Excitation Dynamics in Phycoerythrin 545: Modeling of Steady-State Spectra and Transient Absorption with Modified Redfield Theory

Abstract

We model the spectra and excitation dynamics in the phycobiliprotein antenna complex PE545 isolated from the unicellular photosynthetic cryptophyte algae Rhodomonas CS24. The excitonic couplings between the eight bilins are calculated using the CIS/6-31G method. The site energies are extracted from a simultaneous fit of the absorption, circular dichroism, fluorescence, and excitation anisotropy spectra together with the transient absorption kinetics using the modified Redfield approach. Quantitative fit of the data enables us to assign the eight exciton components of the spectra and build up the energy transfer picture including pathways and timescales of energy relaxation, thus allowing a visualization of excitation dynamics within the complex.

Quantum Chemical Description of Absorption Properties and Excited‐State Processes in Photosynthetic Systems

“…The lower value reported in ref. [440] also fits to the excitonic coupling of 99 cm À1 obtained for the strongly coupled dimer in R-PE, a light-harvesting complex found at the top of phycobilisomes of red algae. [302] For this system, delocalized excited states were proposed, whereas a similar approach led to mainly localized states in the corresponding dimer in APC.…”

“…In view of the heterogeneous dielectric screening effect of the environment on the excitonic coupling by an average factor of about 0.3, [278] these deviations between unscreened calculated values [430] and those obtained by spectral modeling [440] are not surprising. The lower value reported in ref.…”

“…[430] In a later combined spectral modeling-quantum chemical study, Novoderezhkin et al noted that these coupling strengths should be significantly lower to be able to model the exciton dynamics in PE545 with modified Redfield theory. [440] The largest excitonic coupling determined in this study amounts to 92 cm…”

Estimation of damped oscillation associated spectra from ultrafast transient absorption spectra

“…When structural information is available, the dynamics of complex molecular systems can be studied with quantum mechanical methods, and time resolved spectra can be predicted, which has been done for the photosynthetic antenna complexes PE545 49,50 and PC577. 26 Here no structural information is used, nor quantum mechanical methods.…”

A coherent modified Redfield theory for excitation energy transfer in molecular aggregates

“…As a result, it correctly and smoothly interpolates between the Förster and the Redfield theories at their respective limits [27,28]. Thus, the modified Redfield theory has been a popular tool in studies of light harvesting EET [29,28,3,30]. Recently, Novoderezhkin and van Grondelle have comprehensively evaluated the validity of the modified Redfield theory by benchmarking it against other theoretical methods [28,4] and by applying it to fit experimental data [31,29,32].…”