Theoretical study of thiolysis in penicillins and cephalosporines

Semiempirical calculations were used to conduct a comprehensive study of the thiolysis of the fundamental core of penicillins and cephalosporins. The significance of the intramolecular protonation of the β‐lactam nitrogen in the formation and cleavage of the tetrahedral intermediate ( T in Scheme 1 ) was examined in two thiols bearing substituents of different basicity in β with respect to the thiol group in the attacking nucleophile, namely 2‐mercaptoethanol ( 6 ) and 2‐mercaptoethylamine ( 7 ). Based on the results, the rate‐determining step in the reaction of penicillins is the cleavage of the tetrahedral intermediate, consistent with an intramolecular acid catalysis process in their thiolysis by 2‐mercaptoethylamine. On the other hand, the rate‐determining step in the reaction of cephalosporins, which possess an appropriate leaving group at position 3', is the formation of the tetrahedral intermediate, so the desolvation energy of the nucleophile is a major contributor to the overall energy of the process. This differential behavior between the two types of β‐lactam bicycles arises from the presence of the acetate group at 3' and the delocalization of π electrons over the N₅–C₄–C₃ system in cephalosporins; this favors the formation of a thiolate with the 5‐ethoxymethylene‐1,3‐thiazine group in the cleavage of the tetrahedral intermediate, which is stabilized by an intramolecular hydrogen bond between N₅ and the alcohol or amine group in β of the attacking thiol. The theoretical results are consistent with previous experimental data showing that, unlike penicillins, cephalosporins undergo no intramolecular acid catalysis in their thiolysis. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 434–443, 2005     

Preparation of vesicles composed of 2-(hexadecyloxy) cinnamic acid and N-[3-(dimethylamino) propyl]-octadecanamide and their photo- and pH-responsive release property

Vesicles composed of N-[3-(dimethylamino) propyl]-octadecanamide (DMAPODA) and 2-(hexadecyloxy) cinnamic acid (HOCA) in an equimolar ratio were prepared by taking advantage of salt bridge formed between the amino group and the carboxylic group. The structure of vesicle was observed on a Transmission electron microscopy (TEM), and the size was determined on a dynamic light scattering equipment. The phase transition of the vesicular membrane was found to be around 35 °C on a differential scanning calorimeter. HOCA of the vesicular membrane was readily dimerized under the irradiation of a UV light (λ?=?254 nm, 6 W). The release degree of rhodamine B from vesicle suspended in distilled water (pH 6.8) was about 70 % in 1 h at 25 °C, and the UV irradiation during the release experiment had little effect on the release degree. However, it had a significant effect when the temperature of release medium was 40 °C. Upon the photodimerization, HOCA would readily change its orientation in the vesicular membrane vesicle at 40 °C, possibly because the vesicular membrane is in a liquid crystalline state at the temperature, which is higher than the phase transition temperature (around 35 °C). In addition, the vesicle released rhodamine B in a pH-dependent manner. The release degrees were the highest at pH 3.0 and the lowest at pH 9.0 among the pH values tested. The salt bridge formed between DMAPODA and HOCA is labile at a strong acidic condition so it would be responsible for the extensive release at pH 3.0.     

Effect of the pH value on the thermal stability of alkaline phosphatase

The thermal stability of bovine and chicken intestinal alkaline phosphatase (IAP) was studied at 60°C over a pH range from 6.7 to 9.0. It was shown that the peaks of stability (at pH 7.5) and activity (at pH 9.0) do not coincide. The pH dependence of the elementary rate constants of dissociative thermal inactivation of the IAP dimers was determined: k ₁ (the rate constant of dissociation of active dimer E₂) and k _d (the rate constant of denaturation of the inactive monomers). At pH 7.5, the stability IAP attains its highest level. As the pH increases, k ₁ increases drastically while k _d does so only slightly. A comparison of the rate constants of dissociation k ₁ and association k ₁ showed that the interrelation between these parameters explains why the stability of the active enzyme is lower at acidic and basic pH values. The pH produces a weaker effect on the stability of the inactive monomers. An analysis of the thermal stability of chicken IAP at an optimum pH value and 55–60°C showed that the thermal inactivation is a three-stage process (including dissociation and denaturation) with an induction period. Measuring the induction period makes it possible to determine the minimum number of latent stages preceding the dissociation of E₂: 6, 4, and 3 at 55, 58, and 60°C, respectively.     

