SPECTRAL CHARACTERIZATION OF MONOMERIC C‐PHYCOERYTHRIN FROM PSEUDANABAENA W 1173 AND ITS α AND β SUBUNITS: ENERGY TRANSFER IN ISOLATED SUBUNITS AND C‐PHYCOERYTHRIN

— Molecular weight determination by gel chromatography and analytical ultracentrifugation revealed that C‐phycoerythrin from Pseudanabaena W 1173 exists as the monomer ∝βat a concentration up to 7μM and as the dimer (∝β) 2 at 40 μM. The purified subunits were monomeric species ∝ or β , respectively at 20 μM. Native monomeric C‐phycoerythrin was compared with the renatured subunits in a number of spectral properties: absorption spectroscopy including determination of molar extinction coefficients, fluorescence emission and excitation at 293 and 4 K, and fluorescence polarization. Whereas spectra of denatured subunits and denatured C‐phycoesythrin are identical, substantial changes occurred upon renaturation. Linear superposition of the absorption spectra of renatured subunits results in a curve identical with the absorption of native monomeric C‐phycoerythrin. The following conclusions were drawn: (a) Chromophore conformation is identical in renatured subunits and in native αβ monomer of C‐phycoerythrin. (b) In C‐phycoerythrin there are two fluorescing chromophores, degenerate in energy, one in the α ‐ and the other in the β ‐subunit and three sensitizing chromophores which differ in their energies and spectral shapes, (c) The energy transfer in C‐phycoerythrin occurs not only within but also between the subunits. (d) A weak fluorescence from sensitizing chromophores observed at low temperature (4 K) in the α ‐subunit and in C‐phycoerythrin shows the limitations of the concept of sensitizing chromophores. C‐PE, C‐phycoerythrin; P, degree of polarization; R, degree of anisotropy; SDS, sodium dode‐cyl sulfate; s 20 , sedimentation coefficient