Solvent effect on concertedness of the transition state in the hydrolysis of p-nitrophenyl acetate

[reaction: see text]. The reaction pathway for the alkaline hydrolysis of p-nitrophenyl acetate is investigated using density functional theory. It is shown that solvent plays an indispensable role in shaping the concerted transition state. The concertedness of this transition state is supported by good agreement with the measured kinetic isotope effects.     

Lipophilic sulfonic acids as the phase transfer catalysts in the hydrolysis of p-nitrophenyl acetate

Lipophilic dialkoxybenzenesulfonic acids effectively catalyze the acid-hydrolysis of p-nitrophenyl acetate in a benzene-H₂O two-phase system. Examination of the effects of varying the alkyl-chain length and catalyst concentration on the rate suggests the reaction involving some micellar aggregates of these catalysts in the organic phase.     

Chain-Cleavage and hydrolysis of activated polyethylene glycol derivatives: Evidence for competitive processes

Studies have been carried out with the tosylate of the monomethyl ether of polyethylene glycol (MeO–PEG–OTs) and with low molecular weight models to assess whether the neighboring oxygen at position 3 or 6 provides the driving force for hydrolytic cleavage of these activated derivatives. Our results reveal that MeO–PEG–OTs undergoes hydrolysis by competitive pathways. Water directly displaces the tosylate group to give the original PEG alcohol and the oxygen at position 6 nucleophilically displaces the tosylate group to give a cyclic oxonium ion as an intermediate. This intermediate can react by three pathways. First, it can lead to the production of the original PEG alcohol by attack of water on a ring carbon; second, dioxane and a lower molecular weight PEG alcohol is produced by water attack at the nonring carbon next to the charged oxygen; and third dioxane can be displaced by the oxygen atom at position 6 in the chain.     

Quantitative proteomics reveals the kinetics of trypsin-catalyzed protein digestion

Trypsin is the popular protease to digest proteins into peptides in shotgun proteomics, but few studies have attempted to systematically investigate the kinetics of trypsin-catalyzed protein digestion in proteome samples. In this study, we applied quantitative proteomics via triplex stable isotope dimethyl labeling to investigate the kinetics of trypsin-catalyzed cleavage. It was found that trypsin cleaves the C-terminal to lysine (K) and arginine (R) residues with higher rates for R. And the cleavage sites surrounded by neutral residues could be quickly cut, while those with neighboring charged residues (D/E/K/R) or proline residue (P) could be slowly cut. In a proteome sample, a huge number of proteins with different physical chemical properties coexists. If any type of protein could be preferably digested, then limited digestion could be applied to reduce the sample complexity. However, we found that protein abundance and other physicochemical properties, such as molecular weight (Mw), grand average of hydropathicity (GRAVY), aliphatic index, and isoelectric point (pI) have no notable correlation with digestion priority of proteins. Graphical Abstract

Sequence logos of four cleavage site types with different kinetics (very fast, fast, slow, and very slow sites)     

The dielectric constant effect upon the acid hydrolysis of ethyl acetate

The dielectric constant effect upon the rate of acid hydroly.sis of ethyl acetate is presented from the standpoint of electrostatics and the results compared to like effects upon the acid hydrolysis of methyl propionate.     

The kinetics of diethylene glycol formation in the preparation of polyethylene terephthalate

For revealing diethylene glycol (DEG) formation in poly(ethylene terephthalate) (PET) synthesis, this research focused on finding the stage most critical for DEG formation. It is found that the esterification stage was the most critical stage for DEG formation during production of PET through the direct esterification process. In addition, the kinetics of the formation of DEG (ether bond), which is mainly produced from hydroxyl end groups of ethylene glycol (EG) and bis-hydroxyethyl terephthalate (BHET) oligomer, was investigated. The results show that the reactivity of BHET-OH functional group is greater than that of EG-OH functional group in the reaction to produce ether bonds. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3073–3080, 1998     

