L-Threonine

Threonine has two chiral centres, which is unusual on this list — only threonine and isoleucine do. The consequence is a diastereomer, allo-threonine, that a simple optical-rotation check will not reliably exclude. Specify allo-threonine as a named related substance, not as part of a generic 'other amino acids' limit.

Its side-chain hydroxyl gives it a second unusual property: an unusually high second dissociation constant. The Merck Index reports pKa1' 2.63 and pKa2' 10.43, against about 9.6 for most amino acids here. In practice that means threonine's buffering shifts more alkaline than its neighbours in a free-form blend.

At 97 g/L in water at 25 °C it is one of the more soluble amino acids in the range — four times leucine — and will not limit a concentrate. It is, however, insoluble in ethanol, ether and chloroform, so aqueous and dry formats only.

It decomposes at about 256 °C without a clean melt, and the literature gives different decomposition figures for the DL form (228-229 °C) than for the natural L form (255-275 °C) — another reason not to accept a DL specification as a proxy.

Commercially, threonine is a feed-grade amino acid first and a nutritional one second (it is the second or third limiting amino acid in swine and poultry diets). Food and supplement buyers should specify explicitly, because the cheap tonnage is feed grade and does not carry food microbiological or heavy-metal limits.

Threonine is the amino acid that intestinal mucin synthesis draws on most heavily, which is why it appears in gut-health positioning. As a supplier statement that is a research context, not a product claim — see the literature section.