Multiresponsive Viscoelastic Vesicle Gels of Nonionic C12EO4 and Anionic AzoNa

Viscoelastic vesicle gels were prepared by mixing a nonionic surfactant, tetraethylene glycol monododecyl ether (C₁₂EO₄), and an anionic dye, sodium 4‐phenylazobenzoic acid (AzoNa). The gels, which were composed of multilamellar vesicles, were analyzed by cryogenic transmission electron microscopy (cryo‐TEM), freeze–fracture transmission electron microscopy (FF‐TEM), ²H NMR spectroscopy, and small‐angle X‐ray scattering (SAXS). The mechanism of vesicle‐gel formation is explained by the influence of anionic molecules on the bilayer bending modulus. Interestingly, the vesicle gels were observed to be sensitive to temperature, pH, and light. The viscoelastic vesicle gels respond to heat; they thin at lower temperatures and become thicker at higher temperatures. The vesicle gels are only stable from pH 7 to 11, and the gels become thinner outside of this range. UV light can also trigger a structural phase transition from micelles to multilamellar vesicle gels.     

Reaction mechanism of the benzoquinone/hydroquinone couple at platinum electrodes in aqueous solutions: Retardation and enhancement of electrode kinetics by single chemisorbed layers

The electrochemical kinetics of the benzoquinone (Q)/hydroquinone (H₂Q) redox couple at platinum electrodes in aqueous solutions has been found to be extremely sensitive to the nature of species adsorbed on the electrode surface at monolayer coverages. Experimental measurements were based on thin-layer cyclic voltammetry; the use of thin-layer electrodes was dictated by the need to minimize surface contamination. Bulky neutral or anionic aromatic adsorbates led to the familiar U-shaped rate-vs.-pH curves; the rate minimum occurred near pH 4. Kinetic effects due to oriental changes of chemisorbed species were noted only when the rate was low. Adsorbed 1 atoms led to comparatively rapid reactivity (rate constant k° > 10^?3 cm s^?1) and virtual independence of pH. Profound retardation resulted from pretreatment ofthe surface with CN^? and SCN^?; total irreversibility (k° < 10^?6 cm s^?1) was observed at pH 4, with a further decrease in rate at pH 7. In contrast, when the surface contained n layer of chemisorbed phenyltriethylammonium cations, the electrode rate increased with increasing pH. The results indicate that different reaction pathways predominate when different absorbates are present.     

In situ monitoring of gold-surface adsorption and acidic denaturation of human serum albumin by an isolation-capacitance-adopted electrochemical quartz crystal impedance system

Combined measurements of piezoelectric quartz crystal impedance (PQCI) and electrochemical impedance spectrum (EIS) using a suitable isolation capacitance is reported for the first time to monitor in situ adsorption and acidic denaturation of human serum albumin (HSA) on gold electrodes in Britton-Robinson (B-R) buffers. This method provides simultaneously mutual-interference-free and accurate parameters of EIS and PQCI. Effects of surface thiol-modification, electrode-potential and solution pH on HSA adsorption were examined and discussed. Comparative experiments of HSA adsorption in a B-R buffer of pH 6.42 on bare, cysteine- and 1-dodecanethiol-modified gold electrodes revealed that HSA adsorption is more significant on a hydrophobic (1-dodecanethiol-modified) surface. Insignificant electrode-potential effect implied minor electrostatic effects on HSA adsorption. The adsorption amount of HSA at pH 3.28 was found to be notably greater than those at pH 4.84 and 6.42. To characterize HSA adsorption, electrode standard rate constants (k_s) of the Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ couple were measured before and after HSA adsorption. The k_s-pH curves on an HSA-modified Au electrode revealed that k_s increased abruptly with the decrease of solution pH below pH ∼4. Moreover, pH-dependent responses of the resonant frequency, the motional resistance, the double-layer capacitance, the capacitance of adsorbed HSA layer and the peak absorbance of HSA solutions at 278 nm all exhibited an inflexion change at pH ∼4, and these findings have been explained on the basis of acidic denaturation of HSA and electrical charges carried by HSA molecules.     

