Selective interaction between xanthophylls and chlorophylls in LHCII probed by femtosecond transient absorption spectroscopy

We have performed femtosecond transient absorption measurements on trimeric light-harvesting complex II from spinach. Either chlorophyll (Chl) a (675 nm) or Chl b (650 nm) was excited, and the spectral response was probed for wavelengths longer than 470 nm. Excitation of Chl b led to instantaneous bleaching of two distinct Chl b bands with absorption peaks at 473 and 486 nm, corresponding to different Chl b subpopulations. The latter band probably also contains a contribution from a neoxanthin molecule, which is strongly coupled to Chl b. Most of the subsequent energy transfer from Chl b to Chl a occurs well within a picosecond, but energy transfer from the Chl b subpopulation with an absorption peak at 473 nm occurs faster (several hundreds of femtoseconds). Excitation of Chl b does not lead to a detectable instantaneous response of the lutein molecules with absorption peaks at 494 and 510 nm. This is in contrast with excitation of Chl a, which in turn does not lead to a detectable response of the neoxanthin or Chl b molecules. Excitation of Chl a leads to concomitant small bleachings of both lutein molecules at 494 and 510 nm (2% and 1% respectively of the Chl a bleaching) which we ascribe to excitonic mixing of the S, states of the lutein molecules and the Q(y) states of the Chl a molecules.