On the Stability of the Schiff Bases of Pyridoxal 5′-Phosphate with Polypeptides Containing L-Lysine

We determined the apparent equilibrium constant of formation, K_pH, of the Schiff bases of pyridoxal 5′-phosphate (PLP) and poly- and copolymers containing L -lysine, as a function of pH at 25° and a constant ionic strength of 0.1 M . The K_pH values obtained at acidic and neutral pH were larger that those reported for Schiff bases of PLP and hexylamine. We determined calorimetrically ΔH of formation of Schiff bases of PLP and poly(L -lysine) (?4.5′kcal/mol), and PLP and hexylamine (?3.4 kcal/mol) at pH 7.00. Semi-empirical theoretical calculations (INDO and AMI methods) of a model compound of Schiff base of PLP and polypeptide containing L -lysine show the capability of specific interactions between groups of PLP and the peptide skeleton.     

Kinetic Study on Stability of Schiff Base of Pyridoxal 5′-Phosphate and Leucine in Water Media with Cationic Surfactants

We studied the stability of the Schiff bases formed between pyridoxal 5′-phosphate (PLP) and leucine in the presence of (hexadecyl)trimethylammonium bromide (CTAB) over a wide pH range by determining the kinetic constants of formation and hydrolysis of these compounds. The results show that the stability of the Schiff bases is increased by the presence of CTAB as a result of increased rates of formation and decreased hydrolysis rate constants. The ionic head groups of CTAB favour the formation of the bases, while its hydrophobic rests protect the imine double bond from hydrolysis. This model system permits one to obtain partially hydrophobic media with no need for any non-aqueous solvents.     

Kinetic and thermodynamic parameters for Schiff-rase formation between 5′-deoxypyridoxal and hexylamine

The thermodynamic parameters involved in the formation of Schiff bases between 5′-deoxypyridoxal and hexylamine were determined at different pH values and a constant ionic strength (0.1 M). The overall and individual rale constants of formation and hydrolysis at 10, 15, 20, 25, and 30° were also determined. The enthalpy of the overall formation process was found to be negative at all the pH values assayed except the neutral, while its entropy was always positive. The results obtained show the great relevance of the phosphate group at C(5′) to the stabilization of the Schiff bases of pyridoxal 5′-phosphate.     

Schiff bases of pyridoxal and polyallylamine. The formation rate determining step

The apparent rate constants of formation (k₁) and hydrolysis (k₂) of the Schiff bases formed between pyridoxal and polyallylamine has been fitted to a kinetic scheme that involve the different protonated species in the reaction medium and the individual rate constants of formation (k₁ⁱ) and hydrolysis (k₂ⁱ). The (k₁ⁱ) values precludes an acid catalyzed intramolecular process. The effects of hydrophobic medium due to the presence of the macromolecule on the reaction is also discussed. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 1–6, 1998.     

Determination of the rates of formation and hydrolysis of the Schiff bases formed by pyridoxal 5′-phosphate and copolypeptides containing l-lysine

We determined the apparent rate constants of formation (k₁) and hydrolysis (k₂) of the Schiff bases formed between pyridoxal 5′-phosphate (PLP) and l-lysine and l-alanine copolymers of different compositions, as well as those formed between PLP and l-lysine and l-glutamic acid copolymers, at various pH values, a temperature of 25 °C and an ionic strength of 0.1 M. The k₁ values obtained in neutral and acidic media were independent of the copolymer composition. The efficiency of the intramolecular acid catalysis for the formation of the Schiff bases was found to be somewhat higher than that of PLP—primary amine systems (the slope of the Brøwted plot was α=0.77). The most stable of the Schiff bases studied was that with a protonated imine nitrogen and phosphate group and a unprotonated pyridine nitrogen.     

