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.