy. The results indicate that the fluorescence quenching mechanisms between BSA and CCSCPs were static quenching at low concentrations of CCSCPs or combined quenching (static and dynamic) at higher CCSCP concentrations of 298, 303 and 308 K. The binding constants, binding sites and corresponding thermodynamic parameters (ΔH, ΔS, ΔG) were calculated at different temperatures. All ΔG values were negative, which revealed that the binding processes were spontaneous. Although all CCSCPs had negative ΔH and positive ΔS, the contributions of ΔH and ΔS to ΔG values were different. When the 4'‐substituent was fluorine or chlorine, van der Waals interactions and hydrogen bonds were the main interaction forces. However, when the halogen was bromine, ionic interaction and proton transfer controlled the overall energetics. The binding distances between CCSCPs and BSA were determined using the Förster non‐radiation energy transfer theory and the effects of CCSCPs on the conformation of BSA were analyzed by synchronous fluorescence spectroscopy. Copyright © 2012 John Wiley & Sons, Ltd.     

Novel 6-N-arylcarboxamidopyrazolo[4,3-d]pyrimidin-7-one derivatives as potential anti-cancer agents

A novel series of 3,5,6-trisubstituted pyrazolo[4,3-d]pyrimidin-7-one derivatives, especially 6-N-arylcarboxamidopyrazolo[4,3-d]pyrimidin-7-ones were synthesized and evaluated for their in vitro anticancer activities against various human cancer cell lines. The inhibitory activities for several kinases have also been tested. The prepared compounds library exhibited significant anticancer activity towards HT-29 colon and DU-145 prostate cancer cell lines. The structure–activity relationships of the 6-N-arylcarboxamidopyrazolo[4,3-d]pyrimidin-7-one scaffold at R¹, R² and R³ have been elucidated. Among the synthesized compounds, 12b was the most active compound with GI₅₀ value of 0.44 μM and 1.07 μM against HT-29 and DU-145 cell lines, respectively, and 13a was the most selective compound towards colon cancer cell line.     

Energetics of Acidianus ambivalens growth in response to oxygen availability

All life requires energy to drive metabolic reactions such as growth and cell maintenance; therefore, fluctuations in energy availability can alter microbial activity. There is a gap in our knowledge concerning how energy availability affects the growth of extreme chemolithoautotrophs. Toward this end, we investigated the growth of thermoacidophile Acidianus ambivalens during sulfur oxidation under aerobic to microaerophilic conditions. Calorimetry was used to measure enthalpy (ΔH_inc) of microbial activity, and chemical changes in growth media were measured to calculate Gibbs energy change (ΔG_inc) during incubation. In all experiments, Gibbs energy was primarily dissipated through the release of heat, which suggests enthalpy‐driven growth. In microaerophilic conditions, growth was significantly more efficient in terms of biomass yield (defined as C‐mol biomass per mole sulfur consumed) and resulted in lower ΔG_inc and ΔH_inc. ΔG_inc in oxygen‐limited (OL) and oxygen‐ and CO₂‐limited (OCL) microaerophilic growth conditions resulted in averages of ?1.44 × 10³ kJ/C‐mol and ?7.56 × 10² kJ/C‐mol, respectively, and average ΔH_inc values of ?1.11 × 10⁵ kJ/C‐mol and ?4.43 × 10⁴ kJ/C‐mol, respectively. High‐oxygen experiments resulted in lower biomass yield values, an increase in ΔG_inc to ?1.71 × 10⁴ kJ/C‐mol, and more exothermic ΔH_inc values of ?4.71 × 10⁵ kJ/C‐mol. The observed inefficiency in high‐oxygen conditions may suggest larger maintenance energy demands due to oxidative stresses and a preference for growth in microaerophilic environments.     

Thermodynamic interaction between urea and the peptide group in aqueous solutions at 25°C

Isopiestic vapor pressure measurements of the ternary systems water + triglycine + urea and water + glycine-L-alanine + urea were made and used to calculate the Gibbs free energy of these systems. Together with recently published analogous results on systems, in which the first solute was glycine or alanine or diglycine, and measurements of the excess enthalpy of all these solutions, it is possible to calculate the Gibbs free energy of transfer and the enthalpy of transfer of the peptide group from water to aqueous urea solutions. The transfer can be described as a binding of urea to the peptide group with ΔG = ?1.85 kJ mol^?1 and ΔH = ?18.7 kJ mol^?1 at 298.1 K.     

