The flow of excitation energy in LHCII monomers: Implications for the structural model of the major plant antenna

Spectral and kinetic information on energy transfer within the light-harves ting complex II (LHCII) monomer was obtained from this subpicosecond transi ent absorption study, by using selective excitation (663, 669, 672, 678, an d 682 nm) of various Chl a absorption bands and detecting the induced chang es over the entire Q(y) region (650 -700 nm). It is shown that transfer fro m the pigment(s) absorbing around 663 nm to the low energy ones occurs in 5 +/- 1 ps, whereas the 670-nm excitation is delivered to the same "destinat ion" in two phases (0.30 +/- 0.05 ps, and 12 +/- 2 ps) and a fast equilibra tion (lifetime 0.45 +/- 0.05 ps) takes place within the main absorption ban d (675-680 nm). From comparison with results from similar time-resolved mea surements on trimeric samples, it can be concluded that the intramonomeric energy transfer completely determines the spectral equilibration observed i n native LHCII complexes. To correlate the measured lifetimes and their ass ociated spectra with the pigment organization within the available structur al model of LHCII (Kuhlbrandt et. al. 1994. Nature. 367:614-621), extensive but straightforward theoretical modeling was used. Thus it is demonstrated that the pigment assignment (Chl a or Chl b) given by Kuhlbrandt and co-wo rkers cannot simultaneously describe the dichroic spectra and the transient absorption results for the rather homologous LHCII and CP29 proteins. A mo re recent assignment for CP29, in which a Chl b molecule ("Chl b5") is iden tified as a Chl a (Dr. R. Bassi, personal communication), leads to a much b etter description of both CP29 and LHCII. Furthermore, the orientations of the transition dipole moments, which have not been obtained in the crystal structure, are now assigned for most of the Chl's.