Effect of dialkyldimethylammonium vesicles on the thiolysis of p-nitrophenyl acetate

Cationic surfactant vesicles, prepared by an alcohol injection technique and characterized by gel filtration, are extremely effective at increasing the rate of ester thiolysis at near neutral pH.     

The effect of pH on some outer-sphere electrode reactions at carbon electrodes

The apparent heterogeneous rate constant, k°_app, for three couples known to undergo outer-sphere electron transfer reactions has been investigated at carbon, gold, and platinum electrodes as a function of solution pH. The rates are found to be pH dependent at two different types of glassy carbon and at carbon paste but not at metallic electrodes. The rate is found to decrease with increasing pH for Fe(CN)^{$\text{3?}\text{4?}$}₆ and IrCl^{$\text{2?}\text{3?}$}₆, and the opposite effect is observed for the Ru(NH₃)^{$\text{3+}\text{2+}$}₆ couple. For the three compounds examined, the magnitude of the pH dependent rate changes can be associated with a change in the magnitude of the potential of the reaction layer induced by a change in surface charge of ～ 2 × 10^?7 C cm^?2. The pH dependence of the observed rate constant at glassy carbon can be attenuated by heat treatment, chemical reduction, or esterification of the surface. This behaviour is consistent with the theory that the pH dependence of k°_app arises from the presence of functional groups on the carbon surface.     

Difunctional aliphatic arsenic acids as reagents for liquid-liquid extraction of Sc(III)

The liquid-liquid extraction of Sc³⁺ at 50°C from HCl solutions by [C₈H₁₇As(O)OH]₂(CH₂)_n in octanol where n = 6, 8, 12 was investigated. Sc(III) was extracted from HCl acidified solutions containing NaCl and potassium acid phthlate to control the Cl-ionic strength and pH, respectively, in the pH range 1.8–5.3. The E_a⁰ reached a maximum of 14.6 for n = 6 at an aqueous equilibrium pH 5.10 corresponding to the extraction of 94% of the Sc³⁺ initially present in the aqueous solution, 11.0 for n = 8 at pH 4.45 corresponding to 92% extraction, and 12.4 for n = 12 at pH 4.70 corresponding to 93% extraction. The log E_a⁰ was found to be first power dependent on the log of the aqueous equilibrium [H⁺] for n = 6, 8 and 12. The extractions were done at 50°C using organic reagent solutions (4.72 × 10⁻⁴ M for n = 12, 4.63 × 10⁻⁴ M for n = 8,4.62 × 10⁻⁴ M for n = 6) with aqueous solutions of 1.06 × 10⁻⁵ M ScCl₃ having their Cl⁻ ionic strengths adjusted to 0.100 with NaCl and the pH adjusted using HCl and KHP as buffer. The reagent dependence studies at 50°C show that 1.5 reagent (n = 6) molecules are bound to each Sc³⁺ ion. For n = 8 and n = 12, there are mixtures of 1:1 and 2:1 reagent molecules complexed per Sc³⁺. It is assumed that the complex is hexacoordinated.     

