Photoprocesses of merocyanine 540 bound to serum albumin and lysozyme

The binding of merocyanine 540 to either lysozyme or bovine serum albumin (BSA) in aqueous solution and the related photodecomposition processes were studied. Absorption, fluorescence and trans → cis photoisomerization demonstrate a shift from free dimers to monomers upon binding to BSA, in contrast to lysozyme, where the binding appears spectroscopically less pronounced. The quantum yield (Φred) of merocyanine damage is generally small (⩽0.0004). However, Φred is markedly enhanced upon binding and was found to be comparable to the quantum yields of protein oxidation, which are ca. 0.002. The mechanisms of protein oxidation were discussed. The major effect is electron transfer from aromatic amino acid residues of the protein to the triplet state of merocyanine 540.

► We found that the quantum yield of damage of merocyanine 540 is small, Φred ⩽ 0.0004.
► Upon binding of the dye to proteins Φred is markedly enhanced.
► The quantum yield of protein oxidation is also low and comparable to Φred.
► The mechanism of damage is attributed to electron transfer from aromatic amino acid residues to the triplet state of the dye.