Absorption and fluorescence spectra of open-chain tetrapyrrole pigments–bilirubins, biliverdins, phycobilins, and synthetic analogues

Open-chain tetrapyrroles are ubiquitous and abundant in living organisms (algae, animals, bacteria, and plants), including examples such as bilirubin, biliverdin, phycocyanobilin, phycoerythrobilin, and urobilin. The open-chain tetrapyrroles, collectively termed bilins, arise from biosynthesis or degradation of tetrapyrrole macrocycles. Bilins are now known to play a wide variety of biological roles encompassing light-harvesting (in phycobiliproteins), photomorphogenesis, signaling, and redox chemistry. The absorption spectra of bilins spans the ultraviolet (UV), visible, to near-infrared (NIR) regions depending on the degree of conjugation, thereby providing a wide range of colors from red/orange to blue/green. The fluorescence intensity of bilins is often quite low and hence fewer spectra are available, but can be increased substantially by structural rigidification, as evidenced by the wide use of biliproteins as fluorescent labels. The present article describes a database of absorption and fluorescence spectra of bilins from natural and synthetic origins for 220 compounds (270 absorption and 13 fluorescence spectral traces). Spectral traces of bilins published over the past ∼50 years have been digitized and assembled along with information concerning solvent, photochemical properties (molar absorption coefficient and fluorescence quantum yield), and literature references. The spectral traces (xy-coordinate data files) can be viewed, downloaded, and accessed at www.photochemcad.com. The accessibility of spectral traces in digital format should facilitate identification and quantitative calculations of interest in diverse scientific areas.