Propentdyopents: Brief history of a family of dipyrrolic pigments

Propentdyopents are naturally occurring dipyrroles deriving from the metabolism of heme and characterized by a dipyrrin-1, 9-dione motif. The unusual name propentdyopent is due to the first colorimetric method (the Stokvis reaction) for the detection of these compounds, which were initially isolated from urine samples. Upon reduction in alkaline solutions, they produced red species that were termed pentdyopents to describe with Greek numerals their absorption maximum (525 nm) in the visible range. The precursors to the red pentdyopents were thus indicated as propentdyopents. Over the course of several decades, these dipyrrolic compounds have appeared in several studies of human physiology, typically associated to conditions of abnormal heme metabolism and/or oxidative stress. Concurrently, synthetic investigations have confirmed their chemical structure, reactivity, and ability to coordinate metals as bidentate monoanionic ligands. Notably, the planar dipyrrindione platform can undergo reversible one-electron redox processes and thereby act as an electron reservoir in metal complexes. In combination with the documented ability of the carbonyl groups to act as hydrogen-bonding acceptors, the coordination chemistry of propentdyopents could lead to new applications for this old class of pigments. Furthermore, the observation of these pigments in several clinical contexts could potentially delineate a role of propentdyopents as diagnostic biomarkers. This mini-review summarizes both the chemistry and biology of propentdyopents while highlighting the ample space for new discoveries.