Enantiomerization of Environmentally Significant Overcrowded Polychlorinated Biphenyls (PCBs)

The rotational barriers of overcrowded PCBs are predicted by ab initio methods including electron correlation, thus settling the controversy between theory and experiment. For 2,2′,3,3′,4,6′-hexachlorobiphenyl ( PCB 132 ), an enantiomerization barrier of 185 kJ/mol is calculated by B3LYP/6-31G*, in excellent agreement with the experimental data. Chirality 9:350–353, 1997. © 1997 Wiley-Liss, Inc.     

Enantiomerization of 2,2′-diisopropylbiphenyl during chiral inclusion gas chromatography: Determination of the rotational energy barrier by computer simulation of dynamic chromatographic elution profiles

By computer simulation of experimental dynamic gas chromatographic elution profiles, the rotational energy barrier ΔG⁼ of racemic 2,2′-diisopropylbiphenyl has been determined as 114.6–115.0 kJ/mol (75–100°C). These data are in good agreement with a value that was determined previously by measuring the racemization kinetics of an enriched sample. This indicates that there is no measurable catalytic or inhibitory effect of the stationary phase. © 1994 Wiley-Liss, Inc.     

Exposure to polychlorinated biphenyls causes endothelial cell dysfunction

Environmental chemicals, such as polychlorinated biphenyls (PCBs), may be atherogenic by disrupting normal functions of the vascular endothelium. To investigate this hypothesis, porcine pulmonary artery-derived endothelial cells were exposed to 3,3′,4,4′-tetrachlorobiphenyl (PCB 77), 2,3,4,4′,5-pentachlorobiphenyl (PCB 114), or 2,2′,4,4′,5,5′-hexachlorobiphenyl (PCB 153) for up to 24 hours. These PCBs were selected for their varying binding avidities with the aryl hydrocarbon (Ah) receptor and differences in their induction of cytochrome P450. PCB 77 and PCB 114 significantly disrupted, in a dose-dependent manner, endothelial barrier function by allowing an increase in albumin transfer across endothelial monolayers. These PCBs also contributed markedly to cellular oxidative stress, as measured by 2,7-dichlorofluorescin (DCF) fluorescence and lipid hydroperoxides, and caused a significant increase in intracellular calcium ([Ca²⁺]_i) levels. Enhanced oxidative stress and [Ca²⁺]_i in PCB 77- and PCB 114-treated cells were accompanied by increased activity and content of cytochrome P450 1A and by a decrease in the vitamin E content in the culture medium. In contrast to the effects of PCB 77 and PCB 114, cell exposure to PCB 153 had no effect on cellular oxidation, [Ca²⁺]_i, or endothelial barrier function. These results suggest that certain PCBs may play a role in the development of atherosclerosis by causing endothelial cell dysfunction and a decrease in the barrier function of the vascular endothelium. It is possible that interaction of PCBs with the Ah receptor and activation of the cytochrome P450 1A subfamily are involved in this pathology. © 1995 John Wiley & Sons, Inc.     

Degradation of Polychlorinated Biphenyls by UV-Catalyzed Photolysis

Photolysis of five polychlorinated biphenyl (PCB) congeners [2,4,4′-trichlorobiphenyl (PCB 28), 2,2′,5,′5-tetrachlorobiphenyl (PCB 52), 2,2′,4,5,5′-pentachlorobiphenyl (PCB 101), 2,2′,4,4′,5,′5-hexachlorobiphenyl (PCB 153) and 2,2′,3,4,4′,5,′5-heptachlorobiphenyl (PCB 180)] individually and in combination were carried out in the solvents methanol, ethanol, and 2-propanol. The disappearance of parent congener generally increased with UV intensity. The solvents had significant or limited effect on the removal of PCBs depending on the congener used. Because 2-propanol was highly toxic and methoxylated products were formed when methanol was used, ethanol was selected as the optimum solvent. The results of photolysis of the PCB mixture showed that PCB 52 was formed and accumulated after 4 h of photolysis. The addition of sodium hydroxide increased the rate of photolysis of the PCB mixture. One hundred percent removal can be obtained of the PCB in mixture in 90 min under optimized conditions. Gas chromatography–mass spectrometry was used to determine the intermediates of the photolysis of PCBs under optimized conditions. For the PCB congeners and mixture studied, the major photolytic intermediates were less chlorinated congeners, and biphenyl was the major product with minor amounts of hydroxylated PCBs, ethylated, dimethylated, and methylated biphenyls. Biphenyl could be further degraded by a prolonged photolysis. Toxicity of the PCB mixture during photolysis was monitored by the Microtox^® test. It was found that the toxicity increased at the early stage of photolysis, and gradually decreased as the reaction proceeded. After 90 min, the EC₅₀ of the reaction mixture was similar to that of the untreated sample.     