Chemical transformations of pyridoxal and pyridoxal 5′-phosphate condensation products with amino acids

The mechanism of chemical transformations of pyridoxal and pyridoxal 5′-phosphate condensation products with amino acids is studied by kinetic measurements. The Schiff bases are shown to be fairly stable in neutral media. In acid media, the Schiff bases are hydrolyzed into the initial components. In alkaline media, cleavage of α-hydrogen from the amino acid fragment and structural rearrangement into the quinoid form followed by hydrolysis of the latter with elimination of pyridoxamine and keto acid take place. The rate constants of the chemical transformations of the Schiff bases are found to depend on the pH of the medium. It is shown for the first time that the phosphate group in the pyridoxal 5′-phosphate fragment catalyzes the α-hydrogen cleavage and strongly accelerates alkaline decomposition of the Schiff bases.     

Fluoroimines from the reaction of fluoro amino acids or fluoroketo acids with the aldehyde or amine form of vitamin B6: 2—Stereochemical aspects of their formation from β-fluoroaspartates or β-fluorooxaloacetate,using 19F NMR

The fluorine NMR spectra of systems initially containing 0.05–0.1 M of pyridoxal 5′-phosphate and erythro-β-fluoroaspartate (or threo-β-fluoroaspartate) in D₂O solution were examined over the pD range 1–12. The formation of the aldimine Schiff base gave only one stereoisomer, whcih was trapped with sodium borohydride. Reactions of pyriodoxamine 5′-phosphate and fluorooxaloacetate were examined under the same conditions. A mixture of two products was given, identified as two ketimine Schiff bases (E and Z isomers) with well characterized fluorine chemical shifts and ²J(DF) values. This mixture, trapped with sodium borohydride, gives the two reduced erythro and threo aldimines. The configurations and conformations of all reaction products were determined using the ³J(HF) values and their correlation with the fluorine chemical shift. The role of the 3-phenolate oxygen of the pyridine ring, of the conjugate acid of the iminium nitrogen and of the carboxylate oxygen in ionic or hydrogen bond interactions in the determination of the stereochemistry is discussed.     

Hydrolysis of linear polyurethanes and model monocarbamates

Alkaline hydrolysis of model carbamates, polyurethanes, and poly(urethane-ureas) has been investigated. The model carbamates were based upon phenyl, benzyl, and cyclohexyl isocyanates. The polyurethanes and poly(urethane-ureas) were prepared from tolylene diisocyanate (TDI), xylylene diisocyanate (XDI), and 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI) and a poly(oxyethylene)glycol of 6000 molecular weight. Pseudo-first-order rate constants of hydrolysis were obtained in aqueous pyridine solution at 110°C, and second-order rate constants were obtained in aqueous KOH solution for the model biscarbamates. Pseudo-first-order rate constants of hydrolysis were obtained in alcoholic KOH solution for the polyurethanes and poly(urethane-ureas). The hydrolysis of the model carbamates showed that the stability increased in the following manner: phenyl < benzyl < cyclohexyl. The pseudo-first-order rate constants were dependent upon the pK_b of the corresponding amines. The hydrolysis of the polyurethanes and poly(urethane-ureas) showed that the stability increased in the following manner: aromatic < aralkyl < cycloaliphatic. It was shown that polyurethanes are more susceptible to alkaline hydrolysis than to acidic hydrolysis.     

Kinetic study on the alkaline hydrolysis of 1-aryl-2-phenyl-2-imidazolines. Basicity-rate of hydrolysis and structure relationships

Rate constants (k_obs) of hydrolysis in boiling alkaline ethanolic solution for six 1-aryl-2-phenyl-2-imidazolines were determined. The influence of substituents in the phenyl group at N-1 upon rate of hydrolysis was studied. When the imidazoline ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. It was observed that reaction rates were enhanced by electron-releasing phenyl substituents of N-1 and reduced by electron-withdrawing groups, providing a change in the mechanism of the reaction in the first case that was not observed in the second. Agreement with the Hammett equation allowed comparison between experimental and “calculated” rate constants which are nearly equal. An equation relating the rate constants with the ionization constants of imidazolinium ions is given.     

