Biliverdin–copper complex at physiological pH

Abstract

Biliverdin (BV), a product of heme catabolism, is known to interact with transition metals, but the details of such interactions under physiological conditions are scarce. Herein, we examined coordinate/redox interactions of BV with Cu²⁺ in phosphate buffer at pH 7.4, using spectrophotometry, HESI-MS, Raman spectroscopy, ¹H NMR, 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 long-standing question of high energy investment in the catabolism of BV, which represents a relatively harmless molecule per se.