Photoprocesses in biliverdin dimethyl ester in ethanol studied by laser-induced optoacoustic spectroscopy (lioas)

Abstract

Laser-induced optoacoustic spectroscopy was applied to biliverdin dimethyl ester (BVE) in 95 % ethanol. The acoustic wave generated after dye-laser irradiation (λ exc = 630–680 nm) provided a measure of α, the “prompt” heat dissipation. The latter is denfied as the fraction of energy converted to heat by excited BVE in a time shorter than that (t 1, ca 1 μs) required to generate the acoustic wave. The value of α varied from 0.4 to 1 with the BVE concentration increasing from 10 −6 to 1.3 x 10 −5 M. The loss of heat at the low BVE concentrations is explained in terms of a photoisomerization of BVE, followed by thermal reversion to ground-state BVE in a time longer than t 1. Structural possibilities for the previously unknown photoisomer(s) are discussed. Static inhibition of the formation of the photoisomer(s) through some sort of association of an unidentified quencher with ground state BVE is proposed to cause the concentration dependence of α. The previously published scheme, which attributed the fluorescence of BVE to monomeric conformers, is thus extended to include the associated species which account for the concentration dependence of the optoacoustic data.