Bilirubin and its crystal forms

Abstract

Bilirubin IXα is the major product of the degradation of heme from red blood cells and is of biological and medical relevance. It is insoluble in aqueous media leading to crystals in the gall bladder and urine indicating liver dysfunction. The degradation process leads to enantiomers – the M- and P-helical conformers – which are mirror versions of each other. The molecule is rigid because of strong intramolecular hydrogen bonding. We have grown two forms of bilirubin and analyzed another that was published in 1980. All forms crystallize in the space group P. Form I was first published in 1978, but the original authors mentioned that the molecules showed unresolved disorder. We have regrown this form and resolved the disorder. There are two molecular sites in the asymmetric unit in Form I, and both sites are a disordered mixture of M- and P-helical conformers, with no voids in the structure. Form II is a solvate (void space: 5% of the unit cell volume) and is a new form. It contains two molecular sites in the asymmetric unit with the same helical conformer in both sites, with an ordered molecule in one site and rotationally disordered molecules in the other site. Form III is also a solvate (void space: 25% of the unit cell volume) and contains methanol and chloroform. There is no strong intermolecular hydrogen bonding in any of the three forms, and the molecules aggregate through weak interactions. Despite this, CLP-PIXEL calculations showed that the three most stable molecule⋯molecule arrangements between the three forms are geometrically consistent.