3. Chemistry and Biology of H2S
H2S is recognized by its odor of rotten eggs. Humans can detect as low as 0.0005-0.3 ppm, whereas concentrations higher than 10 ppm start to cause discomfort and higher than 500 ppm cause rapid loss of consciousness and death [49].
The recommended names by IUPAC are dihydrogen sulfide and sulfane for H2S, and sulfanide or hydrogen(sulfide)(1-) for HS- [20]. In this review, “H2S” will be used for the gas and for the mixture of H2S and HS- in aqueous solution.
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H2S may be considered at first sight to be similar to water, but there are several important differences that lead to dissimilar physical and chemical properties. Sulfur is larger than oxygen (van der Waals radii of 1.80 and 1.42 Å, respectively) [50], has a lower electronegativity (2.58 and 3.44 in the Pauling scale, respectively), and is more polarizable [51]. Hence, the dipole moment of H2S is lower than that of water, and hydrogen bonds in H2S are not formed at room temperature [51]; this explains why H2S is a gas at ambient temperature and pressure. H2S can dissolve in water with a relatively high solubility (101.3 mM/atm at 25 °C) [52], and the solvation is dominated by dispersive forces with no hydrogen bonds with water [51]. The slightly hydrophobic character of H2S is further supported by its twice higher solubility in organic solvents such as octanol and hexane than in water [12].
H2S is a weak diprotic acid with pKa1 = 6.98 at 25 °C and 6.76 at 37 °C (Equation 1) [53]. The second pKa2 has been more difficult to determine exactly but lies between 17 and 19 (Equation 2) in aqueous solution at 25 °C [20,53]. Therefore, S2- concentrations will be very low in biological settings and will not contribute significantly to H2S reactivity. At pH 7.4 and 25 °C, total sulfide exists 28% as H2S, 72% as HS-, and as insignificant amounts of S2-.
The hydrophobic character of H2S results in a higher solubility in membrane lipids than in water that results in a rapid diffusion through cell membranes without assistance by protein channels [11,12,13]. Permeability coefficients are estimated to be larger than 0.5 cm/s and probably larger than 10 cm/s [11,12,13], which indicates that membranes could slow down H2S diffusion 10-100 times compared to an equally thick layer of water. This is a low barrier that does not impede free diffusion across cellular membranes and only becomes significant when considering diffusion across a large number of membranes such as in a tissue [12].
The oxidation state of sulfur in H2S and in HS- is -2. Thus, H2S can only be oxidized; it cannot act as an oxidant. Therefore, the statements in the literature regarding H2S oxidation of protein cysteines to persulfides are not correct; either the thiol or H2S need to have undergone previous oxidation for cysteine persulfide formation to occur. Oxidation of H2S can lead to various products in which the sulfur can have oxidation numbers up to +6. The oxidation products include sulfate (SO42−), sulfite (SO32−), thiosulfate (S2O32−), persulfides (RSS−), organic (RSSnSR) and inorganic (HSSnS−) polysulfides, and elemental sulfur (Sn).
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