A temperature-dependent change in the mechanism of acid catalysis of the hydrolysis of p-nitrophenyl β-D-glucopyranoside indicated by oxygen-18 and solvent deuterium kinetic isotope effects

Abstract

The l80 kinetic isotope effect for hydrolysis of p-nitrophenyl [l -la,] -P-glucopyranoside in Z.OM-HCI, measured by the isotopic quasi-racemate method, is 1.025, at 65.5 "C and 1.023 at 75.1 "C; there is a literature value of 1.0355 0.001 5 at 50.0 "C, measured mass spectrometrically. That this apparent discrepancy arises from a change in the mode of acid catalysis as the temperature is lowered is shown by the strongly temperature-dependent solvent deuterium isotope effect [log (kO2JkH2$ = 1.84 -0.595 x 103/T], and by the greater effect of trifluoroacetate buffers at 45.0 than at 85.0 C: a rough catalytic constant for general acid catalysis by trifluoroacetic acid of ca. 1 O-7.5 I mol-' s-' can be estimated at the former temperature.

Purification and mechanistic properties of an extracellular α-l-arabinofuranosidase from Monilinia fructigena

“…The observed isotope effect for AFIII-catalysed hydrolysis ofp-nitrophenyl arabinoside is similar to that observed for the acid-catalysed hydrolysis of the same compound (1.023 + 0.003 at 80°C) (Bennet et al, 1985), and, at this temperature, for the acid-catalysed hydrolysis of p-nitrophenyl f-D-glucopyranoside (Bennet et al, 1987). However, at 50°C, the latter effect increases to 1.0355+0.0015 (Rosenberg & Kirsch, 1981b) as the mode of catalysis by H30+ changes from specific acid to general acid (Bennet et al, 1987;Rosenberg & Kirsch, 1981b).…”

A solvent-isotope-effect study of proton transfer during catalysis by Escherichia coli (lacZ) β-galactosidase

“…These measurements are undoubtedly experimentally correct (cf. Bennet et al, 1987). They could (just) be reconciled with the mechanism above on the assumption that there was a virtual transition state governing V for 4-nitrophenyl f8-D-galactopyranoside, to which loss of nitrophenolate without acid catalysis and the conformation change (i.e.…”

Mechanisms of glycopyranosyl and 5-thioglycopyranosyl transfer reactions in solution

“…For aryl β-glucopyranosides the mechanistic change occurs when the leaving group is 4-nitrophenol. 83 It is remarkable that the reactions of 4-nitrophenyl β-glucopyranoside (48) appear to be balanced finely between the two types of acid-catalysis as demonstrated by both 1-18 O and solvent KIEs. At a temperature of 50 ЊC, the two KIEs are consistent with the occurrence of general-acid catalysis (k 16 /k 18 = 1.035 84 and k H 3 O ϩ/k D 3 O ϩ = 1.01 83 ) yet at 80 ЊC, the 18 O-KIE value is in the range normally associated with specific-acid catalysis (1.025), 83 and the solvent effect is significantly smaller at this higher temperature (0.71).…”

“…83 It is remarkable that the reactions of 4-nitrophenyl β-glucopyranoside (48) appear to be balanced finely between the two types of acid-catalysis as demonstrated by both 1-18 O and solvent KIEs. At a temperature of 50 ЊC, the two KIEs are consistent with the occurrence of general-acid catalysis (k 16 /k 18 = 1.035 84 and k H 3 O ϩ/k D 3 O ϩ = 1.01 83 ) yet at 80 ЊC, the 18 O-KIE value is in the range normally associated with specific-acid catalysis (1.025), 83 and the solvent effect is significantly smaller at this higher temperature (0.71). 83 The acid-catalysed reaction of neutral aryl α--N-acetylneuraminides exhibits a β Lg value of 0 and, when taken together with the observed solvent KIE (k H 3 O ϩ/k D 3 O ϩ) value for the 4nitrophenyl compound (49) at a pH of 1.36, is consistent with a general-acid catalysed D N ϩ A N mechanism as depicted in Scheme 20.…”