The species- and site-specific acid–base properties of biological thiols and their homodisulfides

• Cysteamine, cysteine, homocysteine and their disulfides.
• Site-specific protonation constants determined.
• Interactivity parameters determined.
• pH-dependent distribution of protonation microspecies.
• Oxidizability of thiolates assessed.

Cysteamine, cysteine, homocysteine, their homodisulfides and 9 related compounds were studied by 1H NMR-pH titrations and case-tailored evaluation methods. The resulting acid–base properties are quantified in terms of 33 macroscopic and 62 microscopic protonation constants and the concomitant 16 interactivity parameters, providing thus the first complete microspeciation of this vitally important family of biomolecules. The species- and site-specific basicities are interpreted by means of inductive and hydrogen-bonding effects through various intra- and intermolecular comparisons. The pH-dependent distribution of the microspecies is depicted. The thiolate basicities determined this way provide exclusive means for the prediction of thiolate oxidizabilities, a key parameter to understand and influence oxidative stress at the molecular level.

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