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 inve stigated by Fourier-transform infrared spectroscopy (FT-IR). Differenc e spectra between the parent states P566 and P507 mere obtained in (H2 O)-H-1 and 2H(2)O for the first time. The spectra are generally charac terised 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 alp ha-subunit and the (a beta)(3) trimer are compared, showing distinct d ifferences which may be of relevance for the chromophore-protein and p rotein-protein interactions. The difference spectra demonstrate many s imilarities to the spectra recently obtained for the P-r --> meta-R(c) transition of phytochrome [Foerstendorf et al. (1996) Biochemistry 35 , 10793-10799]. In particular, a band around 1710 cm(-1) which was ten tatively assigned to the C = O stretch of ring d is also observed in t he spectra of PEC. It strongly supports this identification and the de duced molecular interpretation on the protonation state of the chromop hore.