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.