Vibronic effects in the spectroscopy and dynamics ofC-phycocyanin

Abstract

Femtosecond laser spectroscopies are used to investigate the influence of intramolecular nuclear modes on electronic relaxation in the cyanobacterial light harvesting protein, C-phycocyanin (CPC). Of particular interest are sub-ps dynamics localized on pairs of closely spaced phycocyanobilin pigments (i.e. dimers). Experiments conducted under different polarization conditions are used to distinguish isotropic and anisotropic vibrational modes within the dimers. Two isotropic nuclear modes are detected near 185 and 260 cm −1 using two-dimensional photon echo spectroscopy. In addition, a transient absorption anisotropy measurement reveals vibrational resonances associated with (out-of-plane) anisotropic nuclear modes near 640 and 815 cm −1. We investigate two possible origins for the recurrences in the anisotropy. A mechanism involving ground state nuclear coherences in the Condon approximation is ruled out by comparing the potential energy surfaces of the excitons to the direction of wavepacket motion. Electronic structure calculations suggest that non-Condon effects are the most likely explanation for the beats observed in the anisotropy. Such non-Condon effects also hold interesting implications for the vibronic exciton electronic structure of CPC. We calculate non-Condon intermolecular couplings in the dimer as large as 10 cm −1 , which suggests that these effects are not negligible and deserve further consideration. Our findings provide additional insights into the sub-100 fs vibronic relaxation channel found in the closely related protein, allophycocyanin, whose pigment dimers possess nearly the same geometry and intermolecular Coulombic interactions as CPC. This study underscores the complex interplay of intramolecular vibronic coupling and site energy tuning in photosynthetic light harvesting.

Disentangling Electronic and Vibrational Coherence in the Phycocyanin-645 Light-Harvesting Complex

“…26 2.067 eV , 601 nm Conversely, where two states involving different electronic states are involved the coherence decays in <300 fs. These results are not only self-consistent with the data presented here, but also with data obtained with other wavelength combinations and reported by other groups 15,19,[22][23][24] in the same or similar complexes.…”

“…Furthermore, the 100 meV energy difference between the states involved is consistent with previous work that has identified a HOOP (Hydrogen out of plane) wagging mode at 815 cm −1 (101 meV) in the related C-phycocyanin 22 and the work of Turner et al, 19 who identified a coherence at 26 THz (108 meV) that they attributed to a vibrational coherence based on rephasing and non-rephasing 2D spectra.…”

“…22 Similar modes have also been found in the traces of transient grating experiments on Allophycocyanin (APC) at 185-216 cm −1 (23-26 meV) 23 and 205 cm −1 (25 meV), 24 and again assigned to isotropic vibrational modes.…”

Vibronic enhancement of excitation energy transport: Interplay between local and non-local exciton-phonon interactions

“…29,[60][61][62][63] Using a vibronic exciton model, Womick and Moran suggested that vibronic resonance between the hydrogen out-of-plane mode and the electronic energy gap is responsible for much faster relaxation rate observed for APC despite the almost identical structures of pigment dimers (α84 and β84) in APC and CPC.…”

Energy transfer in structured and unstructured environments: Master equations beyond the Born-Markov approximations

“…[16][17][18][19] In addition to the (often low frequency) continuum, the environmental spectral densities of pigment-protein complexes are generally structured; that is, there are specific underdamped vibrational modes of the environment that couple strongly to the excitonic degrees of freedom. There is now increasing evidence to suggest that these underdamped modes are an important contributing factor to the long-lived coherences observed in photosynthetic systems, [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] as well as links between the quantum mechanical nature of these vibrational modes and enhanced transfer rates. 25,37,39,40 A multitude of powerful computational methods have been developed to deal with the difficulties faced in modelling strongly dissipative quantum systems.…”