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Controlling the Size, Shape and Stability of Supramolecular Polymers in Water
Published on: August 2, 2012
Stereochemical control of polymorph transitions in nanoscale reactors
Qi Jiang1, Chunhua Hu, Michael D Ward
1Molecular Design Institute, Department of Chemistry, New York University, 100 Washington Square East, New York, New York 10003-6688, USA.
Journal of the American Chemical Society
|February 2, 2013
Summary
Metastable beta-glycine nanocrystals formed in nanopores transform to alpha-glycine at high humidity. Amino acid auxiliaries can suppress this phase transition by blocking water access to critical crystal faces.
Area of Science:
- Materials Science
- Crystallography
- Nanotechnology
Background:
- Glycine, an amino acid, exists in multiple polymorphic forms, including metastable beta-glycine and stable alpha-glycine.
- Nanoconfinement can alter crystallization pathways and polymorph stability.
- Anodic aluminum oxide (AAO) nanopores provide a controlled environment for studying nanocrystal formation.
Purpose of the Study:
- To investigate the crystallization of glycine in AAO nanopores.
- To determine the phase transition behavior of beta-glycine nanocrystals under varying humidity.
- To explore the role of amino acid auxiliaries in controlling glycine polymorph transitions.
Main Methods:
- Crystallization of glycine within AAO nanopores of varying diameters.
- Two-dimensional X-ray microdiffraction for structural analysis.
- Controlled humidity experiments to induce phase transitions.
- Use of racemic and enantiopure amino acid auxiliaries during crystallization.
Main Results:
- Metastable beta-glycine formed in AAO nanopores < 200 nm, with the [010] axis aligned with the pore direction.
- Beta-glycine nanocrystals transformed to alpha-glycine at 90% relative humidity, with a reorientation of the [010] axis.
- Racemic amino acid auxiliaries suppressed the beta to alpha transition by selectively binding to {010} faces.
- Enantiopure auxiliaries did not prevent the transition, indicating stereoselective binding and the importance of water access.
Conclusions:
- The [010] axis orientation of beta-glycine in AAO nanopores is similar to that in polymer monoliths.
- High humidity induces a phase transition from beta-glycine to alpha-glycine with significant molecular reorientation.
- Amino acid auxiliaries can act as crystallization modifiers, preventing phase transitions by sterically hindering water molecules.
- The stereoselective binding of amino acids to specific crystal faces is crucial for controlling glycine polymorphism in nanoconfined environments.
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