Z,E isomerization of the alpha-84 phycoviolobilin chromophore of phycoerythrocyanin from Mastigocladus laminosus investigated by Fourier-transform infrared difference spectroscopy.

The photoreaction of the phycoviolobilin (PVB) chromophore-containing part of phycoerythrocyanin (PEC) from Mastigocladus laminosus was investigated by Fourier-transform infrared spectroscopy (FT-IR). Difference spectra between the parent states P566 and P507 were obtained in 1H2O and 2H2O for the first time. The spectra are generally characterised by large changes in the range between 1710 and 1590 cm(-1) and by a strong difference band around 1270 cm(-1). In order to study the influence on the PVB chromophore upon aggregation, spectra of the alpha-subunit and the (alphabeta)3 trimer are compared, showing distinct differences which may be of relevance for the chromophore-protein and protein-protein interactions. The difference spectra demonstrate many similarities to the spectra recently obtained for the Pr --> meta-Rc transition of phytochrome [Foerstendorf et al. (1996) Biochemistry 35, 10793-10799]. In particular, a band around 1710 cm(-1), which was tentatively assigned to the C = O stretch of ring D is also observed in the spectra of PEC. It strongly supports this identification and the deduced molecular interpretation on the protonation state of the chromophore.