The apparent contradiction in the common observation that bound metal ions inhibit H (2) exchange in imidazoles and catalyze H(8) exchange in the imidazole ring of purines is resolved. The specific second-order rate constant for hydroxide ion attack on protonated imidazole rings to yield an ylide is compared with that for attack on metalated rings. For both the bis(3-methylhistidine) complex of Pd(II) and the inosine and guanosine complexes of Pt(II) at N(7), the specific second-order rate constant for carbon-bound hydrogen exchange is 105 times faster for protonated than for metalated rings. The factor is more than 102 times greater than that expected from the basicity of the adjacent nitrogen. The many times greater basicity of N(1) or N(3) in imidazoles compared to N(7) in nucleic bases results in a greater concentration product of protonated species and hydroxide ion in neutral solutions. Coordination of a metal ion at N(7) of a nucleic base places a positive charge where a proton does not appear until quite low pH and results in catalysis of exchange in neutral solutions. In contrast, the much more basic imidazoles partially protonate even in neutral solutions and the greater polarizing power of the proton over a metal ion reduces the apparent relative role of the metal ion.
ASJC Scopus subject areas
- Colloid and Surface Chemistry