Table 2.

Formation constants of MeHg⁺ or Hg²⁺ with nucleophilic centers found in biomacromolecules. The nucleophilic centers can be categorized in hard (HNu⁻) or soft (SNu⁻) centers. −SeH is the softest of the SNu⁻ centers found in living cells, followed by −SH. The −NH and −COOH-containing molecules are HNu⁻ and the centers containing O are harder than those containing N (Hancock and Martel 1996; Martin 2002).

Functional group	MeHg+	Hg2+
R-COOH	≈ 2.5–3.0	≈ 4–7
R-(COOH)2	---	≈ 5–9
R-(COOH)3 (citrate)	---	≈ 11
R-NH2	≈ 7–8	≈ 4–5
R-(NH2)2	---	≈ 11
R2-NH	≈ 6.7	≈ 4–5
R-S-R	≈ 7–8	≈ 6–12
R-SH	≈ 15–26	≈ 40–50
R-SeH	≈ 16–18	≈ 50–60

The values represent the log of the constant and a high value indicates that a given complex has high thermodynamic stability. The values were obtained from Stricks and Kolthoff 1953; Rabenstein et al. 1974; Rabenstein and Fairhurst 1975; Rabenstein 1978; Jawaid and Ingman 1981; Arnold and Canty 1983; Arnold et al. 1986; Berthon 1995; Seby et al. 2001; Anderegg et al. 2003; Mousavi 2011; Liem-Nguyem et al. 2017.