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

Abstract— 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; s₂₀, sedimentation coefficient