CHROMOPHORE ASSIGNMENT IN PHYCOERYTHROCYANIN FROM MASTIGOCLADUS-LAMINOSUS

The component spectra (maxima of absorption, circular and linear dichr oism) of individual chromophores have been assigned for phycoerythrocy anin (PEC) trimer, monomer(s), and its subunits (cr-PEC and P-PEC) by titration with p-chloromercury-benzene-sulfonate (PCMS), linear dichro ism and photochemical transformations, as well as by deconvolution usi ng a 'bilin' line-shape spectrum based on the alpha-84 phycoviolobilin -chromophore in the alpha-subunit. The level ordering PVB-alpha-84 --> PCB-beta-155 --> PCB-beta-84 is the same irrespective of aggregation. Two different monomers (alpha beta) were observed. In 4 M urea, the s pectra are appropriately weighted sums of the subunit spectral whereas in the monomer obtained in 1 M KSCN, both beta-chromophores are red-s hifted by 4-5 nm. Formation of trimer (alpha beta)(3) gives considerab le spectral changes: (1) the absorption is narrowed, which has been ra tionalized by excitonic coupling between neighbouring monomers, (2) th e short wavelength part in the CD spectrum is missing and (3) a fourth band (+) at 528 in the LD spectrum appears. A deconvolution of the tr imeric aggregation state using only the 'bilin' line-shape model is no t possible.