Hydrogen-bonded complexes formed in the body may lead to the formation of kidney stones. One food contaminant that can form these insoluble complexes is melamine. Several such compounds have been characterized: melamine cyanurate, melamine urate, and melamine oxalate. Spartan software was used to model complexes formed between melamine and oxalic acid in the presence of urea, creatine, or cystine as well as between melamine and a precursor consisting of fumaric acid, maleic acid, gentisic acid, or phthalic acid. Raman microscopy was used to analyze crystallites formed in these solutions. Infrared spectra for these compounds were compared to computationally predicted spectra to understand how hydrogen-bonding affected the structure of the complexes. Both Raman microscopy and scanning electron microscopy show that the crystallites formed were not uniform and contained different shapes and sizes ranging from 10-960 microns. For crystallites with dimensions that were greater than 0.1-0.2 mm, structural characterization using X-ray diffraction was carried out. Figure 1 shows the structures of melamine-maleic acid and melamine-fumaric acid and their space groups and unit cell parameters are listed in Table 1. Although maleic acid and fumaric acids are isomeric, the cis configuration of maleic acid allows intramolecular bonding and accounts for its monoclinic crystal system as opposed to the triclinic classification for melamine-fumaric acid.

Table 1: Comparison of crystal parameters for the complexes of melamine-maleic acid and melamine-fumaric acid.

Fig. 1: (Left) Structure of Melamine-Maleic Acid; (Right) Structure of Melamine-Fumaric Acid

Beng Ooi

Appearances