The First Example of a Hoogsteen Basepaired DNA Duplex in Dynamic Equilibrium with a Watson-Crick Basepaired Duplex—A Structural (NMR), Kinetic and Thermodynamic Study

Abstract

A single-point substitution of the O4′ oxygen by a CH₂ group at the sugar residue of A ⁶ (i.e. 2′-deoxyaristeromycin moiety) in a self-complementary DNA duplex, 5′- d(C¹G²C³G⁴A⁵A⁶T⁷T⁸C⁹G¹⁰C¹¹G¹²)₂ ^?3, has been shown to steer the fully Watson-Crick basepaired DNA duplex (1A), akin to the native counterpart, to a doubly A ⁶:T⁷ Hoogsteen basepaired (1B) B-type DNA duplex, resulting in a dynamic equilibrium of (1A)→←(1B): K_eq = k₁/k_-1 = 0.56±0.08. The dynamic conversion of the fully Watson-Crick basepaired (1A) to the partly Hoogsteen basepaired (1B) structure is marginally kinetically and thermodynamically disfavoured [k₁ (298K) = 3.9± 0.8 sec^?1; δH°? = 164±14 kJ/mol;-TδS°? (298K) = ?92 kJ/mol giving a δG₂₉₈°? of 72 kJ/mol. Ea (k1) = 167±14 kJ/mol] compared to the reverse conversion of the Hoogsteen (1B) to the Watson-Crick (1A) structure [k-1 (298K) = 7.0±0.6 sec-1, δH°? = 153±13 kJ/mol;-TδS°? (298K) = ?82 kJ/mol giving a δG₂₉₈°? of 71 kJ/mol. Ea (k-1) = 155±13 kJ/mol]. A comparison of δG₂₉₈°? of the forward (k1) and backward (k-1) conversions, (1A)→←(1B), shows that there is ca 1 kJ/mol preference for the Watson-Crick (1A) over the double Hoogsteen basepaired (1B) DNA duplex, thus giving an equilibrium ratio of almost 2:1 in favour of the fully Watson-Crick basepaired duplex. The chemical environments of the two interconverting DNA duplexes are very different as evident from their widely separated sets of chemical shifts connected by temperature-dependent exchange peaks in the NOESY and ROESY spectra. The fully Watson-Crick basepaired structure (1A) is based on a total of 127 intra, 97 inter and 17 cross-strand distance constraints per strand, whereas the double A ⁶:T⁷ Hoogsteen basepaired (1B) structure is based on 114 intra, 92 inter and 15 cross-strand distance constraints, giving an average of 22 and 20 NOE distance constraints per residue and strand, respectively. In addition, 55 NMR-derived backbone dihedral constraints per strand were used for both structures. The main effect of the Hoogsteen basepairs in (1B) on the overall structure is a narrowing of the minor groove and a corresponding widening of the major groove. The Hoogsteen basepairing at the central A ⁶:T⁷ basepairs in (1B) has enforced a syn conformation on the glycosyl torsion of the 2′- deoxyaristeromycin moiety, A ⁶, as a result of substitution of the endocyclic 4′-oxygen in the natural sugar with a methylene group in A ⁶. A comparison of the Watson-Crick basepaired duplex (1A) to the Hoogsteen basepaired duplex (1B) shows that only a few changes, mainly in α, σ and γ torsions, in the sugar-phosphate backbone seem to be necessary to accommodate the Hoogsteen basepair.     

