Alternative explanation for a saturation phenomenon exhibited by poly-4(5)-vinylimidazole and an anionic ester

The hydrolysis of a negatively charged ester, sodium 3-nitro-4-acetoxybenzenesulfonate (NABS), catalyzed by poly-4(5)-vinylimidazole (PVIm) was reinvestigated under the conditions in which the substrate concentration was in excess of the catalyst concentration. The reaction was carried out under the same reaction conditions as previous work (28.5 vol-% ethanol–water, pH 7.1, μ = 0.02, 26°C). Careful studies on the initial rate with a stopped-flow spectrophotometer revealed that the initial rate at high NABS concentration did not level off (saturate) as much as expected from the previous results. This difference was ascribed to two factors. One was that in the previous study, we treated up to 5% hydrolysis, but in the present work up to 1% in order to determine the initial rate. The other was the relatively slow deacylation reaction of the intermediate, partially acetylated PVIm, which resulted in less efficiency of the catalysis. The slow deacylation was confirmed by the kinetics of the hydrolysis of NABS and also independently by measuring the deacylation rate of the acetylated (or acylated) PVIm. The deacylation was an intramolecular catalysis and the rate constant was about 0.1 min^—1 in 28.5 vol-% ethanol–water at pH 7.1 at 26°C. The whole process of the hydrolysis could be explained by a two-step pathway: an initial burst acylation followed by the slow deacylation. The electrostatic interaction between the cationically charged imidazole groups and NABS seemed not strong enough to form a stable complex. Thus, potassium p-toluenesulfonate showed no effect as a competing agent (inhibitor) against NABS.     

4(5)-vinylimidazole by dehydrobromination of 1-triphenylmethyl-4-(2-bromoethyl)imidazole

1-Triphenylmethyl-4-(2-bromoethyl)imidazole undergoes elimination upon basic treatment providing an easy approach to 4(5)-vinylimidazole whereas the N-unsubstituted analogue leads only to substitution products.     

Esterolytic activity of imidazole-containing polymers. Hydrophobic influences in copoly[1-alkyl-4- or 5-vinylimidazole/4(5)-vinylimidazole]-catalyzed hydrolysis of 3-nitro-4-acyloxybenzoic acids

Water-soluble copolymers containing imidazole and N-alkylated imidazole pendant groups have been synthesized in order to investigate the hydrophobic interactions between polymeric catalysts and long alkyl chain ester substrates. Copoly[1-methyl-4-vinyl-imidazole/4(5)-vinylimidazole],copoly[1-methyl-5-vinylimidazole/4(5)-vinylimidazole], copoly[1-ethyl-5-vinylimidazole/4-(5)-vinyl-imidazole] and copoly[1-propyl-5-vinylimidazole/4(5)-vinylimidazole] were synthesized and their catalytic activity toward 3-nitro-4-acyloxybenzoic acid substrates (S_n^?) was determined in 28.5% ethanol–water and in water and compared with that of the mixtures of homopolymers. Hydrophobic interactions were important for rate enhancement of the hydrolysis of long-chain ester substrates compared to that of short-chain ester substrates. The copolymers catalyzed the hydrolysis of 3-nitro-4-dodecanoyloxy-benzoic acid (S₁₂^?) about two times faster than the mixtures at pH 7.11 in 28.5% ethanol–water. The hydrolysis of S₁₂^? by the copolymers was about five times faster in water than 28.5% ethanol–water.     

The esterolytic activity of poly(1-methyl-4- and -5-vinylimidazole) in water.

The water solubility of poly(1-Me-5-VIm) has made it possible to achieve phenomenal rate enhancements and to gain even greater insight into the mechanism of catalysis by polymeric imidazoles. The poly(1-Me-5-VIm)-catalyzed hydrolysis of S12- exhibited saturation in excess catalyst nad in excess substrate. Inhibition of the poly(1-Me-5-Vim) catalyzed hydrolysis of Sn- type substrates by analog inhibitors was also observed. The saturation apparently did not follow a simple Michaelis-Menten mechanism; however, the results could be rationalized by analogy to certain enzymatic systems. Multisite enzymes have long been known to display kinetic patterns different from that exhibited by enzymes with only one active site, i.e. such phenomena as sigmoidal rate vs [S] plots. These phenomena may arise entirely as a result of the multisite nature of the enzyme. Consequently, a synthetic macromolecular catalyst with multiple sites might also be expected to display such characteristics. The poly(1-Me-5-VIm)-catalyzed hydrolysis of S12- is apparently the first synthetic system in which such phenomena have been observed. The intermediacy of an apolar polymer substrate complex for the poly(1-Me-5-VIm)-catalyzed hydrolysis of S12- in water was given support by studies of the effect of temperature on the rate of hydrolysis. Activation parameters were determined for catalysis by 1,5-DMIm and by poly(1-Me-5-VIm). These results showed that the rate enhancement exhibited by the polymer was due entirely to a favorable entropy term.     

