POLARIZED SITE-SELECTION SPECTROSCOPY OF CHLOROPHYLL-A IN DETERGENT

Monomeric chlorophyll a (Chl a) was obtained from the isolated core antenna complex CP47 of photosystem II after incubation with the detergent triton X-100 and was studied by low-temperature polarized light spectroscopy with the aim to obtain model spectra for Chl a in intact photosynthetic complexes. Evidence is presented by circular dichroism and anisotropy measurements that the isolated chlorophyll is monomeric. The absorption bandwidths are relatively large compared to those found in photosynthetic complexes due to inhomogeneous broadening introduced by the detergent. By selective laser excitation at low temperature, considerable narrowing can be achieved. A number of vibrational bands are resolved in the site-selected, polarized absorption and fluorescence emission spectra. The emission spectrum of Chl a in detergent-damaged CP47 is compared with that of Chl a in the intact light-harvesting complex of photosystem II (LHC-II) from green plants. The spectra are remarkably similar indicating that the low-temperature thermal emitter in LHC-II has spectral properties that are very similar to those of monomeric Chl a.