Resonance light scattering technique for determination of polychlorinated biphenyls with silver nanoparticles

In this paper, a simple and novel method for the determination of polychlorinated biphenyls (PCBs), using silver nanoparticles (AgNPs) as a resonance light scattering (RLS) probe, is proposed. Under optimized conditions, there existed linear relationships between the enhancing RLS intensity of the system and the concentrations of PCBs in the range 8.0 × 10^?8?1.0 × 10^?6 g mL^?1 for 2,4,4′‐trichlorbiphenyl (PCB28), 9.0 × 10^?8?1.0 × 10^?6 g mL^?1 for 2,2′,5,5′‐tetrachlorbiphenyl (PCB52) and 4.0 × 10^?8?1.0 × 10^?6 g mL^?1 for 3,3′,4,4′‐tetrachlorobiphenyl (PCB77). The corresponding detection limits (S/N = 3) were 2.6 × 10^?8 g mL^?1 for PCB28, 3.3 × 10^?8 g mL^?1 for PCB52 and 6.3 × 10^?9 g mL^?1 for PCB77, respectively. Finally, the mechanism of RLS enhancement was also studied. The results indicated that PCBs were adsorbed on the surface of AgNPs to form larger AgNP–PCB aggregates, resulting in the RLS enhancement of the system. Copyright © 2011 John Wiley & Sons, Ltd.     

Different molecular capacity in the induction of apoptosis by polychlorinated biphenyl congeners in rat renal tubular cell cultures

The effects of a commercial polychlorinated biphenyl (PCB) mixture (Aroclor 1248) and two individual PCB congeners were evaluated on rat renal proximal tubule culture cell viability and internucleosomal DNA fragmentation (DNA ladder) characteristic of apoptosis. Treatment with Aroclor 1248 caused the loss of cell viability and promoted apoptosis in a concentration- and time-dependent manner. The two PCB congeners assessed can also induce apoptosis. However, the extent of apoptosis generated was greater for the non-ortho-substituted planar congener (3,3,4,4-tetrachlorobiphenyl) than for the di-ortho-substituted nonplanar congener (2,2,4,4,5,5-hexachlorobiphenyl). This correlated with the loss of cell viability since the planar compound is much more cytotoxic. The results suggest a different molecular mechanism in the induction of apoptosis by planar or nonplanar PCB congeners.     

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.     

Study on the binding characteristics of hydroxylated polybrominated diphenyl ethers and thyroid transporters using the multispectral technique and computational simulation

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are a class of toxic environmental pollutants that are persistent, bioaccumulative, and difficult to degrade. Their structure is very similar to the thyroid hormone (T4) and uses the body’s thyroid transporter (TTR) binding to interfere with the endocrine balance, disrupting the body’s normal physiological activity. According to Fourier transform infrared spectroscopy and dynamics simulation of do_dssp module analysis, there are three kinds of OH-PBDEs that can induce TTR secondary structural changes. Fluorescence spectra and UV–Vis spectra show that for the three kinds of OH-PBDEs for TTR, the main methods of quenching are static quenching and non-radiative energy transfer. According to thermodynamic analysis, ΔG < 0, ΔH > 0, and ΔS > 0 combine to show that the hydrophobic interaction is the main driving force of the combination. From the molecular docking analysis, it was found that 4′-hydroxy-2,2′,4,5′- tetrabromodiphenyl ether (4′-OH-BDE49) and 4 hydroxy-2,2′,3,4′,5,6,6′- heptabromodiphenyl ether (4-OH-BDE188) had a cationic–π interaction with TTR, whereas 4 hydroxy-2,2′,3,4,5,5′,6- heptabromodiphenyl ether (4-OH-BDE187) was bonded to TTR by hydrogen bonds to form stable complexes. In this paper, we highlight the consistency of spectroscopic experiments and computer simulations so as to provide a reliable analytical method for the toxicological properties of small molecule contaminants.     

