L-cysteinium semioxalate: a new monoclinic polymorph or a hydrate?

Area of Science:

• Crystallography
• Structural Chemistry
• Biochemistry

Background:

• L-cysteinium semioxalate exists in an orthorhombic form.
• Crystalline amino acid derivatives often exhibit polymorphism.
• Understanding crystal structures is crucial for material properties.

Purpose of the Study:

• To characterize a new crystalline form of L-cysteinium semioxalate.
• To compare its structure and conformation with the known orthorhombic form.
• To investigate potential hydration and structural similarities.

Main Methods:

• Single crystal X-ray diffraction analysis.
• Crystallographic space group determination (monoclinic C2).
• Structural comparison with related compounds.

Main Results:

• A new monoclinic form (I) of L-cysteinium semioxalate (1:1 stoichiometric ratio) was identified.
• Form (I) displays a layered structure, differing from the orthorhombic form.
• Conformations of the cysteinium cation and oxalate anion in form (I) are substantially different from the orthorhombic form.
• The structure contains voids capable of hosting disordered water molecules (≤0.3 per void).

Conclusions:

• The newly identified monoclinic form of L-cysteinium semioxalate presents a unique crystal structure.
• Structural differences, including cation-anion conformation, are observed compared to the orthorhombic form.
• The observed conformational variations in cysteinium cations are analogous to those seen in DL-cysteine under different conditions.