Blue transition metal complexes of a natural bilin-type chlorophyll catabolite

Abstract

“Non-fluorescent” chlorophyll catabolites (NCCs) are ubiquitous, colourless bilane-type natural products, first identified about 20 years ago. In various senescent leaves NCCs are oxidized, in part, to yellow chlorophyll catabolites (YCCs), which undergo further oxidation to unique pink chlorophyll catabolites (PiCCs). The present work presents the crystal structure of a PiCC, the first of a natural chlorophyll catabolite from a higher plant. The PiCC binds (divalent) zinc-, cadmium-, copper- and nickel-ions with high affinity. Binding of these metal ions to the PiCC is rapid at room temperature. The resulting deep blue complexes represent the first transition metal complexes of a bilin-type chlorophyll catabolite. The structure of the metal complexes has been deduced from spectroscopic analyses, which has revealed an effective tridentate nature of the tetrapyrrolic PiCC ligand. The zinc and cadmium complexes show bright red luminescence, the nickel and copper complexes are non-luminescent. Binding of Zn- and Cd-ions to the PiCC ‘lights-up’ the intensive red fluorescence of the metal-complexes, which is detectable at nM levels of these closed shell metal ions. Formation of transition metal complexes with PiCCs, and related chlorophyll catabolites, may thus also occur in the tissues of plants, notably of ‘heavy metal (hyper)-accumulating’ plants.