Esterolytic activity of imidazole-containing polymers. Synthesis and characterization of copoly[1-alkyl-4- or 5-vinylimidazole/4(5)-vinylimidazole] and its catalytic activity in the hydrolysis of p-nitrophenyl acetate

The water-soluble monomers, 1-methyl-4-vinylimidazole, 1-methyl-5-vinylimidazole, 1-ethyl-5-vinylimidazole, and 1-propyl-5-vinylimidazole have been synthesized, polymerized, and copolymerized with 4(5)-vinylimidazole. The copolymers were characterized by ¹⁴C-labeling, NMR, pK_a determination and viscosity measurements. The monomer reactivity ratios determined by ¹⁴C counting are r₁ = 1.04; r₂ = 0.94 [M₁ = 4(5)-vinylimidazole, M₂ = 1-methyl-4-vinylimidazole] and r₁ = 1.01; r₂ = 0.86 [M₁ = 4(5)-vinylimidazole, M₂ = 1-methyl-5-vinylimidazole]. The esterolytic activity of the copolymers for the hydrolysis of p-nitrophenyl acetate (PNPA) at pH 7–8 in 28.5% ethanol–water was higher than that of the mixtures of homopolymers. At pH 5–6 the esterolytic activities of the copolymers and the mixtures were similar. The most efficient esterolytic activity for PNPA hydrolysis at pH 7.11 in 28.5% ethanol–water occurred for copolymers containing 75 mole % 4(5)-vinylimidazole and for copolymers containing 1-methyl-4-vinylimidazole rather than 1-methyl-5-vinylimidazole.     

EPR studies of copper(II) complexes with poly-2-methyl-5-vinyl-pyridine and poly-2-vinylpyridine

Cu(II) complexes of poly-2-vinylpyridine (P2VP) and poly-2-methyl-5-vinylpyridine (P2M5VP), partially quaternized by dimethylsulphate, and of the analogues (2-methyl-5-ethylpyridine, 2-ethylpyridine) were studied by EPR spectroscopy in a mixture of methanol and water. Peculiarities of the complex formation reaction were observed for the polymers compared to the analogues. At ratios of [Py]:[Cu²⁺] > 40, the predominant formation of tetrapyridinate-Cu(II) species [(CuL₄)]²⁺ was found for P2M5VP. However, differences were found between the parameters of EPR-spectra for the [CuL₄]²⁺ in the polymer from that of the [Cu(2M5EPy)₄]²⁺. It was suggested that, in the polymer, [CuL₄]²⁺ complexes with structure intermediate between square planar and tetrahedral are formed. Moreover, the maximum value of the pyridine fraction forming [CuL₄]²⁺ in P2M5VP was found to be about 10% and it is appreciably less the value of the fraction in P4VP (about 40%). For P2VP at [Py]:[Cu²⁺] > 40, an insignificant amount of [CuL₁]²⁺ and [CuL₂]²⁺ are formed in the solution. It follows that the main chain position relative to the ligand nitrogen atom in these polyvinylpyridines affects profoundly the complexation between the macromolecules and Cu(II) ions. The steric hindrances due to the chain are likely to change the [CuL₄]²⁺ structure and to prevent complex formation for P2VP.     

Selective fluorescence determination of chromium(VI) with poly-4-vinylaninline nanoparticles

The strong fluorescence poly-4-vinylaniline nanoparticles (PVN) has been prepared under ultrasonic radiation. Based on the fluorescence quenching of PVN by Cr(VI), a method for the selective determination of Cr(VI), without separation of Cr(III) in water, was developed. The reaction conditions between Cr(VI) and PVN were investigated in detail. The assay is characterized by short reaction time (<1 min even at 0 degrees C temperature), very few interference stable fluorescence signals (at least 2.5 h), simple instrument (common spectrofluorometer) and simplicity (a one-step assay). Under optimal experimental conditions, a limit of detection of 0.02 microg ml(-1) was achieved. The calibration curve was linear over the concentration range 0.1-13.0 microg ml(-1) with a correlation coefficient of 0.9984. The proposed method has been applied to the selective quantification of Cr(VI) in synthetic samples and waste-water samples with the satisfactory results.     

