Bilirubin and its congeners: conformational analysis and chirality from metadynamics and related computational methods

The conformational properties of natural bilirubin, bisbutyricmesobilirubin-XIIIα, and their chiral derivatives (βS,β′S)-
dimethylmesobilirubin-XIIIα and 8,12-(γR,γ′R)- dimethylbisbutyricmesobilirubin-XIIIα were investigated by metadynamics,
an improved molecular dynamics computational methodology useful for conformational analysis. Applied to linear
tetrapyrroles, which may be treated as two blade molecular propellers, our results are represented by conformational energy
hypersurface maps built as functions of the two central dihedral torsion angles (φ1 and φ2) following rotations of two dipyrrinone
units about the connecting CH2,
and are compared successfully with maps previously obtained from molecular
mechanics. With an emphasis on the conformationally more flexible 8,12-(γR,γ′R)-dimethylbisbutyricmesobilirubin-XIIIα,
a conformationally mobile optically active bilirubinoid with butyric acid replacing propionic, metadynamics reveals global
and nearby minima. Its electronic circular dichroism and UV–Vis spectra predicted by DFT calculations correlate well with
the experimental spectra and are explained on the basis of equilibrium between the predicted conformational energy minima.