5 research outputs found
The Rich Solid-State Phase Behavior of dl-Aminoheptanoic Acid: Five Polymorphic Forms and Their Phase Transitions
The
rich landscape of enantiotropically related polymorphic forms
and their solid-state phase transitions of dl-2-aminoheptanoic
acid (dl-AHE) has been explored using a range of complementary
characterization techniques, and is largely exemplary of the polymorphic
behavior of linear aliphatic amino acids. As many as five new polymorphic
forms were found, connected by four fully reversible solid-state phase
transitions. Two low temperature forms were refined in a high <i>Z</i>′ crystal structure, which is a new phenomenon for
linear aliphatic amino acids. All five structures consist of two-dimensional
hydrogen-bonded bilayers interconnected by weak van der Waals interactions.
The single-crystal-to-single-crystal phase transitions involve shifts
of bilayers and/or conformational changes in the aliphatic chain.
Compared to two similar phase transitions of the related amino acid dl-norleucine, the enthalpies of transition and NMR chemical
shift differences are notably smaller in dl-aminoheptanoic
acid. This is explained to be a result of both the nature of the conformational
changes and the increased chain length, weakening the interactions
between the bilayers
The Rich Solid-State Phase Behavior of dl-Aminoheptanoic Acid: Five Polymorphic Forms and Their Phase Transitions
The
rich landscape of enantiotropically related polymorphic forms
and their solid-state phase transitions of dl-2-aminoheptanoic
acid (dl-AHE) has been explored using a range of complementary
characterization techniques, and is largely exemplary of the polymorphic
behavior of linear aliphatic amino acids. As many as five new polymorphic
forms were found, connected by four fully reversible solid-state phase
transitions. Two low temperature forms were refined in a high <i>Z</i>′ crystal structure, which is a new phenomenon for
linear aliphatic amino acids. All five structures consist of two-dimensional
hydrogen-bonded bilayers interconnected by weak van der Waals interactions.
The single-crystal-to-single-crystal phase transitions involve shifts
of bilayers and/or conformational changes in the aliphatic chain.
Compared to two similar phase transitions of the related amino acid dl-norleucine, the enthalpies of transition and NMR chemical
shift differences are notably smaller in dl-aminoheptanoic
acid. This is explained to be a result of both the nature of the conformational
changes and the increased chain length, weakening the interactions
between the bilayers