TABLE 3.

Calculated Quantum Mechanical Parameters for Nucleophilic Amino Acids

Amino acid residuea	ELUMO (ev)	EHOMO (ev)	μ (ev)	σ (ev)
Cysteine thiolate (− 1)	4.76	− 0.35	2.21	0.391
Cysteine thiol (0)	0.14	− 5.87	− 2.87	0.330
Histidine (0)	0.30	− 5.75	− 2.75	0.331
Lysine (+ 1)	− 2.98	− 10.39	− 6.69	0.270

^aFor each nucleophile, quantum mechanical parameters were calculated based on the predominant ionization state (in parentheses) at pH 7.4. To model a cysteine catalytic triad, quantum mechanical parameters were also calculated for the anionic thiolate state (− 1). E_LUMO = energy level (ev) of the LUMO; E_HOMO = energy level (ev) of the HOMO. E_LUMO and E_HOMO values were used to calculate the chemical potential (μ) of the nucleophile and corresponding softness (σ) as described in the “Materials and Methods” section.