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Some substances can exist in various crystal modifications with different melting points (
polymorphism).  In the case of tripalmitin, the first heating run shows the sample as received is in the stable form.  On cooling, a metastable modification crystallizes out, whose melting point lies lower than that of the thermodynamically stable form.  In the second heating run, the metastable modification first melts and then immediately begins recrystallization into the stable modification, which melts again at the higher temperature.  The lower onset temperature and slightly lower heat of fusion observed for the stable form in the second heating experiment are caused by incomplete crystallization.


Courtesy of Mettler Toledo.

peptide biochemistry polymerization