Reactivity of imidazolidin-4-one derivatives of primaquine: implications for prodrug design

In contrast to peptide-based imidazolidin-4-ones, those synthesized from N-(α-aminoacyl) derivatives of the antimalarial drug, primaquine and ketones are unexpectedly stable in pH 7.4 at 37 °C. The kinetics of hydrolysis of primaquine-based imidazolidin-4-ones were investigated in the pH range 0.3-13.5 at 60 °C. The hydrolysis to the parent α-aminoacylprimaquine is characterized by sigmoidal-shaped pH-rate profiles, reflecting the spontaneous decomposition of both unionized and protonated (at N-1) forms of the imidazolidin-4-one. The kinetically determined pK_a values are ca. 3.6-4.0, i.e., 4 pK_a units lower than those of amino acid amides, thus implying that hydrolysis of imidazolidin-4-ones at pH 7.4 involves the unionized form. Reactivity of this form decreases with the steric crowding of the amino acid α-substituent. In contrast, the rate constant for the spontaneous decomposition of the unionized form increases sharply for imidazolidin-4-ones derived from cyclic ketones, an observation that can be explained by the I-strain (internal strain) effect. These results are consistent with a mechanism of hydrolysis involving an S_N1-type unimolecular cleavage of the imidazolidin-4-one C2-N3 bond with departure of an amide-leaving group. The mechanism for the decomposition of the protonated imidazolidin-4-one is likely to involve an amide-carbonyl oxygen protonated species, followed by the C2-N3 bond scission, as supported by computational studies. The results herein presented suggest that imidazolidin-4-ones derived from simple N-alkyl α-aminoamides are too stable and therefore, may be useful as slow drug release prodrugs.     

Antioxidant activity of lipophilic vitamin c derivative in dipalmitoyl phosphatidylcholine vesicles — A stopped-flow esr kinetic study

The antioxidant reactivity of the lipophilic derivative of vitamin C, i.e., ascorbyl-6-palmitate (VC-16) was remarkably enhanced in dipalmitoyl phosphatidylcholine (DPPC) vesicles. The rate enhancement is tentatively explained by a vesicle fusion mechanism and subsequent lateral diffusion of reactants within the vesicle. The reduction of nitroxides incorporated into the vesicle by VCs was employed to examine the distribution of the nitroxide inside and outside the vesicle as well as the dynamic behaviour of the inside-outside transition (flip-flop).     

pH Dependent Elasticity of Polystyrene‐block‐poly(acrylic acid) Vesicle Shell Membranes by Atomic Force Microscopy

We assess the elastic properties of PS‐b‐PAA vesicle membranes under different pH values by AFM force measurements. We find that based on the shell deformation theory, the values of the estimated apparent Young's modulus of the vesicle membranes decrease as the pH of the solution increases. The onset of the decrease of E coincides with the surface pK_a determined from ζ‐potential measurements. This decrease of E at higher pH is attributed to electrostatic repulsion between the deprotonated PAA chains resulting in the thinning of the vesicle membrane.

     

p-sulfonatocalix[8]arene and chitosan based vesicle formation studies by spectroscopic and thermal methods

p-sulfonatocalix [8]arene and chitosan based supramolecular amphiphilic vesicles were formed by electrostatic interaction between anionic character of calixarene and cationic chitosan molecules. UV–visible, fluorescence, dynamic light scattering and thermogravimetric analysis were performed for the characterization of p-SCX8 with chitosan. Increase in absorption and enhancement of fluorescence emission intensity attributed to vesicle formation between both calixarene and chitosan in aqueous media. Results obtained from size distribution curve of dynamic light scattering experiment gives clear evidence for vesicle formation at pH 4.98. Positive zeta potential values obtained from dynamic light scattering experiment gives the evidence for vesicle formation. Thermogravimetric analysis gives quantitative results for purity and stability of the vesicles. From overall studies we can conclude that degree of deacetylation as well as pH 4.98 of the solutions and anionic nature of pSCX8 plays important role in electrostatic interaction of vesicle formation.     

