Stereoelectronic properties and reactivity of the tetrahedral intermediate in amide hydrolysis. Nonempirical study of aminodihydroxymethane and relation to enzyme catalysis

Effect of cationic micelles on the decomposition of alpha-aminophenyl cephalosporins

The intramolecular rates of degradation of alpha-aminophenyl cephalosporins were determined with and without hexadecyltrimethylammonium bromide (CTAB). Micellar-derived spectral shifts were used to measure the bind of the ionic forms as well as to determine the effect of CTAB on the apparent dissociation constant of the antibiotics. The rate of the degradation of cephalexin (Cp), cefadroxil (Cf), and cephradine (Cph), increased with surfactant concentration reaching a plateau at high surfactant concentrations. In the plateau region, the rate constant was salt sensitive decreasing with NaBr concentrations. These effects were quantitatively analyzed within the framework of the pseudo-phase model with explicit considerations of ion exchange. All the experimental results were fitted to this model. The intramolecular degradation of Cf, Cp and Cph was catalyzed by 96-, 59-, and 29-fold, respectively. A working hypothesis to rationalize these effects was suggested. The obtained results demonstrate that the quantitative analysis can be used to assess, predict and control the effects of surfactants on the drug stability.

Is there stereoelectronic control in hydrolysis of cyclic guanidinium ions?

To assess stereoelectronic effects in the cleavage of tetrahedral intermediates, a series of five-, six-, and seven-membered cyclic guanidinium salts was synthesized. If stereoelectronic control by antiperiplanar lone pairs is operative, these are expected to hydrolyze with endocyclic C-N cleavage to acyclic ureas. However, hydrolysis in basic media produces mixtures of cyclic and acyclic products, as determined by 1H NMR analysis. The results show that in the six-membered ring antiperiplanar lone pairs provide a weak acceleration of the breakdown of the tetrahedral intermediate, but in five- and seven-membered rings there is no evidence for such acceleration, which instead can be provided by syn lone pairs.

Stereoelectronic control in addition of nucleophiles to an amidinium ion

Nucleophilic addition to 1,3-dimethyl-5-phenyl-1,4,5,6-tetrahydropyrimidinium ion provides a quantitative measure of stereoelectronic control. This amidinium ion presents the nucleophile with two distinct paths for attack. Axial attack is favored by interaction between the orbital of the developing bond and antiperiplanar lone pairs on the nitrogens. Reaction of the amidinium salt with diverse nucleophiles (D(-), H3C(-), n-Bu(-), PhCH2(-), allyl(-), Ph(-), C5F6(-), CH2=CH(-), HC triple bond C(-), PhC triple bond C(-), CN(-)) produces mixtures of cis and trans stereoisomers. Both kinetic and thermodynamic product distributions were measured by 1H NMR, before and after acid-catalyzed equilibration. The values provide insight into the roles of steric and stereoelectronic forces at the transition state and in products. Stereoelectronic effects on reactivity are found to be weak (ca. 1 kcal/mol).

RNA hydrolysis via an oxyphosphorane intermediate

From calculations of a model reaction scheme for base-catalyzed RNA hydrolysis, a pentacoodinate dianionic intermediate 2a (Storer, et al., J. Am. Chem. Soc., 1991, 113, 5216-5219) as well as two transition states, TS1 and TS2, to the intermediate have been located by ab initio calculations at the 3-21G* level. Although the intermediate, which has the well depth on the order of kBT, is unlikely to be kinetically significant, the overall rate-limiting transition state structure TS2 obtained at 3-21G* level is very close to the corresponding structure at the STO-3G level; it has an extended P-O(5') bond breaking character. These gas-phase calculation results are used to qualitatively interpret mutagenesis results of Barnase and RNase T1 where water molecules are absent from the active site.

Stereoelectronic control in peptide bond formation. Ab initio calculations and speculations on the mechanism of action of serine proteases

Ab initio molecular orbital calculations have been performed on the reaction profile for the addition/elimination reaction between ammonia and formic acid, proceeding via a tetrahedral intermediate: NH3 + HCO2H----H2NCH(OH)2----NH2CHO + H2O. Calculated transition state energies for the first addition step of the reaction revealed that a lone pair on the oxygen of the OH group, which is antiperiplanar to the attacking nitrogen, stabilized the transition state by 3.9 kcal/mol, thus supporting the hypothesis of stereoelectronic control for this reaction. In addition, a secondary, counterbalancing stereoelectronic effect stabilizes the second step, water elimination, transition state by 3.1 kcal/mol if the lone pair on the leaving water oxygen is not antiperiplanar to the C-N bond. The best conformation for the transition states was thus one with a lone pair antiperiplanar to the adjacent scissile bond and also one without a lone-pair orbital on the scissile bond oxygen or nitrogen antiperiplanar to the adjacent polar bond. The significance of these stereoelectronic effects for the mechanism of action of serine proteases is discussed.

Catalysis and leaving group binding in anilide hydrolysis by chymotrypsin

The influence of the leaving group on the reactivity of specific anilides in alpha-chymotrypsin-catalyzed hydrolysis (chymotrypsin, EC 3.4.21.2) involves both its binding to the enzyme (steric effect) and electronic nature (electronic effect). These effects are considered in terms of the stereoelectronic theory for the formation and cleavage of the tetrahedral intermediate in acyltransfer reactions. The application of this theory to the enzyme hydrolysis leads to the conclusion that the nature of the reaction products and the effectiveness of the catalysis are controlled by the orientation of the leaving group nitrogen lone pair orbital. The leaving group binding affects the formation of a reactive conformation of the enzyme tetrahedral intermediate that is presumed to intervene between the Michaelis complex and the acylenzyme. The steric and electronic effects could be separated in a straightforward fashion only in the case of equal binding of the leaving groups to the leaving-group-binding site of alpha-chymotrypsin.

Kinetic investigation of the alpha-chymotrypsin-catalyzed hydrolysis of peptide-ester substrates. The relationship between the structure of the peptide moiety and reactivity

A number of peptide-ester substrates of the general structure Ac-Lxn-...-Lx2-Lx1-OMe have been synthesized and their alpha-chymotrypsin-catalyzed hydrolysis studied. The kinetic analysis involved varying the concentration of substrate and methanol product, and measuring rates along the entire progression curve. For the dipeptide esters Ac-Lx2-Lx1-OMe and the amino-acid derivatives Ac-Lx1-OMe the following constants could be determined: the dissociation constant of the enzyme-substrate complex, KEA, both rate constants of the acylation step, k23 and k32, and the forward rate constant of the deacylation step, k31. For the tripeptide ester Ac-Ala-Ala-Tyr-OMe it appears that the rate constant for the dissociation of the enzyme-substrate complex, k21, is smaller than the rate constant for acylation, k23. Thus, for this substrate only the association and dissociation rate constants k12 and k21 could be determined and the values of k23, k32 and k31 only indirectly estimated. The influence of structural changes in the peptide moiety of the substrates on reactivity has been established by comparing the rate constants of appropriate pairs of substrates. It was found that the substrate reactivity, as measured by k23/KEA, increase with the number and strength of the secondary interactions in a manner consistent with the binding scheme which has been proposed on the basis of crystallographic studies. The effect of a particular interaction on k23 and on KEA is dependent on the nature of the other interactions. However, the effect of k23/KEA appears to be independent of the presence of the other interactions and therefore characteristic of that particular interaction. The results for these substrates are compared with those found previously for a series of peptide substrates of the structure Ac-Lxn-... Lx2-...-Lx1-Gly-NH2 which have the same acyl moiety as the peptide esters studied in this work.