The compound 2-[(1E)-2-(1H-pyrrol-2-yl)ethenyl]-quinoxaline (PQX) is a promising fluorescent chromophore for the estimation of protein binding site polarity, due to its full-color solvatochromic fluorescence. A linear relationship was obtained between the peak emission wavenumber and E(T)(N) (normalized solvent polarity). The BSA binding site polarity was estimated from the solvatochromic plot.

The quantum yields and the photostationary state isomer ratios in the photoisomerization of bilirubin are strongly governed by the properties of the reaction media. Among the reaction media examined, the quantum yields for ZZ→ZE isomerization (φZZ–ZE) and for cyclization (φLR) were the highest in buffered aqueous solution (potassium phosphate buffer, pH 7.4) containing human serum albumin (HSA) in a 1 : 1 molar ratio to ZZ-BR, 0.11 and 2 × 10−3, respectively, on 436 nm irradiation. The effects of the reaction media surrounding bilirubin are discussed.

The quantum yields of the isomerization and photostationary state isomer ratios of the photoisomerization of (4Z,15Z)-bilirubin IXα (ZZ-BR) were determined in an aqueous buffered solution in the presence of human serum albumin (HSA) at a molar ratio of [ZZ-BR]/[HSA] from 0.5 to 2. The BR isomer compositions in the photostationary state were constant at [BR]/[HSA] = 0.5—0.7. With increasing [ZZ-BR]/[HSA] from 0.8 to 2, the ZZ-BR composition in the photostationary state increased from 58 to 75%, but the ZE-BR composition decreased from 39 to 22%. The quantum yields in the isomerization of ZZ-BR to ZE-BR (ΦZZ→ZE) and a cyclized product, lumirubin (LR), (ΦZZ→LR), remained unvaried up to [ZZ–BR]/[HSA] = 1, but ΦZZ→ZE decreased while ΦZZ→LR increased along with a further increase of [ZZ-BR]/[HSA]. These results are explained by the existence of two binding sites, a first-class site and a second-class site, for bilirubin binding to HSA; ΦZZ→ZE in the second-class site (= 0.035) was as low as 1/3 of that in the first-class site (= 0.11), but ΦZZ→LR in the second-class site (= 4.2 × 10−3) was nearly two-times higher than that in the first-class site (= 2.4 × 10−3).

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