Determination of the activation rate constants of alkyl halide initiators for atom transfer radical polymerization

The second-order activation rate constants k_A for low-molar-mass alkyl halides catalyzed by cuprous halide complexes Cu(I)X/2L (X = Cl or Br; L = 4,4′-diheptyl-2,2′-bipyridine) were determined by the nitroxide capping method along with ¹H NMR. The k_A for 1-phenylethyl bromide, a typical initiator for atom transfer radical polymerization (ATRP), with the Cu(I)Br complex was found to be close to the known value of the k_A for a polystyryl bromide, being large enough for the initiation to be completed at an early stage of polymerization. It was also found that k_A strongly depends on the kind of halogen and the steric factor of the alkyl halide in question.     

Radical and cationic polymerizations of 3‐ethyl‐3‐methacryloyloxymethyloxetane

3‐Ethyl‐3‐methacryloyloxymethyloxetane (EMO) was easily polymerized by dimethyl 2,2′‐azobisisobutyrate (MAIB) as the radical initiator through the opening of the vinyl group. The initial polymerization rate (R_p) at 50 °C in benzene was given by R_p = k[MAIB]^0.55 [EMO]^1.2. The overall activation energy of the polymerization was estimated to be 87 kJ/mol. The number‐average molecular weight (M?_n) of the resulting poly(EMO)s was in the range of 1–3.3 × 10⁵. The polymerization system was found to involve electron spin resonance (ESR) observable propagating poly(EMO) radicals under practical polymerization conditions. ESR‐determined rate constants of propagation (k_p) and termination (k_t) at 60 °C are 120 and 2.41 × 10⁵ L/mol s, respectively—much lower than those of the usual methacrylate esters such as methyl methacrylate and glycidyl methacrylate. The radical copolymerization of EMO (M₁) with styrene (M₂) at 60 °C gave the following copolymerization parameters: r₁ = 0.53, r₂ = 0.43, Q₁ = 0.87, and e₁ = +0.42. EMO was also observed to be polymerized by BF₃OEt₂ as the cationic initiator through the opening of the oxetane ring. The M?_n of the resulting polymer was in the range of 650–3100. The cationic polymerization of radically formed poly(EMO) provided a crosslinked polymer showing distinguishably different thermal behaviors from those of the radical and cationic poly(EMO)s. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1269–1279, 2001     

Kinetic and ESR studies on radical copolymerization of p-tert-butoxystyrene and di-n-butyl maleate in benzene

The copolymerization of p-tert-butoxystyrene (TBOSt) (M₁) and di-n-butyl maleate (DBM) (M₂) with dimethyl 2,2′-azobisisobutyrate (MAIB) in benzene at 60°C was studied kinetically and by means of ESR spectroscopy. The monomer reactivity ratios were determined to be r₁ = 2.3 and r₂ = 0 by a curve-fitting method. The copolymerization system was found to involve ESR-observable propagating polymer radicals under practical copolymerization conditions. The apparent rate constants of propagation (k_p) and termination (k_t) at different feed compositions were determined by ESR. From the relationship of k_p and f₁ (f₁ = [M₁]/([M₁] + [M₂])) based on a penultimate model, the rate constants of five propagations of copolymerization were evaluated as follows; k₁₁₁ = 140 L/mol s, k₂₁₁ = 3.5 L/mol s, k₁₁₂ = 61 L/mol s, k₂₁₂ = 1.5 L/mol s, and k₁₂₁ = 69 L/mol s. Thus, a pronounced penultimate effect was predicted in the copolymerization. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1449–1455, 1998     

Kinetics of the reaction of 4,4′‐biphenol with bromoethane and 1‐bromobutane by phase‐transfer catalysis