Thermodynamics of melting of the circular dumbbell d〈pCGC-TT-GCG-TT〉

The conformational behavior of DNA minihairpin loops is sensitive to the directionality of the base pair that closes the loop. Especially tailored circular dumbbells, consisting of a stem of three Watson–Crick base pairs capped on each side with a minihairpin loop, serve as excellent model compounds by means of which deeper insight is gained into the relative stability and melting properties of hairpin loops that differ only in directionality of the closing pair: C-G vs G-C. For this reason the thermodynamic properties of the circular DNA decamers 5′-d〈pCGC-TT-GCG-TT〉-3′( I ) and reference compounds 5′-d〈pGGC-TT-GCC-TT≤-3′( II ) and 5′-d(GCG-TC-CGC)-3′( III ) are studied by means of nmr spectroscopy. Molecules I and II adopt dumbbell structures closed on both sides by a two-membered hairpin hop. At low temperature I consists of a mixture of two slowly exchanging forms, denoted L2L2 and L2L4 . The low-temperature L2L2 form is the fully intact minihairpin structure with three Watson–Crick C-G base pairs. The high-temperature form, L2L4 ,contains a partially disrupted closing G-C base pair in the 5′-GTTC-3′ loop, with the cytosine base placed in a syn orientation. The opposite 5′-CTTG-3′ loop remains stable. A study of the noncircular hairpin structure III shows similar conformational behavior for the 5′-GTTC-3′ loop as found in I a syn orientation for C(6) and two slowly exchanging imino proton signals for G(3). The melting point T_m of II was estimated to lie above 365 K. The T_m value of the duplex stem and the 5′-CTTG-3′ loop of the L2L4 form ofIis 352 ± 2 K. The ΔH° is calculated as ?89 ± 10 kJ/mol. The T_m value determined for the individual residues of the 5′-GTTC-3′ loop lies 4°–11° lower. The enthalpy ΔH° of melting the thymine residues in the 5′-GTTC-3′ loop is calculated to be -61± 7 kJ/mol. Thermodynamic data of the equilibrium between the slowly exchanging two- and four-membered loop conformers of I reveal an upper limit for ΔH° of +30 kJ/mol in going from a two-memberedto a four-membered loop, in agreement with the enthalpy difference of +28 k.j/mol between the two loops at the T_m midpoint. For hairpin III the upper limit for ΔH° going from a two-membered to a four-membered loop amounts to ±21 kJ/mol. The mutual exchange rate between the L2 and L4 form in III is estimated as 13.6 s^?1. Our results clearly suggest that small four-way DNA junctions(model for immobilized Holliday junctions) can be designed that consist of a single DNA strandthat features -CTTG-caps on three of the four arms of the junction. © 1995 John Wiley & Sons, Inc.     

Degradation of anaerobic reductive dechlorinationproducts of Aroclor 1242 by four aerobic bacteria

We studied the aerobic degradation of eight PCB congeners which comprise from 70 to 85% of the anaerobic dechlorination products from Aroclor 1242, including2-, 4-, 2,4-, 2,6-, 2,2'-, 2,4'-, 2,2',4-, and2,4,4'-chlorobiphenyl (CB), and the biodegradation of their mixtures designed to simulate anaerobic dechlorination profiles M and C. StrainsComamonas testosteroni VP44 and Rhodococcus erythreus NY05 preferentially oxidizeda para-substituted ring, while Rhodococcus sp. RHA1, similar to well known strain Burkholderia sp. LB400, preferably attackedan ortho-chlorinated ring. Strains with ortho-directed attack extensively degraded2,4'- and 2,4,4'-CB into 4-chlorobenzoate, while bacteria with para-directed attack transformed these congeners mostly into potentially problematicmeta-cleavage products. The strains that preferentiallyoxidized an ortho-substituted ring readily degradedseven of the eight congeners supplied individually; only 2,6-CB was poorly degraded. Degradationof 2,2'- and 2,4,4'-CB was reduced when present in mixtures M and C. Higher efficiencies of degradation of the individual congeners and defined PCB mixtures M and C and greater production of chlorobenzoates were observed with bacteria that preferentially attackan ortho-substituted ring. PCB congeners 2,4'-, 2,2',4-, and 2,4,4'-CB canbe used to easily identify bacteria with ortho-directed attack whichare advantageous for use in the aerobic stage of the two-phase (anaerobic/aerobic)PCB bioremediation scheme.     

