Table 2.
Interactions with type-2 alkenes with potential amino acid target: calculated HSAB parameters.
| Residue | Side chain group | σ × 10–3/eV | ACR ω– × 10–3 eV (relative) | Acrolein ω– × 10–3 eV (relative) | ||||
|---|---|---|---|---|---|---|---|---|
| CYS (–1) | –CH2S– | 382 | 146 (1.00) | 266 (1.00) | ||||
| LYS (0) | –(CH2)4NH2 | 285 | 56.6 (0.39) | 126 (0.47) | ||||
| HIS (0) | 313 | 48.5 (0.33) | 114 (0.43) | |||||
| CYS (0) | –CH2SH | 282 | 40.0 (0.27) | 98.4 (0.37) | ||||
| LYS (+1) | –(CH2)4NH3+ | 271 | 35.3 (0.24) | 90.0 (0.34) | ||||
| For each amino acid nucleophile, HSAB parameters were calculated on the basis of selected ionization states (in parentheses). Data show that the sulfhydryl thiolate state is a significantly softer (σ) nucleophile than either the corresponding thiol state or the other amino acid residues such as histidine or lysine. This characteristic indicates that the thiolate state will react selectively with comparably soft electrophiles such as acrolein. The nucleophilic index (ω–), which reflects the propensity of adduct formation, indicates that the sulfhydryl thiolate state is the preferential target of the type-2 alkenes. Relative to the thiolate state (1.00), thiol groups and the lysine and histidine residues are substantially less competitive targets for type-2 alkene adduct formation (mean relative value, 0.35). | ||||||||