The fluorescence of biliverdin dimethyl ester

Freshly prepared solutions of biliverdin dimethyl ester (2) in ethanol showed fluorescence maxima at 710 and 770 nm [ΦF = 1.1. 10−4 (room temperature) and 5.0 10−4 (77 K)]. The maxima of monoprotonated 2 at 77 K were shifted to 725 and 806 nm and the quantum yield was increased to 2.6. 10−2. This acid effect was reversible by neutralization with base. When a neutral solution was kept standing in the dark at room temperature, or when an acidic solution was neutralized by base, an additional fluorescence maximum at 500 nm with a mirror image excitation spectrum with λmax = 470 nm developed, which disappeared on addition of acid and which is attributed to a chemical change of 2.