Table 2.
Reaction pKa and second-order rate constant kb for 76u and 76D cleavage in the presence of 4-substituted imidazole analogues
| pKa | m.v.* | C76u
|
C76Δ
|
|||
|---|---|---|---|---|---|---|
| pKa | kb | pKa | kb | |||
| 4-Methylimidazole | 7.8 | 79 | 7.9 | 33 | 7.8 | 76 |
| Imidazole | 7.0 | 62 | 7.0 | 63 | 7.1 | 30 |
| 4-Hydroxymethylimidazole | 6.5 | 84 | 6.6 | 5.0 | 6.6 | 14 |
| 3-Amino-1,2,4-triazole | 5.5 | 67 | 5.4 | 1.3 | 5.5 | 5.3 |
| Histamine | 6.0 | 104 | — | — | 6.7 | 3.7 |
The second-order rate constants kb (× 103 min−1⋅M−1) were determined from a plot of first-order rate constants against free base concentration. — indicates the numbers were not determined.
*
Molecular volume of each base (m.v., Å3).