Phytochrome Models, II. The Fluorescence of Biliverdin Dimethyl Ester1)

✍ Scribed by Holzwarth, Alfred R. ;Lehner, Harald ;Braslavsky, Silvia E. ;Schaffner, Kurt

• Publisher: Wiley (John Wiley & Sons)
• Year: 1978
• Weight: 843 KB
• Volume: 1978
• Category: Article
• ISSN: 0074-4617
• DOI: 10.1002/jlac.197819781215

✦ Synopsis

Abstract

Biliverdin dimethyl ester (2) in freshly prepared ethanolic solution showed fluorescence maxima at 710 and 770 nm which were shifted to 725 and 806 nm on monoprotonation. Φ~F~ for 2 and 2 H ^+^ was similar at room temperature (1.1 · 10^−4^ and 2.7 · 10^−4^, respectively), and it increased at 77 K to 5.0 · 10^−4^ for 2 and to 2.6 · 10^−2^ for 2H^+^. This acid effect was reversed on neutralization. The p__K__~a~ values of 2 were 4.3 ± 0.1 at room temperature and ca. 5.3 at 77 K, and those of the fluorescent state were higher by Δp__K__ = 1.5 and 4.4, respectively. Certain differences between the absorption and fluorescence excitation spectra reflect either anomalous relaxation properties of excited *2 and *2H^+^ or non‐identity of the emitting species with the predominant ground state absorbers, or both. The changes of Φ~F~ with temperature and protonation of 2 are interpreted in terms of radiationless deactivation induced by twisting around the methine‐ring bonds and by intramolecular proton exchange in 2. These conclusions are supported by fluorescence measurements of the pyrromethenes 4, 5, and 8, the pyrromethenone 9, and bilirubin dimethyl ester (10). When a degassed neutral ethanolic solution of 2 was left to stand in the dark at room temperature, or when an acidic solution was neutralized, a new fluorescence maximum at 500 nm (excitation spectrum at 77 K: Δ~max~ = 470 nm) developed. This emission disappeared again on addition of acid. It is ascribed to 10‐ethoxy‐ or 10‐hydroxybilirubin dimethyl ester (12) whose formation by nucleophilic addition of solvent or trace water to 2 is reversed on protonation, affording 2H^+^.