Structure of biliverdin and its interaction with copper

Abstract

The structure of biliverdin (BV) and its complexion capacity with Cu2+ in dependence of solvent choice has been examined. We first examined coordinate/redox interactions of BV with Cu2+ in phosphate buffer at pH 7.4, using spectrophotometry, HESI-MS, Raman spectroscopy, 1HNMR, EPR, fluorimetry, and electrochemical methods. BV formed a stable coordination complex with copper in 1 : 1 stoichiometry. The structure of BV was more planar and energetically stable in the complex. The complex showed strong paramagnetic effects that were attributed to an unpaired delocalized e−. The delocalized electron may come from BV or Cu2+, so the complex is formally composed either of BV radical cation and Cu1+ or of BV radical anion and Cu3+. The complex underwent oxidation only in the presence of both O2 and an excess of Cu2+, or a strong oxidizing agent, and it was resistant to reducing agents. The biological effects of the stable BV metallocomplex containing a delocalized unpaired electron should be further examined, and may provide an answer to the longstanding question of high energy investment in the catabolism of BV, which represents a relatively harmless molecule per se. The complex possibility was quite different after changing the solvent. BV structure in DMSO was analyzed by using NMR techniques and unrestricted density function theory simulations to explain the incapacity of BV to build coordination complex(es) with Cu2+ in dimethyl sulfoxide, which was confirmed by UV-Vis, EPR and NMR spectroscopy. NMR showed that N atoms of BV are protonated in all four pyrrole rings. The structure is stabilized by two hydrogen bonds between NH moieties and carbonyl oxygens from opposite terminal pyrrole rings, and by the bending of propionyl chain with carboxyl group out of the plain toward central position of BV. The simulations of deprotonated BV, which builds copper complexes in water and chloroform as described previously, showed a different conformation and organization of hydrogen bonds. Taking into account that deprotonation represents a critical step in coordinate bonds formation, the protonation of an additional N atom may represent a key difference between the interactions of BV with copper in different solvents.