Fig. 3.

Model of the molecular changes in LH2 upon switching between the observed states. Each BChl in the B850 ring is represented as a brown ellipse. State A shows photoactivated switching to state B. According to the model, this is a conformational change to a quenched form, with increased vibrational dissipation of excitation energy, that thermally reverts to the ground state. State A also shows photoactivated switching to state C, but with a nonlinear dependence on excitation fluence. According to the model, this is a photobleaching of a BChl in the B850 ring, represented by a gray ellipse, lower right. The state C to A transition is represented as a dotted line because of the rarity of this process. Switching between states C and B also is observed and most likely corresponds to a conformational change in an LH2 with a photobleached BChl in the B850 ring. The two forms of state B (with and without a photobleached pigment) overlap spectroscopically and so cannot be separated clearly. Finally, because state C appears only at high excitation fluence, the intensity dependence of the transitions to state B cannot be characterized.