Identification and quantification of polyethylene glycol types in polyethylene glycol methyl ether and polyethylene glycol vinyl ether

Quantitative determination of polyethylene glycol (PEG) impurities in two monofunctional polyglycol types, PEG methyl ether (M-PEG) and PEG vinyl ether (V-PEG), has been carried out by reversed-phase liquid chromatography with evaporative light scattering detection (ELSD). In addition to optimizing the resolution between PEG and monofunctional PEG peaks, the major focus has been to determine the molecular weights of PEG impurities in M-PEG and V-PEG of diverse molecular weights. The latter is achieved by examining liquid chromatography–mass spectrometry (LC–MS) mass spectra of both monofunctional PEG and PEG in several cases, and matching peak retention times with those of available PEG standards for all M-PEG and V-PEG sample types. This information is helpful in selecting the appropriate PEG standard to determine PEG content in each sample type. ELSD response factors for various PEG standards have also been compared. It has been found that PEG standards with molecular weights from 1000 Da to 8000 Da show responses that are within 10% of each other. However, a low molecular weight PEG such as PEG 400, provides approximately 30% less response compared to its higher molecular weight counterparts.     

Radiation chemistry of polyethylene. XIV. Allyl radical decay kinetics in different types of polyethylene

The decay kinetics of the chain allyl free radical has been studied in the following morphological forms of polyethylene (PE): Marlex bulk film, hydrogenated PE, and extended-chain PE. Coupled with previous work on single-crystalline PE it can be seen that the decay rate is greater the more amorphous the sample. In the Marlex bulk film and hydrogenated PE the decay can be interpreted in terms of a simultaneous fast and slow decay process by means of our Q-function equation, but with rising temperature the decay approximates a single rate process. With extended-chain PE the allyl decay rate does not become appreciable until the melting range is approached. The fraction of allyl radicals decaying by the slow process is 2 to 10 times greater than the fraction of fast decaying radicals. The ratio of the fast decay rate constant to that of the slow rate constant is greater for the bulk Marlex film than for the hydrogenated PE. All ratios decrease with rising temperature. For times up to about 150 min the allyl decay in the extended-chain PE accurately follows a single second-order decay law with a time-independent diffusion controlled reaction rate constant.     

The influence of detergentless microemulsion on the kinetics of hydrolysis of di(p-nitrophenyl) methylphosphonate catalyzed by amines

The influence of the detergentless water/oil microemulsion formed in then-hexane-water-2-propanol system on the kinetics of the hydrolysis of di(p-nitrophenyl) methylphosphonate under the conditions of general base catalysis by primary, secondary, and tertiary aliphatic amines was investigate. The leveling of the catalytic rate constants and basicities of the amines was found for this reaction in a microemulsion. The catalytic rate constants for the amines containing hydrophilic groups increase in microemulsions as compared to those in an aqueous medium, while the catalytic constants for the amines containing hydrophobic groups decrease. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1339–1342, July, 1998.     

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.     

Eco-friendly polyethylene glycol promoted Michael addition reactions of α,β-unsaturated carbonyl compounds

Intra- and inter-nucleophilic addition reactions of different α,β-unsaturated carbonyl compounds were found to be highly effective without any additives in PEG-400 as a recyclable reaction medium under neutral conditions.     

Catalysis of the hydrolysis of di(p-nitrophenyl) methylphosphonate by polyamines

Conclusions The kinetics of the hydrolysis of di(p-nitrophenyl)methylphosphonate in aqueous solutions of triethylenetetramine and tetraethylenepentamine have been studied at various pH values. The catalytic rate constants for the neutral and monoprotonated forms of the polyamines are represented by the Brønsted equation as with the primary aliphatic amines and for the diprotonated forms they are represented together with the secondary aliphatic amines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 756–759, April, 1987.     

