Sergio Alunni

Mechanisms of inhibition of phenylalanine ammonia-lyase by phenol inhibitors and phenol/glycine synergistic inhibitors

Phenylalanine ammonia-lyase (PAL) catalyzes the beta-elimination of ammonia from L-phenylalanine to trans-cinnamic acid. A study of inhibition of PAL by phenol, ortho-cresol, and meta-cresol gave mixed inhibition; para-cresol is not an inhibitor. The calculated values of K(i) and alphaK(i) are phenol, K(i)=2.1+/-0.5 mM and alphaK(i)=3.45+/-0.95 mM; ortho-cresol, K(i)=0.8+/-0.2 mM and alphaK(i)=3.4+/-0.2 mM; meta-cresol, K(i)=2.85+/-0.15 mM and alphaK(i)=18.5+/-1.5 mM. The synergistic inhibition of the same inhibitors with glycine showed a lack of inhibition with the para-cresol/glycine pair, while mixed inhibition was observed with the ortho-cresol/glycine pair (K(i)=0.038+/-0.008 mM, alphaK(i)=0.13+/-0.04 mM) and phenol/glycine pair (K(i)=0.014+/-0.003 mM, alphaK(i)=0.058+/-0.01 M). The meta-cresol/glycine pair gave competitive inhibition (K(i)=0.36+/-0.076 mM). The strong synergistic inhibition observed implies that the inhibitors bind at the active site: in fact, the inhibitors used imitate the structure of the substrate. The order of synergistic inhibition is the same for the sites related to K(i) and alphaK(i). These results are in agreement with the inhibitors entering two active sites located in two different subunits.

A study on the inhibition of adenosine deaminase

Adenosine deaminase (ADA, EC 3.5.4.4) catalyses the irreversible deamination of adenosine and 2′-deoxyadenosine to inosine and 2′-deoxyinosine, respectively. In this study the inhibition of ADA from bovine spleen by several molecules with structure related to that of the substrate or product has been quantified. The inhibitors adenine, purine, inosine, 2-aminopurine, 4-aminopyrimidine, 4-aminopyridine, 4-hydroxypyridine and phenylhydrazine are shown to be competitive inhibitors with KI (mM) values of 0.17, 1.1, 0.35, 0.33, 1.3, 1.8, 1.4 and 0.25, respectively. Synergistic inhibition by various combinations of molecules that imitate the structure of the substrate has never been observed. Some general conclusions are: i) the enzyme ADA from bovine spleen we have used is appropriate for kinetic studies of inhibition and mechanistic studies; it can be a reference catalytic system for the homogeneous comparison of various inhibitors; ii) this enzyme presents very rigid requirements for binding the substrate: variations in the structure of adenosine imply the loss of important interactions.

Use of the Hammett equation in substituted thiophenes

The application of multivariate statistics to linear free-energy relationships in the thiophene series increases the understanding of the Hammett equation. Principal components analysis shows that α-substituted thiophenes require the σ constants used for para-substituted benzene derivatives, owing to a strict proportionality of the substituent effects in the heterocyclic and homocyclic systems. Partial least-squares analysis indicates that two independent effects are linearly transferred from benzenes to thiophenes.

Reactivity and Mechanism of 1-X-2-(O-Nitrophenyl)Ethanes in Base Induced β-Elimination Reactions With Formation of O-Nitrostyrene

Studies of acid-base catalysis in acetohydroxamic/acetohydroxamate buffers, H/D exchange and solvent isotope effect show that the mechanism of the β-elimination with the formation of o-nitrostyrene from the N-[2-(o-Nitrophenyl)ethyl] quinuclidinium ion (1) is E 1 cb, involving an intermediate carbanion formed with a high degree of reversibility. The reactivity of 1-X-2-o-nitrophenylethanes with various leaving groups such Br, Cl, F, quinuclidine, and tetrahydrothiophene in OH - /H 2 O is reported. At 25 °C in OH - /H 2 O, μ = 1 M KCl, the second order rate constant for the elimination reaction from (1) is k OH = 4.9. 10 -5 M -1 s -1 and it is 15.7 times lower than that of the corresponding p-nitro substituted analogue [1] (k OH = 0.77 .10 -3 M -1 s -1 ).