Electrochemically Modifying the Electronic Structure of IrO2 Nanoparticles for Overall Electrochemical Water Splitting with Extensive Adaptability

Designing the electrocatalysts that are stable and active for extensively adaptable water splitting is highly desirable for developing hydrogen based energy. IrO₂ is a promising and widely used catalyst for the oxygen evolution reaction in commercial applications, but is rarely used for the hydrogen evolution reaction (HER), due to the high Gibbs free energy for hydrogen adsorption (ΔG_H*). Herein, an approach to modify the electronic structure of IrO₂ via cyclic voltammetry is proposed. In this process, Ir(+4) is partially reduced and trace Pt is simultaneously deposited on IrO₂, which greatly lowers the ΔG_H* and thus accelerates the reaction kinetics. The as‐prepared Pt–IrO₂/CC with low noble metal loading (36.6 µg cm^?2_(Ir+Pt)) exhibits excellent HER activity with overpotentials of 5, 22, and 26 mV at 10 mA cm^?2 in 0.5 m H₂SO₄, 1 m KOH, and 1 m phosphate buffer solution, respectively, making it possible to organize an all‐IrO₂ based water electrolyzer. The Pt–IrO₂/CC||IrO₂/CC couple exhibits a promising activity and stability in pH‐universal conditions as well as natural seawater for H₂ production. Density function theory calculations reveal that the optimized electronic structure of IrO₂ balances the ΔG_H*, resulting in a much enhanced HER performance.     

Steroid metabolism by purified adult rat leydig cells in primary culture

To characterize Leydig cell steroidogensis, we examined the metabolism of (³H)pregnenolone (3β-hydroxy-5-pregnen-20-one) to androgens in the presence and absence of human chorionic gonadotropin (hCG) as a function of culture duration. Approximately 20–30% of the (³H)pregnenolone was converted to testosterone (17_β-hydroxy-4-androsten-3-one) by purified Leydig cells at 0, 3 and 5 days (d) of culture. Androstenedione (4-androstene-3,17-dione) and dihydrotestosterone (17_β-hydroxy-5_α-androstan-3-one) were also produced while on day 5 of culture, significant amounts of progesterone (4-pregnene-3, 20-dione) were isolated. The Δ⁵ intermediates, 17-hydroxypregnenolone (3β, 17-dihydroxy-5-pregnen-20-one) and dehydroepiandrosterone (3β-hydroxy-5-androsten-17-one), accounted for less than 1% of substrate conversion, indicating a clear preference for Leydig cells to metabolize (³H)pregnenolone via the Δ⁴ pathway. On day 0 of culture, unidentified metabolites consisted of predominately polar steroids while on day 5 of culture, the unidentified metabolites consisted of predominately nonpolar steroids. In the presence of hCG, (³H)pregnenolone metabolism did not differ from basal on day 0 or 3 of culture. HCG increased the conversion of pregnenolone to progesterone and 17-hydroxyprogesterone (17-hydroxy-4-pregnene-3, 20-dione) on 5d. This suggests that Leydig cells cultured for 5d have decreased C_17–20 desmolase activity or that hCG acutely stimulates 3β-hydroxysteroid dehydrogenase and Δ⁴-Δ⁵ isomerase activities.     

Design,synthesis and pharmacological screening of a series of N1-(substituted)aryl-5,7-dimethyl-2-(substituted)pyrido(2,3-d)pyrimidin-4(3H)-ones as potential histamine H1-receptor antagonists

A series of N₁-(substituted)aryl-5,7-dimethyl-2-(substituted)pyrido(2,3-d)pyrimidin-4(3H)-one was designed on the basis of the triangular pharmacophoric requirement of histamine H₁-receptor antagonists. The designed series was synthesized by cyclo-condensation of monoaryl thiourea with ethyl cyanoacetate in the presence of dry HCl gas to give N₁-(substituted aryl)-2-mercaptopyrimidine-4(3H)-one, which on cyclo-condensation with acetylacetone gave the pyridopyrimidinone. Further methylation of the mercapto group at C-2 with methyl iodide followed by nucleophilic displacement of the methylmercapto group by various amines gave the targeted compounds. All the synthesized compounds were screened for histamine H₁-receptor antagonistic activity by the in vitro method of inhibition of the isotonic contraction induced by histamine on isolated guinea pig ileum using cetirizine as a standard drug. All the compounds exhibited potent histamine H₁-receptor antagonistic activity with pA₂ values from 7.30– 9.75 (cetirizine, pA₂ value 9.40). The potent compounds were screened for their in vivo antihistaminic activity by protection of animal from asphyxic shock. The sedative potential of potent compounds was checked on albino mice by photoactometer and they had comparative sedative potential to the standard drug cetirizine. None of the compound exhibited anticholinergic activity in the in vitro rat ileum model.