Configurational photoisomerization of bilirubin. I.Quenching of Z-->E isomerization by two-wavelength irradiation

A simple rate-equation theory of configurational photoisomerization of bilirubin (BR) is presented and used to discuss the Z→E photoisomerization of BR in HSA solutions irradiated with laser light in the blue-green spectral region. The analytical expression of Z, E-BR:HSA (PBR) concentration at photoequilibrium (PE) is used to derive the PBR concentration from the spectrophotometric data on absorbance difference spectra. The PBR values are in good agreement with those recently reported in the case of 10 nm-bandwidth filtered light. Both sets of data give a best-fit value of the BR, PBR quantum yield ratio equal to 2.3 in the case of BR:HSA ratio equal to 1:2. Successive irradiation of BR/HSA solutions with different wavelengths (457/488/457 nm) provides further support for the reversibility of the photoreaction. However, differences between the intermediate PE values have been found, and may reveal departures from the simple Z,Z→Z,E isomerization in BR/HSA solutions. The possibility of controlling the formation of Z, E-BR/HSA has been demonstrated using simultaneous irradiation with blue (457 nm) and green (514 nm) laser beams. The two-wavelength scheme takes advantage of the reversibility of the BRIPBR reaction, and of the differences between BR/PBR absorption spectra and quantum yields. Efficient quenching of PE PBR is achieved when green substantially exceeds (10–40 times) blue light intensity. This experiment may simulate what actually occurs during clinical phototherapy with white and green fluorescent lamps, where the quenching of PBR at the formation sites may prevent diffusion of this slowly excreted isomer and, at the same time, increase the formation of the more rapidly excretable structural, non-reversible isomers, such as lumirubin.