Kinetics and mechanism of large rate enhancement in an acidic aqueous cleavage of the tertiary amide bond of N-(2-methoxyphenyl)-N′-morpholinophthalamide (1)

Can a Typical Protein Assist the Rate of its Own Aqueous Cleavage?

A recent finding of a large rate enhancement in the intramolecular secondary amide group-assisted cleavage of an adjacent tertiary amide bond predicts the possibility of the cleavage of the peptide bond of a protein through a similar reaction mechanism. Based upon enzymatic partial model reactions, the usual proton-switch mechanism has been suggested for the acylation step of the chymotrypsin–catalysed cleavage of the peptide bond which does not favour a His57-shift mechanism - an essential component of the classical charge relay mechanism. Also, the proton-switch mechanism does not necessarily require the two proton-transfer of the classical charge relay mechanism. The unique structural feature of the imidazole moiety of His57 is concluded to be essential in decreasing the rate of collapse of the proposed reactive tetrahedral intermediate back to the reactants. The proposed intramolecular intimate ion-pair formation between anionic Asp102 and cationic His57 is attributed to the energetically preferred location of the proton at Nδ1 of the imidazole moiety of His57. Thus, the analysis described in this review does not favour the necessary requirements of a two proton-transfer and His57-shift as proposed in the classical charge relay mechanism as well as the relatively recently proposed His57-flip mechanism.

Kinetics and Mechanism of Hydrolysis of N-Arylphthalimides

The values of pseudo-first-order rate constants, kobs, for hydrolysis of N-(2'-methoxyphenyl)phthalimide (1) increase linearly with increase in [HCl] over the range 5 × 10−3 - 1.0M at 8.0 × 10−5 M 1, 0.5M μ (by NaCl) and in the temperature range 30-40°C. The rate of acidic hydrolysis of N-(4'-methoxyphenyl)phthalimide (2) reveals behaviour similar to that found for 1, N-(2'-hydro-xyphenyl)phthalimide (3) and N-(4 ‘-aminophenyl)phthalimide (4). The values of kobs at 35°C and varying [HCl] give the respective rate constants for the [HCl]-independent ( k0) and the [HCl]-dependent ( kH) hydrolysis of 1 (at [10] = 8.0 × 10−5 M, μ = 0.5 M) as 6.74 ×10−7 s−1 and 5.47 × 10−6 M−1 s−1, 2 (at [20] = 3.0 × 10−5M, μ = 0.5 M) as 12.2 × 10−7 s−1 and 4.61 × 10−6 M−1 s−1 and 4H+ (at [40] = 2.0 × 10−4 M, μ = 1.0 M) as 5.83 × 10−7 s−1 and 15.2 × 10−6 M−1 s−1. The values of kw/ k0min are of the order of 102 for 1, 2 and 4 where k0min represents the sum of the contributions of H+ - and HO- -catalysed hydrolysis towards k0 (first-order rate constant for pH-independent hydrolysis of imide) and kw is the pseudo-first-order rate constant for the uncatalysed reaction of H2O with imide.