In cyanobacteria, the orange carotenoid protein (OCP) mediates nonphotochemical quenching of bilin excited states in the phycobilisome. An intrinsic ketocarotenoid serves as an ambient light-intensity sensor by triggering the conversion of the resting, orange form (OCPO) to the active, red form (OCPR), which can then bind to the phycobilisome's core. In this study, the photophysical response following optical preparation of the second excited singlet state, S2, of the ketocarotenoid canthaxanthin (CAN) in OCP isolated from Synechococcus sp. PCC 6803 is probed with broadband multidimensional electronic spectroscopy under excitation conditions that photoaccumulate the OCPR ground state. Time evolution of off-diagonal cross peaks in the two-dimensional electronic spectrum (2DES) reveals that nonradiative decay from the bright S2 state to the dark S1 state via the Sx intermediate is almost ten times faster in OCPR than for CAN in toluene solution. This pathway involves a coherent nonadiabatic mechanism mediated by out-of-plane vibrational modes of the isoprenoid backbone. Fourier transform amplitude spectra and oscillation maps show that rapidly damped wavepacket motions with respect to out-of-plane coordinates of the isoprenoid backbone are launched in the S2 state. These motions include a 400-cm-1 mode assigned to delocalized torsions. Quantum coherent mixing of S2 (and Sx) with S1 near a seam of conical intersections is indicated by upshifting of the C=C stretching mode frequency into the 1800–2100-cm-1 range, which suggests a contribution from intramolecular charge-transfer character. S1 then decays in 5.5 ps to yield a long-lived (8.6 ps) intermediate, S1', which exhibits a broad stimulated-emission spectrum not previously observed in CAN solution. These results indicate that the electronic structure of CAN in the OCPR state is significantly perturbed by the translation into the N-terminal domain that accompanies the photoactivation reaction.

Vibronic coherence of canthaxanthin in the orange carotenoid protein in the photoactivated red state

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