Photoacoustic spectra of biliverdin dimethyl ester

Abstract

Biliverdin dimethylester (BVE) in ethanol and BVE incorporated into dipalmitoyl lecithine (DPL) vesicles were irradiated from 555 to 690 nm with a 15 ns tunable dye-laser pulse. A spectrum was obtained by plotting the first amplitude of the photoacoustic (PA) signal, measured with a piezoelectric detector and corrected for the different laser energies, as a function of the excitation wavelength. The system was calibrated using euso .. either in HzO-NHs or incorporated into DPL-Tris buffer vesicles. The PA signal of BVE in ethanol, on one hand, showed a "defect" at ca. 640 nm with respect to the absorption spectrum. This wavelength corresponds to the fluorescence excitation maximum of BVE in ethanol. However, the magnitude of the difference between the PA and absorption spectra indicates an additional occurrence of thermally reversible photoisomerizations. On the other hand, intensity of the PA spectrum for BVE-DPL, measured over a period of 20 h, decreased with time upon incorporation of the pigment into the vesicles. The absorption spectrum, showed no such time dependence. The time behaviour of the PA signal is consistent with our previous observations of an increase in fluorescence yield with time in BVE-DPL. The PA change is, however, of a much larger magnitude. These results are interpreted in terms of a chronological decrease in the radiationless transitions of BVE upon incorporation into the more rigid lipid bilayer, and an additional increase of a photoisomerization reaction of BVE which thermally reverts to its original state.