Crystal structure and conformation of 10-aryl-bilatrienes-abc

Abstract

Crystal structure determinations of etiobiliverdin-IVγ (1) and of twomeso-substituted arylbilatrienes-abc (the 2-imidazolylderivative2 and the 4-nitrophenylderivative3) are reported. All three molecules have aZ,Z,Z configuration and occur in the crystal in anall-syn conformation, which results in a helical overall molecular shape. As a consequence of the steric repulsion between the aryl substituent and the two methyl groups on positions 8 and 12, the pitch and the curvature of the helix are larger in the twomeso-substituted derivatives2 and3 compared to1. In each structure, the packing is dominated by the formation of centrosymmetric, S-shaped dimers via two hydrogen bonds between the pyrrolic H-atom and the carbonyl oxygen of ring A of two molecules related by the operation of an inversion center.