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.     

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.     

Reconstituted human breast milk PCBs as potent inducers of aryl hydrocarbon hydroxylase

The major polychlorinated biphenyl (PCB) components identified in human breast milk have been synthesized and a reconstituted breast milk PCB mixture representing the average levels determined in the Osaka Prefecture in Japan has been prepared. The dose effecting the half-maximal (ED₅₀) induction of rat hepatic microsomal aryl hydrocarbon hydroxylase (AHH) for the reconstituted breast milk PCBs (ED₅₀ ～12 μmol·kg^?1) was approximately seven times less than the ED₅₀ for the commercial PCB mixture, Kanechlor 500. The increased biological potency of the former mixture reflects the preferential bioconcentration of the toxic PCB congeners, 2,3,3′,4,4′-penta-, 2,3′,4,4′,5-penta- and 2,3,3′,4,4′,5-hexachlorobiphenyl.     

Effects of cavity-creating mutations on conformational stability and structure of the dimeric 4-α-helical protein ROP: Thermal unfolding studies

The structural and energetic perturbations caused by cavity-creating mutations (Leu-41 → Val and Leu-41 → Ala) in the dimeric 4-α-helical-bundle protein ROP have been characterized by CD spectroscopy and differential scanning calorimetry (DSC). Deconvolution of the CD spectra showed a decrease in α -helicity as a result of the amino acid exchanges that follows qualitatively the overall decrease in conformational stability. Transition enthalpies are sensitive probes of the energetic change associated with point mutations. ΔH⁰ values at the respective transition temperatures, T_1/2 (71.0, 65.3, and 52.9°C at 0.5 mg/ml) decrease from 580 ± 20 to 461 ± 20 kJ/(mol of dimmer) and 335 ± 20 kJ/(mol of dimmer) for wildtype ROP (Steif, C., Weber, P., Hinz, H.-J., Flossdorf, J., Cesareni, G., Kokkinidis, M. Biochemistry 32:3867-3876, 1993), L⁴¹V, and L⁴¹A, respectively. The conformational stabilities at 25°C expressed by the standard Gibbs energies of denaturation, ΔG, are 71.7, 61.1, and 46.1 kJ/(mol of dimmer). The corresponding transition enthalpies have been obtained from extrapolation using the c(T)and c(T) functions. Their values at 25°C are 176.3, 101.9, and 141.7 kJ/(mol of dimmer) for wild-type ROP, L⁴¹V, and L⁴¹A, respectively. When the stability perturbation resulting from the cavity creating mutations is referred to the exchange of 1 mol of CH₂ group, the average ΔΔG value is ?5.0 ± 1 kJ/(mol of CH₂ group). This decrease in conformation stability suggests that dimeric ROP exhibits the same susceptibility to Leu → Yal and Leu → Ala exchanges as small monomeric proteins. Careful determinations of the partial specific heat capacities of wild-type and mutated protein solutions suggest that the mutational effects are predominantly manifested in the native rather than the unfolded state. © 1995 Wiley-Liss, Inc.     