Hydrolysis study: Synthesis of novel styrenic Schiff bases derived from benzothiazole

Novel styrenic Schiff base derivatives of benzothiazole are synthesized. The condensation of 4-vinylbenzaldehyde 1 with either benzothiazol-2-amine or 4-methoxy-benzothiazol-2-amine leads to a mixture of two isomers. From 4-methyl-benzothiazol-2-amine and 6-fluoro-benzothiazol-2-amine, the N-(4-vinylbenzylidene)-4-methyl-benzothiazol-2-amine 8 and N-(4-vinylbenzylidene)-6-fluoro-benzothiazol-2-amine 9 are isolated, respectively. The structures of the synthesized Schiff bases are confirmed through a combination of various spectroscopic techniques including IR, UV, ¹H and ¹³C NMR. A kinetic study of the hydrolysis process of derivatives 6 and 9 in buffered aqueous medium at pH 4.4, 7.4 and 8.5 is conducted by UV spectroscopy. It is shown that the hydrolysis of these compounds is a first order reaction showing an increasing rate as the medium acidity is enhanced.     

InCl3-catalyzed regio- and stereoselective thiolysis of alpha,beta-epoxycarboxylic acids in water

[reaction: see text] The thiolysis of alpha,beta-epoxycarboxylic acids 1a-e by thiols 2a,b is more efficient in water than in dichloromethane or SFC. At pH 9.0 phenylthiolate generally attacks the C-alpha carbon while at pH 4.0, and in the presence of InCl3 (10 mol %), the thiolysis is exclusively C-beta regioselective. In all cases, the processes are completely anti-diasteroselective, and the corresponding products 3, 4, and 5 have been isolated in good yields. Both water and catalysts have been recovered and reused.     

The synthesis of an α-azaornithine derivative and its reaction with trypsin

N^α-Benzoyl-α-azaornithine phenyl ester has been synthesised as the trifluoroacetate. In aqueous solution the ester salt is stable at pH 3 but decomposes at higher pH values. The analogue ester inhibits trypsin rapidly, and chymotrypsin much more slowly, by acylation at the active site. Spontaneous deacylation of the α-azaacyl-trypsin occurs with a first-order rate constant of 2·1 × 10^?4 s^?1 at 23·8° and pH 7·6.     

NaOH-catalyzed thiolysis of alpha,beta-epoxyketones in water. A key step in the synthesis of target molecules starting from alpha,beta-unsaturated ketones

NaOH (0.02-0.3 molar equiv) is an efficient catalyst for the thiolysis reactions of alpha,beta-epoxy ketones with alkyl and aryl thiols in water. Thiolysis of 3,4-epoxyheptan-2-one (1) with thiols 2a-d has been accomplished in mild conditions (30 degrees C and pH 6 or 9) with complete C-alpha-regioselectivity and anti-stereoselectivity, and the corresponding anti-beta-carbonyl-beta-hydroxysulfides 3a-d have been prepared in excellent yields (95-98%). Compounds 3a-d, depending on their nature and pH conditions, have undergone dehydration, C-3 epimerization reaction, and retroaldol condensation. Dehydration of anti-3a-d has been chemoselectively carried out by in situ acidic treatment at 70 degrees C, giving stereoselectively the related (Z)-vinyl sulfides 4 in 89-94% overall yields. Under NaOH-catalyzed thiolysis conditions, cyclic alpha,beta-epoxyketones 6-9 have shown C-alpha attack only and spontaneously dehydrated to furnish the corresponding vinyl sulfides in high yields (90-96%). The reactions of calchone oxide (10) with thiols 2b-d have exclusively resulted in the formation of beta-carbonylsulfides 10b-d (82-93% yield), coming from the nucleophilic attack at the alpha-position and retroaldol condensation. To highlight the synthetic utility of this procedure, one-pot multisteps preparation of vinyl sulfides 7b and 7c, vinyl sulfoxides 12 and 13, and 1,5,6,7-tetrahydro-4H-1,2,3-benzotriazol-4-one (14) starting from 2-cyclohexen-1-one (11) have also been reported.     