The reaction of 4,4′‐biphenol and two species of bromoalkanes (e.g., bromoethane and 1‐bromobutane) to synthesize two symmetric products (4,4′‐diethanoxy biphenyl and 4,4′‐dibutanoxy biphenyl) and one asymmetric product (4‐ethanoxy, 4′‐butanoxy biphenyl) was successfully carried out under two‐phase phase‐transfer catalysis conditions. A rational mechanism and kinetic model were built up by considering the reactions both in aqueous phase and in organic phase. The first active catalyst (QO(Ph)₂OQ) was also synthesized under two‐phase reaction and was identified by instruments. The experimental data were explained satisfactorily by the pseudo‐steady‐state hypothesis. Two sets of rate constants of organic reactions, i.e. primary (k₁ and k₂) and secondary (k₁₁, k₁₂, k₂₁, and k₂₂) rate constants participate in the kinetic model. The two primary rate constants were obtained individually via experimental data for synthesizing the symmetric products. The ratios of the other four secondary rate constants were obtained from the reaction of synthesizing asymmetric products and determined from the initial yield rates of symmetric products. The effects of the ratio of bromoethane and 1‐bromobutane, temperature, organic solvents, amount of catalyst, and amount of sodium hydroxide on the reaction rate and the selectivity of products were investigated in detail. The results were explained satisfactorily by the interaction between the reactants and the environmental species. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 139–153, 2003     

Reactivity of base catalysed hydrolysis of 2-pyridinylmethylene-8-quinolinyl-Schiff base iron(II) iodide complexes: solvent effects

Three ligands of 2-pyridinylmethylene-8-quinolinyl (L1), methyl-2-pyridinylmethylene-8-quinolinyl (L2), and phenyl-2-pyridinylmethylene-8-quinolinyl (L3), Schiff bases were synthesised by direct condensation of 8-aminoquinoline with 2-pyridinecarboxaldehyde, 2-acetylpyridine, or 2-benzoylpyridine. They coordinated to Fe(II) ion in a 1: 2 mole ratio followed by treatment with iodide ions affording complexes with a general formula [Fe(L)₂]I₂·2H₂O, (L = L1, L2, or L3). Spectrophotometric evaluation of the kinetics of base catalysed hydrolysis of these complex cations was carried out with an aqueous solution of NaOH in different ratios of water/methanol binary mixtures. Kinetics of the hydrolysis followed the rate law (k ₂[OH^?] + k ₃[OH^?]²)[complex]. Reactivity trends and their rate constants were compared and discussed in terms of ligand structure and solvation parameters. The methanol ratio affects the hydrolysis as a co-solvent which was analysed into initial and transition state components. The increase in the rate constant of the base hydrolysis of Fe(II) complexes, as the ratio of methanol increases, is predominantly caused by the strong effect of the organic co-solvent on the transition states.     

Antimony(III) complexes of bifunctional tridentateSchiff bases

Several newSchiff base derivatives of antimony(III) have been synthesized by the reaction of antimony(III) isopropoxide with theSchiff bases having the donor system, O–N–O. The reactions in 1:1 and 2:3 molar ratios [Sb(O-i-C₃H₇)₃: :Schiff base] have yielded Sb(O-i-C₃H₇) (SB) and Sb₂ (SB)₃ type of derivatives (whereSB represents the anion of theSchiff base andSBH₂=o-hydroxyacetophenone-2-hydroxy-1-propylimine, o-hydroxycetophenone-3-hydroxy-1-propylimine, salicylidene-2-hydroxyethylamine, salicylidene-2-hydroxy-1-propylamine and 2-hydroxy-1-naphthylidene-2-hydroxyethylamine) resp. In the resultingSchiff base derivatives, the central antimony atom appears to be tetracoordinated as indicated by their monomeric state determined ebullioscopically. The infrared spectra of the resulting complexes have been recorded and tentative structures indicated. The thermogravimetric analysis of antimony-monoisopropoxysalicylidene-2-hydroxy-1-propylamine has also been carried out.With 1 Figure     

Kinetic study on the acid hydrolysis of 1-aryl-2-phenyl-2-imidazolines. Basicity-rate of hydrolysis and structure relationships

Rate constants (k_obs) of hydrolysis in sulfuric acid-water mixtures at 100° for five 1-aryl-2-phenyl-2-imidazolines were determined. The influence of substituents in the phenyl group at N-1 upon the rate of hydrolysis was studied. When the imidazoline ring is considered to be a substituent of the benzene ring at N-1, a good correlation with the Hammett equation is found. In opposition to the behavior in alkaline media it was observed that reaction rates were enhanced by electron-withdrawing phenyl substituents and reduced by electron-releasing groups providing, similarly, a change in the mechanism of the reaction in the second case that was not observed in the first. Agreement with the Hammett equation allowed comparison between experimental and “calculated” rate constants which are fairly close. An equation relating the rate constants with the pK_a values of the imidazolinium ions is given.     

