Michel Bergon

Transplasma-membrane ferricyanide reduction in sycamore cells. Characterization of the system and inhibition by some phenyl biscarbamates

Abstract Evidence is presented for the presence of a transplasma-membrane redox system in sycamore cells, which were found able to reduce external ferricyanide. This reduction induced a simultaneous acidification of the external medium, with a H+/e− stoichiometry of 0.925. Ferricyanide reduction was accompanied by a slight reduction of potassium uptake (15% inhibition). The transmembrane electron transport was inhibited by oligomycin and quinacrine; the effect of the latter inhibitor was only temporary, owing to its progressive absorption by the cells. At 100 μM, several derivatives of the herbicide phenmedipham were able to inhibit the growth of sycamore cells. These compounds inhibited the external acidification induced by fusicoccin, without interfering with the integrity of the plasmalemma. They had no effect on the phosphohydrolase activity of a microsomal fraction enriched in plasmalemma ATPase. They were not uncouplers of oxidative phosphorylation, and appeared as poor inhibitors of mitochondrial electron transfer (I50 > 100 μM). In intact cell suspension, they inhibited both the reduction of ferricyanide and the accompanying acidification of the external medium (I50 = 32 μM). Moreover, they could induce a reversal of the functioning of the redox system, which consequently induced ferrocyanide oxidation.

A nonlinear hammett relationship as evidence for a change-over in mechanism in the alkaline hydrolysis of methyl carbanilates

Abstract A nonlinear Hammett relationship could be used as evidence for a change-over in mechanism in the alkaline hydrolysis of methyl carbanilates. The electron-withdrawing substituted compounds hydrolyse via an A–E pathway (ϱ 1.06) whereas the hydrolysis of the electron-donating substituted compounds involves an E-A scheme.

Uncoupling activity of some N-phenylcarbamates

Abstract In the N-phenylcarbamate family several compounds inhibit cell division and some others prevent the light-driven electron transfer in thylakoids. In this family (general formula R1-NH-COO-R2 where R1 is a phenyl), 18 compounds were investigated for their possible uncoupling activity. Potent uncouplers, able to uncouple fully the oxidative phosphorylation or the photophosphorylation between 1 and 10 μM, were obtained when R2 was 4-NO2C6H4, CH2CHCl2 or CH2CF3 and R1 was a 3,4-Cl2C6H3. The -NH-group of the carbamate function is probably involved in the proton transfer through the two types of biological membranes studied here (mitochondria and thylakoids).

Kinetics and mechanism of hydrolysis of O-aryl N-phenylthiocarbamates

In kinetic studies of the hydrolysis ofaseries of O-aryl N-phenylthiocarbamates, evidence for the formation of phenyl isothiocyanate is a decisive argument for an E1cB mechanism. The high value of the Hammett parameter (ρ 3.0) and the positive value of the entropy of activation (ΔS‡+ 10.8 cal mol–1 K–1) are discussed as mechanistic criteria which can be used to establish the occurrence of an EIcB mechanism when the intermediate cannot be isolated.

Kinetics and mechanism of hydantoic ring opening. The alkaline hydrolysis of 3-arylimidazolidine-2,4-diones to 5-arylhydantoic acids

The kinetics of hydrolysis of 3-arylimidazolidine-2,4-diones to 5-arylhydantoic acids were studied in aqueous solution over the pH range 8–14 at 25°. In strongly alkaline media (pH > 11.5), an ionisation equilibrium gives rise to an unreactive anion. The pKa values determined both spectrophotometrically and kinetically for the unsubstituted derivative are in good agreement. In mildly alkaline media (pH < 11.5), the observed rates have a first-order dependence on hydroxide ion concentration and no general catalysis is detected. The value of the Hammett parameter (ρ 0.8), the low deuterium isotope solvent effect (kH2O/kD2Oca. 0.7), and the markedly negative values of the entropies of activation (–30 cal mol–1 K–1 < ΔS‡ < –20 cal mol–1 K–1) are rationalized in terms of a rate-determining hydroxide ion attack which is in agreement with the pKa value of the leaving group of 3-arylhydantoins.