Raman spectroscopic analysis of isomers of biliverdin dimethyl ester

The constitutional isomers of biliverdin dimethyl ester, IXα and XIIIα, were studied by resonance Raman spectroscopy. The far-reaching spectral similarities suggest that despite the different substitution patterns, the compositions of the normal modes are closely related. This conclusion does not hold only for the parent state ( ZZZ, sss configuration) but also for the configurational isomers which were obtained upon double-bond photoisomerization. Based on a comparison of the resonance Raman spectra, a EZZ configuration is proposed for one of the two photoisomers of biliverdin dimethyl ester IXα, while a ZZE, ssa configuration has been assigned previously to the second isomer.