Proton assisted nucleophilic mechanism

The occurrence of general acid-catalyzed hydroxylaminolysis or methoxylaminolysis of thiol esters or amides has been described in Section IIB in terms of kinetically important tetrahedral intermediates. Two kinetically indistinguishable mechanisms for general acid-catalyzed aminolysis reactions are represented by transition states 42 and 43. Mechanism 42 involves a prior protonation of the ester followed by a general base-catalyzed aminolysis mechanism 43 is a general acid-assisted nucleophilic reaction of the amine. Mechanism 42 can be ruled out in the hydrazinolysis of phenyl acetates (Bruice and Benkovic, 1964) and in the hydrazinolysis of S-thiolvalerolactone (Bruice et al., 1963) on the basis of a calculated rate constant which is greater than the diffusion-controlled limit. Mechanism 43 is therefore correct. 

The exceptional reactivity of DNA for protonated N-hydroxy arylamines can be rationalized by at least two mechanisms. First, intercalation of the electrophilic intermediate between DNA bases could sterically assist in desolvation and in directing the electrophilic center of the carcinogen over the nucleophilic region of the DNA base. This seems unlikely, however, as pretreatment of DNA with cis-Pt, which decreased the DNA contour length by 50%, failed to reduce the reactivity of N-hydroxy-1-naphthylamine for the DNA (137). A second possibility involves an electrostatic attraction between the electrophile and the phosphate backbone of the DNA (77). This seems more probable since eithe j +high ionic strength or stoichiometric (to DNA-P) amounts of Mg strongly inhibit DNA adduct formation (77,137). In addition, evidence has been presented that N-hydroxy arylamine-DNA/RNA phosphotriesters may be formed which induce strand breaks (137,138) and could serve as a catalyst for desolvation and subsequent adduct formation. 

It is to be noted that there are essentially no principal differences between the catalytic reaction, when the ring is activated by a hydroxyl-containing component, and the noncatalytic one when amine acts as an electrophilic reagent (proton donor). It should be emphasized that for epoxy ring opening by such weakly nucleophilic reagents as amines, an electrophilic assistance is essential. This idea will be confirmed below several times because it is the key idea for understanding the kinetic mechanism of the reaction of epoxy compounds with amines. 

The "oxyanion hole." A third mechanism by which an enzyme can assist in a displacement reaction on a carbonyl group is through protonation of the carbonyl oxygen atom by an acidic group of the enzyme (Eq. 12-21). This will greatly increase the positive charge on the carbon atom making attack by a nucleophile easier and will also stabilize the tetrahedral... 

---