Stereoregularity of poly(2-vinylpyridine) and poly-(4-vinylpyridine)

In order to determine the stereoregularity of poly(4-vinylpyridine), 4-vinylpyridine-β,β-d₂ was synthesized from 4-acetylpyridine. The ¹H-NMR spectra of the deuterated and nondeuterated polymers were measured and analyzed. From the ¹H-NMR spectra of poly(4-vinylpyridine-β,β-d₂), triad tacticity can be obtained, while the ¹H-NMR spectra of nondeuterated poly(4-vinylpyridine) give the fraction of isotactic triad. The ¹³C-NMR spectra of poly(4-vinylpyridine) were also observed, and the spectra of C₄ carbon of polymers were assigned by the pentad tacticities. The fraction of isotactic triad of poly(2-vinylpyridine) and poly(4-vinylpyridine) obtained under various polymerization conditions were determined. The radical polymerization and anionic polymerizations with phenylmagnesium bromide and n-butyllithium as catalysts of 4-vinylpyridine gave atactic polymers.     

Palladium(II) Complexes of 1-vinylimidazole

Two new co-ordination compounds of Pd^II with 1-vinylimidazole of the formulae [PdL₄]Cl₂·3H₂O and trans-[PdL₂Cl₂], where L is a 1-vinylimidazole molecule, have been obtained. The compounds were characterised by spectroscopic, molar conductivity, thermogravimetric and magnetochemical measurements. Single crystal X-ray structure analyses of the complexes were also carried out. The compounds are diamagnetic with square-planar coordinatination around the palladium(II) ions. Other physico-chemical properties of the both complexes are compatible with their structures.     

Poly(1-vinylimidazole) Prospects in Gene Delivery

Polymeric amines are being studied intensively as components of systems for gene delivery in genetic engineering and gene therapy of genetic disorders, including cancer. Despite remarkable achievements in the field, polymeric amines, such as polyethyleneimine, show some disadvantages. Strong interaction between the amine-containing polymer and nucleic acid hampers the release of nucleic acid in the cell cytoplasm. Amine groups can interact with the cell membrane which results in cell death. These limitations of polymeric amines stimulated an investigation of new structures for gene delivery. Imidazole-containing polymers have attracted attention as lesser basic substances, while they are able to interact with polymeric acids. Further development of imidazole-based gene delivery agents requires knowledge about some fundamental aspects of interaction between nucleic acids, and polymeric imidazoles. In this work, we studied the complexation of poly(1-vinylimidazole) and oligomeric DNA. We found that the number of active sites capable of binding with negatively charged phosphate groups is comparable with the number of protonated imidazole units in the case of high molecular weight polymer. The increase in polymer charge by 1-bromopropane quaternizating 1%?5% imidazole units or by decreasing the pH to 6.5?7 considerably increased the ability of poly(1-vinylimidazole) to interact with oligonucleotides. The pH sensitivity of this interaction is interesting for cancer gene therapy because the tumours have a lowered intercellular pH (stable oligonucleotide complex) and a higher extracellular pH which can lead to complex dissociation. Minimal critical length for complexation of quaternized poly(1-vinylimidazole) and DNA is below eight units which corresponds to polymers with amine groups. Fluorescence-tagged poly(1-vinylimidazole) samples were obtained and their potential for monitoring the polymer and polymer-oligonucleotide complex internalization into living cells was demonstrated.     

Photophysical properties of poly-2-phenyl-5-(p-vinyl)-phenyloxazole

2-Phenyl-5-(p-vinyl)phenyloxazole (POS) has been synthesized. Copolymers of POS with styrene and the homopolymer poly–POS have been prepared. The polymers have been characterized through measurements of fluorescence decay times (using synchrotron radiation for excitation) and examination of excimer formation in the homopolymer. Fluorescence decay times of 0.6(±0.4) nsec and 9 (±0.2) nsec have been recorded at 298°K in toluene solution for monomer and excimer, respectively. Poly-POS exhibits excimer formation characterized by an activation energy of 5.9(±0.5) kJ/mole and a binding energy of 17.0(±1.0) kJ/mole.     

Kinetics and mechanism of water substitution in the low-spin Fe(II) complex of 4-octasulfophenylpyrazinoporphyrazine

The substitution reaction of the axial-coordinated water by pyridine, pyrazine and 4-CN-pyridine in the low-spin Fe(II) complex of octasulfophenyltetrapyrazinoporphyrazine was studied. Kinetic and thermodynamic parameters for the different reaction steps of the process were determined. On the basis of NMR data and spectrophotometric titrations, a pronounced non-equivalence of the two coordinated N-donor ligands was observed. The substitution of water by pyridine and 4-CN-pyridine is shown to include the formation of a precursor outer-sphere complex, whereas substitution by pyrazine follows a limiting dissociative mechanism.     