Degradation kinetics of fluvoxamine in buffer solutions: In silico ADMET profiling and identification of degradation products by LC-MS/ESI

The kinetics of hydrolysis of fluvoxamine maleate (FLV) has been investigated over the pH range 1.0–12.0 at 40, 60 and 80 °C. FLV degradation follows pseudo-first-order kinetics which is consistent with the kinetics of drugs that are not readily dissolved in aqueous medium. The hydrolytic degradation rate constant (k_obs) range from 0.92 (pH 6.0) to 13.8 × 10⁻⁴ min⁻¹ (pH 1.0). The k_obs represents the sum of six different degradation rate constants; the k_H has been found to be higher than k_OH. The FLV exhibits a typical rate- pH profile with a flat bottom over the pH range 3.0–6.0 which indicates its maximum stability at pH 6.0. Ten FLV degradants have been predicted by Zeneth software and among them four degradation products (D1, D2, D3 and D4) have been identified in degraded samples. The in-silico pharmacokinetics and toxicity of degradation products have been determined using Swiss ADME and admetSAR software. The toxicity profile reveals that D2 is both AMES toxic and carcinogenic while the rest of the products are non-AMES toxic and non-carcinogenic. All of the degradation products are high in causing fish toxicity thus their presence in pharmaceutical waste is alarming for environmental safety.     

Synthesis and Guest-Binding Properties of pH/Reduction Dual-Responsive Cyclophane Dimer

A water-soluble cyclophane dimer having two disulfide groups as a reduction-responsive cleavable bond as well as several acidic and basic functional groups as a pH-responsive ionizable group 1 was successfully synthesized. It was found that 1 showed pH-dependent guest-binding behavior. That is, 1 strongly bound an anionic guest, 6-p-toluidinonaphthalene-2-sulfonate (TNS) with binding constant (K/M⁻¹) for 1:1 host-guest complexes of 9.6 × 10⁴ M⁻¹ at pH 3.8, which was larger than those at pH 7.4 and 10.7 (6.0 × 10⁴ and 2.4 × 10⁴ M⁻¹, respectively), indicating a favorable electrostatic interaction between anionic guest and net cationic 1. What is more, release of the entrapped guest molecules by 1 was easily controlled by pH stimulus. Large favorable enthalpies (ΔH) for formation of host-guest complexes were obtained under the pH conditions employed, suggesting that electrostatic interaction between anionic TNS and 1 was the most important driving force for host-guest complexation. Such contributions of ΔH for formation of host-guest complexes decreased along with increased pH values from acidic to basic solutions. Upon addition of dithiothreitol (DTT) as a reducing reagent to an aqueous PBS buffer (pH 7.4) containing 1 and TNS, the fluorescence intensity originating from the bound guest molecules decreased gradually. A treatment of 1 with DTT gave 2, having less guest-binding affinity by the cleavage of disulfide bonds of 1. Consequently, almost all entrapped guest molecules by 1 were released from the host. Moreover, such reduction-responsive cleavage of 1 and release of bound guest molecules was performed more rapidly in aqueous buffer at pH 10.7.     

Adsorption of direct black-38 azo dye on p-tert-butylcalix[6]arene immobilized material

The present study demonstrates the adsorption of the direct black-38 (DB-38) azo dye on potential and newly synthesized p-tert-butylcalix[6]arene based silica resin (4). Resin 4 was synthesized via the modification of pure silica 1 as 2 followed by the immobilization of p-tert-butylcalix[6]arene (3) onto modified silica (2). It was characterized by using different analytical techniques such as FT-IR, scanning electron microscopy (SEM) and thermo gravimetric analysis (TGA). The effect of adsorbent dosage, pH and the electrolyte effect on the removal of the DB-38 azo dye were evaluated through batch wise adsorption experiments. Maximum adsorption of 91% was achieved at pH 9.0. The textile wastewater samples were used to ensure the field applicability of the newly synthesized adsorbent 4 for the treatment of dye contaminated effluents. All results regarding the removal of the DB-38 azo dye from the aqueous environment prop up resin 4 as an effective adsorbent and it was found that resin 4 has high adsorption efficiency toward the DB-38 azo dye at a wide range of pH as compared to 1 and 2.