Structures of the oxidized states of some common biochemical reducing agents

The crystal structures of the oxidized states of three of the most common biochemical reducing agents have been determined, by crystallization from aqueous solution where possible: tris(2-carboxyethyl)phosphine oxide (TCEP oxide, 1), bis(2-hydroxyethyl) disulfide (2MESS, 3), and tributylphosphine oxide (Bu4PO, 4). In addition, a less disordered crystal structure of the reducing agent tris(2-carboxyethyl)phosphonium chloride (TCEP·HCl, 2) has been obtained. A greatly simplified method was found for synthesizing 1. An unusual method of obtaining X-ray quality crystals from hard-to-crystallize oily liquids is described, in which the oil is solidified to a glass by rapid cooling, allowed to warm to a supercooled liquid containing a few seed crystals, and finally crystallized just below its melting point. This method was successfully used to crystallize 3, a viscous oil at room temperature that had resisted all other methods of crystallization, for X-ray diffraction. An ab initio quantum chemical analysis of the conformers of 3 showed that the molecular geometry of this flexible molecule in the solid state was 320 kJ/mol higher than that of the lowest-energy conformer, underscoring the fact that the solid-state structure may not always yield the same molecular geometry as that of the minimum-energy conformer, especially for flexible molecules with strong intramolecular nonbonding interactions. The unusual NMR spectral characteristics of 4 are elucidated.