posted on 2015-04-16, 00:00authored byCarl Henrik Görbitz, Pavel Karen
Three quasiracemates, l-norleucine:d-methionine, l-norvaline:d-norleucine, and l-norvaline:d-methionine, were crystallized to see
how they differ from
regular racemates in terms of crystal structure (studied by single-crystal
X-ray diffraction) and of thermally induced phase transitions (studied
by differential scanning calorimetry). Two types of transitions are
detected between 100 and 450 K and structurally characterized: (1)
displacive transitions of the molecular bilayers that form the crystal
and (2) continuous or discontinuous disordering transitions in the
amino acid side chains. Uniquely for the quasiracemates, the displacive
transition proceeds in two close steps as only one surface of each
molecular bilayer slides at first, upon forming an intermediate phase,
while the other surface follows at a slightly higher temperature.
Altogether, 18 new single-crystal structure-refinement data sets are
reported for these three quasiracemates.