Table 1. Room-temperature pKa values (at neutral pH and at room temperature for Mb where otherwise not specified, and for CPO) and calculated proton affinities for relevant deprotonation processes.
For models of the corresponding active sites, see the Experimental section. Abbreviations: Im, imidazole (imidazolate is not abbreviated); P450, cytochrome P450.
| Charge balance | Experimental protonation state and pKa | Calculated proton affinity (kcal/mol) |
|---|---|---|
| +1→0 | ||
| [Im–Fe(III)–OH2]1+→[Im–Fe(III)–OH]0 | Partially protonated (pKa ∼8 in Hb and 9 in Mb) [43] | 258 |
| [Im–Fe(IV)–OH]1+→[Im–Fe(IV)=O]0 | Partially protonated proposed interpretation of Figure 2 data | 256 |
| 0→−1 | ||
| [Im–Fe(III)–OH]0→[imidazolate–Fe(III)–OH]−1 | Protonated, pKa ∼11 [42] | |
| [Im–Fe(III)–OH]0→[Im–Fe(III)=O]−1 | Protonated | |
| [Im–Fe(II)–OO]0→[imidazolate–Fe(II)–OO]−1 | Protonated | 407 |
| [Im–Fe(III)–OOH]0→[Im–Fe(III)–OO]−1 | Protonated [46] | 374 |
| [Im–Fe(III)–OOH]0→[imidazolate–Fe(III)–OOH]−1 | Protonated [46] | 338 |
| [Im–Fe(III)–OO]0→[imidazolate–Fe(III)–OO]−1 | Protonated [46] | 373 |
| [Im–Fe(IV)=O]0→[imidazolate–Fe(IV)=O]−1 | Alternative interpretation of Figure 2 data based on [42] | 337 |
| [Thiolate–Fe(IV)–OH]0→[thiolate–Fe(IV)=O]−1 (CPO) | Protonated [17] | 345 |
| [Thiolate–Fe(III)–OH2]0→[thiolate-Fe(III)–OH]−1 (P450) | Protonated [3] |