trans: cis Olefin ratios in the elimination reactions of 1-benzylethyl halides and toluene-p-sulphonate in different base–solvent systems

trans:cis Olefin ratios have been determined for the elimination reactions of 1-benzylethyl halides and tosylate PhCH2CHXMe (X = Br, I, or OTs) in different base–solvent systems. The trans : cis ratio is substantially the same for X = Br and I, whereas a lower value is observed for X = OTs. The basicity of the nuclecphile does not appear significantly to affect the trans : cis ratio. However, an increase in this ratio is observed in going from a protic to a dipolar aprotic solvent and from EtONa–EtOH to ButOK–ButOH. Ion association increases the trans : cis ratio when X = I and Br. whereas it has practically no effect on the geometrical orientation when X = OTs. The present results are discussed by comparison with those obtained in the corresponding reactions of non-activated substrates.

Deuterium kinetic isotope effects in β-elimination from phenethyl bromide promoted by para-substituted sodium phenoxides in dimethylformamide

Deuterium kinetic isotope effects have been measured for elimination from phenethyl bromide promoted by para-substituted sodium phenoxides in dimethylformamide. Despite the large basicity range considered variations in the magnitude (in the range 7·6–9·0) of the isotope effects were practically within experimental error. Some evidence for proton tunnelling was obtained and this has made difficult a straightforward interpretation of the k2H/k2D values in terms of transition state geometry.

OH−-induced β-elimination reactions with 1-(2-Xethyl)-4-nitrobenzene substrates in solvent mixture H2O/CH3CN and H2O/DMSO: Reactivity and mechanism

The behaviour of 1-(2-bromoethyl) 4-nitrobenzene (1), N,N,N-triethyl-2-(4-nitrophenyl)ethanaminium bromide (2) and N,N-diethyl-N-[2-(4-nitrophenyl)ethyl]octan-1-aminium bromide (3) in the OH−-induced elimination reactions with formation of 1-nitro-4-vinylbenzene in mixtures of DMSO/H2O or CH3CN/H2O has been investigated. With all three substrates an increase in dipolar aprotic solvent content implies a limited increase of the second-order rate constant kOH up to ≅605, and then an exponential increase is observed. The variation of activation parameters ΔH# and dGS#, measured in DMSO/H2O mixtures, is parallel for 1 and 2. This similar behaviour of 1 and 2 with respect to variation in solvent composition is evidence that it is not possible to use this technique of solvent effect for the mechanistic diagnosis of elimination reactions.

Reactivity of ions and ion pairs in the nucleophilic substitution reaction on methyl p-nitrobenzenesulfonate

Nucleophilic substitution on methyl p-nitrobenzenesulfonate has been studied with a series of chloride salts with different structures and solvations: Bu4NCl, PPNCl or KCl complexed by 18-crown-6 or Kryptofix 2,2,2 as well as with a bromide salt (PPNBr) for comparison purposes. The rate constants and the activation parameters are in accordance with an SN2 mechanism. The treatment of the data, following the Acree equation, shows that the process takes place by way of two reaction paths: the first, due to the chloride ion, has the same rate for all the salts, while the second slower path, involving the ion pair, has a rate related to the dissociation constant of the salts.

Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 19. Chemometric investigation of the simultaneous dependence of SN2 rates on alkyl group structure and leaving group nucleofugacity

The preparation and kinetics of nucleophilic displacement are reported for four β-branched primary alkyl groups attached to neutral nucleofuges. Principal-component analysis on a set of 10 nucleophilic substitution reactions with neutral and anionic nucleofuges finds that the first principal component accounts for 70% of the variance and confirms that the tri- and penta-cyclic nucleofuges are similar to chloride ion in leaving-group activity. Partial least-squares analysis shows that the nucleophilic displacement rates for the tricyclic derivatives (2) depends on substituent shape (as measured by the Verloop parameter) rather than on size as measured by ES,. The σ* and polarizability terms are also important.

Dehalogenation reactions of vicinal dihalides. Part V. Kinetic study of the reactions of 1,2-dihalogeno-1,2-diphenylethanes with triphenylphosphine

Triphenylphosphine-promoted dehalogenation reactions of meso-1,2-dibromo-1,2-diphenylethane and some para-substituted derivatives have been studied in dimethylformamide. Substituent effects on the reaction rate are in agreement with a concerted anti-elimination mechanism and do not support a stepwise mechanism involving bridged intermediates. The effect of solvent has also been investigated and the nucleophilicity of triphenylphosphine towards bromine is briefly discussed.