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Thermostability is the quality of a substance to resist irreversible change in its chemical or physical structure at high temperature. (Naturally, the meaning of high temperature will depend upon the type of material.)

Thermostable materials may be used industrially as fire retardants. A thermostable plastic—a perhaps uncommon and unconventional term—is more likely to refer to a thermosetting plastic that cannot be reshaped when heated than to a thermoplastic that can be remelted and recast. Thermostability also commonly refers to a protein resistant to irreversible change in its protein structure due to applied heat.

Thermostable proteins

Most life-forms on Earth live at temperatures of less than 50 °C (commonly from 15 °C to 50 °C). Above this, thermal energy may cause the unfolding of the protein structure to be irreversible, a condition understandably deleterious to continuing life-functions. (The denaturing of proteins in albumen from a clear, nearly colourless liquid to an opaque white, insoluble gel is a common example of this.)

Certain thermophilic life-forms exist which can withstand temperatures above this. Examining homologous proteins present in these thermophiles and other organisms reveal only slight differences in the protein structure. One notable difference is the presence of extra hydrogen bonds in the thermophile's proteins—meaning that the protein structure is more resistant to unfolding. The presence of certain types of salt has been observed to decrease thermostability in the proteins of mesophiles but not of hyperthermophiles which may indicate that salt bridges have more impact on thermostability at high temperature than is currently widely acknowledged.

Thermostable enzymes such as Taq polymerase and Pfu DNA polymerase are used in polymerase chain reactions where temperatures of 94 °C or over are used to melt apart DNA strands.

Thermostable toxins

Certain poisonous fungi contain thermostable toxins, such as amatoxin found in the death cap and autumn skullcap mushrooms.

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