The polymorphs of L-phenylalanine.

Area of Science:

• Crystallography
• Solid-state chemistry
• Biochemistry

Background:

• Phenylalanine (Phe) is an essential amino acid crucial for understanding aromatic compounds.
• Previous structural studies of Phe polymorphs, particularly form I, have yielded conflicting results.
• Obtaining high-quality single crystals of Phe for structural analysis is challenging.

Purpose of the Study:

• To definitively establish the solid-state structure of phenylalanine form I.
• To characterize new polymorphs of phenylalanine and elucidate its structural complexity.
• To investigate the crystallization behavior of racemic dl-phenylalanine.

Main Methods:

• Single-crystal X-ray diffraction analysis.
• Crystal growth from acetic acid solutions.
• Comparison of experimental data with previous structural models.

Main Results:

• The structure of phenylalanine form I was confirmed as P21 with Z'=4, resolving prior discrepancies.
• A new polymorph, form IV, was identified, alongside previously reported forms II and III.
• Racemic dl-phenylalanine was found not to form suitable single crystals for diffraction.

Conclusions:

• The study provides a clear and accurate structural determination for phenylalanine form I.
• The discovery of new polymorphs highlights the complex structural landscape of phenylalanine.
• Understanding Phe's solid-state behavior is vital for its applications and for aromatic compounds.