Near-IR spectra of polyethylene,polyethylene glycol,and polyvinylethyl ether

The near-IR spectrum of polyethylene, polyethylene glycol, and polyvinylethyl ether have been measured. The CH and OH stretch and bend absorptions have been assigned using local mode theory. The CH stretch anharmonicities are about 57 cm⁻¹, typical of CH anharmonicities in molecular samples. The CH bend anharmonicities are 7, 10, and 11 cm⁻¹, respectively, for polyethylene, polyethylene glycol, and polyvinylethyl ether. The CH bend absorption intensities decrease by about a factor of 4 allowing these bending transitions to be observed up to the fourth overtone transition. © 1996 John Wiley & Sons, Inc.     

Kinetic anomalies in chymotryptic hydrolyses of p-nitrophenyl acetate and N-benzoyl-L-alanine methyl ester

Kinetic and thermodynamic parameters were evaluated for the acylation and the deacylation steps in the hydrolysis of p-nitrophenyl acetate by alpha-chymotrypsin at pH 7.8 and at temperatures between 15 and 35 degrees C by the use of stopped-flow and ordinary ultraviolet spectrophotometers. In contrast to the temperature dependencies of k2 and Ks reported in the literature (P.A. Adams and E.R. Swart, Biochem. J., 161, 83 (1977], no kinetic anomaly was observed in either of the steps, but reasonable straight lines were obtained in both Arrhenius and van't Hoff plots. On the other hand, in the chymotryptic hydrolysis of N-benzoyl-L-alanine methyl ester a sharp kinetic anomaly was found. The discrepancy in the case of p-nitrophenyl acetate is discussed in connection with a possible conformational change of the enzyme, an alteration of the rate-limiting step or differences in the experimental procedures. The cause of the anomaly observed in the case of N-benzoyl-L-alanine methyl ester is also discussed in detail.     

Melting behavior of pure polyethylene glycol 6000 and polyethylene glycol 6000 in solid dispersions containing diazepam or temazepam: a DSC study

The purpose of the present study was to investigate the melting behavior of polyethylene glycol 6000 (PEG 6000) as such as well as in solid dispersions containing diazepam or temazepam, prepared by solvent and fusion methods, using differential scanning calorimetry (DSC). It was shown that the melting behavior of pure PEG 6000 is influenced by the crystallization procedure applied. Fusion at 80°C followed by cooling always yielded three different crystal modifications. The rate of cooling (under controlled conditions) was found to have a significant influence on the relative distribution of the three modifications: the lower the cooling rate, the higher the relative amount of the extended modification. Crystallization from organic solution yielded mainly the once folded form. The presence of diazepam and temazepam influenced the relative amount of the different PEG 6000 modifications. Both drugs decreased the formation of the more stable modification, while the formation of the twice folded form was induced. However, in the case of temazepam the contribution of the extended form at higher drug levels increased in dispersions obtained from organic solutions.     

Immobilized stationary phases on the basis of polyethylene glycol

The preparation of the immobilized stationary phase based on Carbowax 20 M is described. Bifunctional isocyanates were used for cross-linking. The properties of the prepared stationary phase were studied.     

Polyesters of phosphoric acid: Synthesis and kinetics of hydrolysis

Three major methods have been elaborated in our laboratory for preparation of polymers with poly(alkylene phosphates) backbones: ring-opening polymerization, poly-condensation and transesterification, and polyaddition. The second method is based on the reaction of the commercially available compounds, namely dialkyl (or diaryl)-H-phosphonates and glycols. Reaction of the aliphatic H-phosphonates with aliphatic glycols is a reversible process, whereas polycondensation of diphenyl H-phosphonates with aliphatic and cycloaliphatic diols is practically irreversible. This latter method has recently been developed and is described in this paper. Poly H-phosphonates with M̄_n up to 40–10³ were prepared. Polymers are easily oxidized and quantitatively converted into the relatively stable poly(alkylene phosphates). Some physical properties of these polymers and kinetics of their hydrolysis is discussed.