Synthesis of analogs of 5(4)-aminoimidazole-4(5)-carboxamide and purines. 9. peculiarities of the reaction of 5(4)-aminoimidazole-4(5)-carboxhydrazide with nitrous acid and of 5-diazoimidazole-4-carboxazide with amines

The reaction of 5(4)-aminoimidazole-4(5)-carboxhydrazide with nitrous acid was investigated. A mixture of four compounds, viz., 5-diazoimidazole-4-carboxazide, 5-diazoimidazole-4-carboxylic acid, 5(4)-aminoimidazole-4(5)-carboxazide, and 2-azahypoxanthine, is formed under all of the investigated conditions, 5(4)-Azidoimidazole-4(5)-carboxamide derivatives were obtained in the reaction of diazoimidazole-carboxazide with various amines in protic and aprotic solvents. 5-N-(Piperidyl)-azoimidazole-4-carboxazide was isolated only in the reaction with piperidine in an aqueous medium.See [1] for Communication 8.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1536–1540, November, 1980.     

Complexes of poly-5-vinyltetrazoles with weak polybases

Formation of interpolymer complexes of poly(5-vinyltetrazole) and poly(5-izopropenyltetrazole) with poly(acrylamide), poly(ethylene oxide), poly(1-vinylpyrrolidone), and poly(1-vinyl-1,2,4-triazole) was investigated by potentiometry and viscometry. The reduced activity of tetrazol-containing polyacids in reaction with poly(1-vinylpyrrolidone) from polyacrylic polyacids was explained by steric hindrances at the expense of bulky tetrazolic rings. There is a strong system of hydrogen bonds in the poly(5-vinyltetrazole) solution that leads to decreasing of its activity in the complexation in comparison with poly(5-izopropenyltetrazole). Hydrophobic interactions between methyl groups of poly(5-izopropenyltetrazole) lead to additional stabilization of the complexes at low degrees of ionization and to fast destruction of them at ionization degrees corresponding to destruction of the compact conformation of poly(5-izopropenyltetrazole) (α = 0.2–0.4). Interaction of poly(5-vinyltetrazole) with poly(1-vinyl-1,2,4-triazole) at elevated pH leads to an interpolymer complex stabilized by donor–acceptor interactions between the π-systems of tetrazolate anions and triazole rings. © 1996 John Wiley & Sons, Inc.     

Effect of copolymers of 4(5)-vinylimidazole and quaternary imidazolium salts on the hydrolysis rates of charged and neutral esters. II

The effects of copolymeric catalysts containing hydrophobic binding sites (quaternary imidazolium salts) and catalytically active nucleophilic sites (imidazole) on the hydrolysis of long-chain anionic esters were investigated. The quaternary imidazolium salt groups contained hydrocarbon chains of varying length. Their solubility relationships and viscosity measurements followed a trend governed by the hydrocarbon chain length of the quaternary imidazolium salt. These investigations revealed that increasing the hydrocarbon chain length of the quaternary imidazole groups along the copolymer chain or the chain length of the ester substrate resulted in an increase of the catalytic rate of ester hydrolysis. Increasing the ethanol content decreased the catalytic activity and greatly reduced the apolar binding between the catalyst and substrate. For the cases of low ethanol content and high chain length of the substrate, the ester hydrolysis reactions in the presence of the copolymeric catalysts preceeded by a Michaelis-Menten kinetic scheme.     

ESR study of radiation damage in TPX polymer (poly-4-methylpentene-1)

Electron spin resonance spectroscopy has been used to study the effects of 15 MeV electrons, x-rays, and ultraviolet radiation on poly-4-methylpentene-1 (TPX) both at 77°K and at room temperature. At least seven identifiably different paramagnetic species are observable in unstabilized oxygen-free TPX after irradiation, and additional species exist in the stabilized grades. The species which predominates under most conditions is interpreted as being due to the loss of hydrogen from a main-chain tertiary carbon atom; interpretations of most of the other species are also given. Oxygen is found to diffuse rapidly into the polymer and to react with the free radicals to form peroxy species. In the absence of oxygen the radiation damage is expected to lead ultimately to crosslinking or double-bond formation, or with oxygen to degradation. The general nature of the free radicals produced by electron or x-ray irradiation is the same, but there are significant differences for ultraviolet irradiation. The observed spectra for irradiated TPX and their interpretations are in good agreement with the spectra and later interpretations for irradiated polypropylene, but are in less satisfactory agreement with the published papers on polybutene-1 and poly-3-methylbutene-1.