Genotoxic effects of PCB 52 and PCB 77 on cultured human peripheral lymphocytes

Polychlorinated biphenyls (PCBs) are known to be carcinogenic, but the mechanisms of this action are uncertain. Most, but not all, studies have concluded that PCBs are not directly mutagenic, and that much if not all of the carcinogenic activity resides in the fraction of the PCB mixture that contains congeners with dioxin-like activity. The present study was designed to determine genotoxic effects of an ortho-substituted, non-coplanar congener, 2,2′,5,5′-tetrachlorobiphenyl (PCB 52), and a non-ortho-substituted coplanar congener with dioxin-like activity, 3,3′,4,4′-tetrachlorobiphenyl (PCB 77) on cultured human peripheral lymphocytes. DNA damage was assessed by use of the comet assay (alkaline single-cell gel electrophoresis). After cell cultures were prepared, test groups were treated with different concentrations of PCB 52 (0.2 and 1 μM) and PCB 77 (1 and 10 μM) for 1 h at 37 °C in a humidified carbon dioxide incubator, and compared to a DMSO vehicle control group. The cells were visually classified into four categories on the basis of extent of migration such as undamaged (UD), low damage (LD), moderate damage (MD) and high damage (HD). The highest concentration of PCBs 52 and 77 significantly increased DNA breakage in human lymphocytes (p < 0.001). Our results indicate that both the non-coplanar PCB 52 and coplanar PCB 77 cause DNA damage, and that the ortho-substituted congener was significantly more potent than the dioxin-like coplanar congener.     

Kinetics of selenite uptake by mononuclear cells from peripheral human blood

The present study deals with the kinetics and thermodynamics of the uptake of⁷⁵Se-labeled SeO ₃ ^2? from incubation media to lymphocytes cultivated from eight normal individuals (14–55 years of age, two females). The uptake of SeO ₃ ^2? was evaluated on the assumption of pseudo-first-order kinetics with regard to a reacting cellular receptor pool. On the basis of the experimental observations, it was assumed that the suggested pool of receptor molecules-symbolically represented by “£H₄”—reacts with SeO ₃ ^2? in the hypothetical reaction: $$\pounds H_4 + SeO_3^{2 - } + 2H^ + \underset{{ - k_1 }}{\overset{{k_1 }}{\longleftrightarrow}}\pounds Se + 3H_2 O$$ The mean value of the change in standard free energy at 25°C was calculated to be ΔG ^o=?141.6±1.3 kJ/mol, while the corresponding mean value of the free energy of activation at 25°C was calculated to be ΔG ²⁺=?7.8±0.9 kJ/mol for the forward reaction. The calculated values of the corresponding individual changes in the respective standard enthalpies and entropies were mutually interdependent for all eight donors. ΔH ^o=?152+315ΔS ^o(kJ/mol) corresponding to the common value ΔG ^o??152 kJ/mol at 315°K. These mutual interdependencies are possibly the effect of variable conformational states (e.g., the macromolecular compactness) of the cellular receptor pools. This suggestion may furthermore be supported by the correlation traced between ΔH ^o vs the biological age in years of the donors: △H ^°?76.7?1.0 (age)kJ/mol (r = ?0.92) The calculated values of activation enthalpy ΔH ²⁺ kJ/mol and activation entropy ΔS ²⁺ (kJ/mol K) also mutually correlated linearly (r=0.998); the regression line was: △H ²⁺ = ?8.9 + 305△S²⁺ (kJ/mol) corresponding to the common value △H ²⁺ △ ?8.9 (kJ/mol) at 305°K Similarly the activation enthalpy ΔH ²⁺ vs the biological age in years correlated linearly: ΔH ²⁺=67.4?0.73(age) (kJ/mol) (r=?0.76) The range of ΔH ²⁺ studied was from 13.8 to 53.9 kJ/mol with a linearly corresponding range in ΔS ²⁺ from 73 to 205 J/mol K. The thermodynamic data reveal the selenite uptake during the hypothetical standard reaction to be exergonic and endothermic. Critical pH dependencies of the selenite uptake were explained.     

Different patterns of hepatic microsomal enzyme activity produced by administration of pure hexachlorobiphenyl isomers and hexachlorobenzene

