TABLE 2.

Softness, electrophilicity, and IC₅₀ values for quinone and α,β-unsaturated carbonyl derivatives

Softness (σ) and electrophilicity (ω) for the selected compounds were calculated as described in the Materials and Methods. The respective IC₅₀ values represent in vitro electrophile concentrations that produce 50% thiol loss and reflect the relative electrophilic potency of each chemical (Zhang et al., 2013).

Electrophile	Softness (σ)a	Electrophilicity (ω)	IC50
		eV	μM
NAPQI	499	6.83	8.4
Benzoquinone	524	7.78	11.6
Acrolein	371	3.82	52.5
4-Hydroxy-2-nonenal	377	3.80	398b

^aValues are ×10⁻³ eV⁻¹.

^bThe IC₅₀ value for HNE is higher than expected based on corresponding electrophilicity (ω). However, the second-order reaction rate for reaction with sulfhydryl groups is affected by the relatively larger size and shape of this molecule (i.e., the reaction rate is slower due to steric hindrance; see LoPachin et al., 2009b).