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