Three hexachlorobiphenyl isomers, 2,2′,4,4′,5,5′-hexachlorobiphenyl (I), 2,2′,3,3′,4,4′-hexachlorobiphenyl (II) and 2,2′,3,4,4′,5′-hexachlorobiphenyl (III), have been administered to rats and the effects of these three compounds upon hepatic microsomal drug metabolism and upon hepatic porphyrins have been studied. Comparisons have been made with hexachlorobenzene and a commercial polychlorinated biphenyl mixture, Aroclor 1254. From measurements of activities of microsomal drug oxidations in vitro, the durations of pharmacological actions of certain drugs in vivo and spectral shifts associated with cytochrome P-450 it is shown that the three pure hexachlorobiphenyl isomers initially produce changes in hepatic microsomal activity which resemble those seen after treatment with phenobarbitone (PB). In contrast, following chronic feeding of the isomers, compounds II and III but not I produce a pattern of hepatic microsomal enzyme activity which shows some characteristics of the 3-methylcholanthrene (3-MC) and some characteristics of the phenobarbitone classes of inducer. Also, compounds II and III, but not I, cause accumulation in the liver of porphyrins containing either seven or eight carboxyl groups. These two responses are similar to those observed following hexachlorobenzene treatment and suggest that a relationship may exist between the mixed pattern of enzyme induction and the onset of hepatic porphyrin accumulation.     

Dynamics of molecular organization in agarose sulphate

Nongelling solutions of structurally regular chain segments of agarose sulphate show disorder–order and order–disorder transitions (as monitored by the temperature dependence of optical rotation) that are closely similar to the conformational changes that accompany the sol–gel and gel–sol transitions of the unsegmented polymer. The transition midpoint temperature (T_m) for formation of the ordered structure on cooling is ～25 K lower than T_m for melting. Salt-induced conformational ordering, monitored by polarimetric stopped-flow, occurs on a millisecond time scale, and follows the dynamics expected for the process 2 coil ? helix. The equilibrium constant for helix growth (s) was calculated as a function of temperature from the calorimetric enthalpy change for helix formation (ΔH_cal = ?3.0 ± 0.3 kJ per mole of disaccharide pairs in the ordered state), measured by differential scanning calorimetry. The temperature dependence of the nucleation rate constant (k_nuc), calculated from the observed second-order rate constant (k_obs) by the relationship k_obs = k_nuc(1 ? 1/s) gave the following activation parameters for nucleation of the ordered structure of agarose sulphate (1 mg mL^?1; 0.5M Me₄NCl or KCl): ΔH* = 112 ± 5 kJ mol^?1; ΔS* = 262 ± 20 J mol^?1 K^?1; ΔG*₂₉₈ = 34 ± 6 kJ mol^?1; (k_nuc)₂₉₈ = (7.5 ± 0.5) × 10⁶ dm³ mol^?1 s^?1. The endpoint of the fast relaxation process corresponds to the metastable optical rotation values observed on cooling from the fully disordered form. Subsequent slow relaxation to the true equilibrium values (i.e., coincident with those observed on heating from the fully ordered state) was monitored by conventional optical rotation measurements over several weeks and follows second-order kinetics, with rate constants of (2.25 ± 0.07) × 10^?4 and (3.10 ± 0.10) × 10^?4 dm³ mol^?1 s^?1 at 293.7 and 296.2 K, respectively. This relaxation is attributed to the sequential aggregation processes helix + helix → dimer, helix + dimer → trimer, etc., with depletion of isolated helix driving the much faster coil–helix equilibrium to completion. Light-scattering measurements above and below the temperature range of the conformational transitions indicate an average aggregate size of 2–3 helices.     

A new dimeric carbazole alkaloid from Murraya koenigii (L.) leaves with α-amylase and α-glucosidase inhibitory activities

A new dimeric carbazole alkaloid, 3,3′,5,5′,8-pentamethyl-3,3′-bis(4-methylpent-3-en-1-yl)-3,3′,11,11′-tetrahydro-10,10′-bipyrano[3,2-a]carbazole, was isolated from the hexane extract of leaves of Murraya koenigii (L.) Sprengel. (Family: Rutaceae). The structure was elucidated based on ¹³C and ¹H NMR, High-Resolution Mass Spectrometry (HRMS), and 2D NMR data. The in vitro antidiabetic activity of the new dimer was investigated in terms of α-amylase and α-glucosidase enzyme inhibition assays. The dimer exhibited significant α-amylase inhibitory activity (IC₅₀ = 30.32 ± 0.34 ppm) and α-glucosidase inhibitory activity (IC₅₀ = 30.91 ± 0.36 ppm).