Determination of the rate of formation of the schiff bases of pyridoxal 5′-phosphate with polyallylamine

The formation constants of the Schiff bases of pyridoxal 5′-phosphate with polyallylamine were determined over the p^H range of the acetic-acetate buffer (3.9–5.5) at an ionic strength of 0.1 M and a temperature of 25°C. The results were consistent with the rapid formation of an ionized carbinolamine, T⁺, followed by deprotonation, in the rate-determining step, to a neutral carbinolamine, T°. Subsequent dehydration of T° in a rapid step yields the final Schiff base. The formation of T⁺ is a concerted process subject to specific acid catalysis. © 1995 John Wiley & Sons, Inc.     

Determination of absolute rate constants for free radical polymerization of ethyl α-fluoroacrylate and characterization of the polymer

The absolute rate constants for propagation (k_p) and for termination (k_t) of ethyl α-fluoroacrylate (EFA) were determined by means of the rotating sector method; k_p = 1120 and k_t = 4.8 × 10⁸ L/mol.s at 30°C. The monomer reactivity ratios for the copolymerizations with various monomers were obtained. By combining the k_p values for EFA from the present study and those for common monomers with the monomer reactivity ratios, the absolute values of the rate constants for cross-propagations were also evaluated. Reactivities of EFA and poly(EFA) radical, being compared with those of methyl acrylate and its polymer radical, were found to be little affected by the α-fluoro substitution. Poly(EFA) prepared with the radical initiator was characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Although the glass transition temperature obtained by DSC for poly(EFA) resembled that of poly(ethyl α-chloroacrylate), its TGA thermogram showed fast chain de polymerization to EFA that was distinct from complicated degradation of poly(ethyl α-chloroacrylate).     

Synthesis of analogs of pyridoxal 5′-phosphate and pyridoxamine 5′-phosphate

The following analogs of pyridoxamine 5′-phosphate (PAMP) have been synthesized by the direct phosphorylation of the corresponding amines: 2-nor-PAMP, 6-methyl-2-nor-PAMP, and 6-methyl-PAMP. A method for the synthesis of analogs of pyridoxal 5′-phosphate (PLP) by the phosphorylation of the Schiff's bases of the corresponding aldehydes with p-phenetidine and subsequent hydrolysis on a sulfonated resin has been worked out. 2-Nor-PLP, 6-methyl-2-nor-PLP, and 6-methyl-PLP have been obtained with yields of 53–73%. The spectral properties of the compounds obtained have been investigated.     

Spectrocoulometric study of the hydrolysis of the tris (1,10-phenanthroline)iron(III) complex ion

The spectrocoulometric technique reported earlier is applied to verify the mechanism and to evaluate the contributions k_Bi of the individual bases to the total rate constant k of the hydrolysis of the tris (1,10-phenanthroline) iron(III) complex, Fe (phen)³⁺₃. Both normal and “open-circuit” spectrocoulometric experiments are used. Partial rate constants for four bases in the acetate-buffered solutions are k_H2O=(3.4±1.2) × 10^?4s^?1 (k_H2O includes the H₂O concentration), k_OH=(1.20±0.06)×10⁷ mol^?1dm³s^?1, k_phen=(1.4±0.2) mol^?1dm³s^?1, k_Ac=(3.8±0.3)×10^?2 mol^?1dm³s^?1, at 25°C and ionic strength 0.5 mol dm^?3. The Fe(phen)³⁺³ hydrolysis, with (phen)₂ (H₂O) Fe-O-Fe (H₂O) (phen)⁴⁺² formation, is first order with respect to Fe (phen)³⁺³ and the bases present in the solution. The rate-determining step in the hydrolysis is the entry of a base to the coordinating sphere of the complex, as in the hydrolysis of the analogous 2,2′-bipyridyl complex.