Sme-seleci’ive Spectroscopy and Level Ordering in C-phycocyanine

Antenna pigments are biopolymers which contain several dye molecules which act as light absorber and energy transmitter (for reviews, see refs. [ 1,2]). Among these antennas, the biliproteins are probably the best characterized systems to date. They contain bile-pigment chromophores which are covalently bound to the apoproteins. The conformation of the polymer chain [ 31 and the chromophore structures [ 1 ] produce certain sites which in turn result in a well defined level ordering of the various S 1 states of the chromophores [ 41. Due to the large energy gaps and the generally weak coupling among the chromophores, the phycobiliproteins have been characterized in detail in recent years. They are antenna pigments of blue-green, red and cryptophyte algae, and are present in the former two as large, supramolecular aggregates, e.g. the phycobilisomes [ 11. The results obtained on these pigments led investigators to classify the dye molecules as s (sentizing) and f (fluorescing) chromophores [ 561 (for a refined treatment, see ref. [ 71). Naturally, the f chromophore is the dye molecule with the lowest S, energy. Clearly, the level ordering within the visible band of the pigments is important with respect to the photophysics and function of antenna pigments [ 81. Among the numerous papers on the photophysics of biliproteins, there are a few studies which are based on the spectral hole-burning technique [ 9111 (for a review, see ref. [ 121). At very low temperatures narrow dips (holes) could be burnt into the absorption of the pigments by irradiating the sample with narrow-bandwidth laser light. The detailed nature of the hole-burning photoreaction is, as yet, unknown. These studies revealed that the optical transition of the pigments are characterized by sharp zero-phonon lines. The occurrence of zero-phonon lines indicates that the excitation process of the chromophore does not lead to a reorganization of the “lattice” structure. Usually, zero-phonon lines are related to a rather rigid “lattice”. It is interesting to note that recent hole-burning studies on photosynthetic reaction centers of Rhodopseudomonas sphaeroides and Rhodopseudomonas viridis [ 13,141 revealed extremely broad holes which originate probably from a strong electron-phonon coupling in these systems [ 151. In the case of the biliproteins where the absorption transitions are characterized by sharp zero-phonon holes, one would expect that the fluorescence would also show a zero-phonon line structure, since absorption and emission are connected by a symmetry relation. However, this is not the case. The fluorescence is essentially broad and structureless, irrespective of the mode of excitation. In this paper we address this problem by comparing hole-burning and fluorescence spectra. We show that this puzzle is a consequence of the special level ordering in the pigment and the concomittant lack of correlation,