Illustrating And Investigating The Electronic Couplings And Electronic Energy Transfer Pathways Within Cyanobacteria Phycocyanin(C-PC) And Allophycocyanin (A-PC)

Electronic Energy Transfer (EET) is widely used to understand photosynthesis systems, also to measure the distance between some molecules and active sights in protein structure, also used to promote the acceleration of photodegradation of polymers. Cyanobacteria employ particular evolved antenna systems to trap the solar energy and convert it into chemical energy which transferred along the network of chromophores and eventually reach the reaction enter to contribute in charge transfer, this collected energy is the backbone of the photosynthesis process in cyanobacteria. The key feature of EET study in Cyanobacteria is providing massively detailed structural data with theory of energy transfer EET (dipole-dipole approximation) and the photo-process measurements.The key illumination of this theoretical work is specifying the detailed structural information (the energy transfer pathways) of antenna complex systems of light-harvesting and indicating to interesting effective sides of Electronic Energy Transfer (EET) that may be measured by obtaining the interplay between electronic coupling among molecules and their interaction with the protein environment of supervening photo-processes.(EET) process has been the main field research because of its fundamental contribution to the photophysical properties of diverse systems.Forster Theory has enabled the efficiency of (EET) to be predicted and analyzed in numerous and various areas of study by the dipole-dipole approximation "Forster Theory".The electronic energy transfer pathways in trimeric and hexameric aggregation state of cyanobacteria C-phycocyanin (C-PC) and the trimeric aggregation state of Allophycocyanin (A-PC) were investigated and determined by adopting Forster theory. This calculating method involves the nonradiative dipole-dipole coupling approximation, to set the delocalized excitations, short range/strong interactions and long range/weak interactions are taken in account along the entire network of the chromophores (Donors and Acceptors).This method assumes that the electronic energy transfer rate from donor (D) to acceptor (A) is definitely smaller than the vibrational relaxation rate of the chromophore during the excitation process.This requirement leads that once the electronic energy is transferred by resonance to the acceptor (A), there is very tiny possibility of a back transfer to the donor (D).The corresponding excited states for each chromophores and transition dipole moments of phycocyanobilins (PCBs) located into C-PC were computationally investigated by model chemistry in gas phase at time-dependent density functional theory (TDDFT), configuration interaction singles (CIS), and Zerner’s intermediate neglect of differential overlap (ZINDO) levels, respectively.This theoretical study proposed that the predominant energy pathway in C-PC protein monomer is β-155to β-84(1/K=13.4ps), in case of trimer, the predominant pathway is a-84of one monomer in the structure to (3-84(1/K=0.3-0.4ps) in an adjacent monomer in C-PC trimer, in the hexameric aggregation, the energy pathway was predicted from (3-155(or α-84) in upper trimer to neighbor β-155(or a-84)(1/K=0.5-2.7ps) which located at the bottom trimer in the structure of C-PC protein.For Allophycocyanin, the compatible excited states and transition dipole moments (PCBs) were investigated by models of crystal structure, optimized gas phase at time-dependent density functional theory (TDDFT), and ONIOM model by implementing optimization for each chromophore in one fragment splitted from the whole protein structure, The depth of low layer reached3A (Shell) from the high layer (PCBs molecule), the equivocation on the fragment structure orientation possibilities were taken in account by using’ NoSymm’keyword.The calculated electronic coupling by utilizing Forster Theory model at TDDFT/B3LYP/6-31+G*level were achieved in outstanding efficiency, it suggested that the faster energy transfer pathways were from one monomer to another adjacent monomer in trimer, such as α1-84â†'β3-84(0.51ps), β1-84â†'α2-84(0.14ps) and α1-84â†'β1-84(0.09ps), while the energy transfer rates in APC monomer were always larger than200ps, these calculated results are in qualitative agreement with experimental finding that a very fast lifetime of0.43-0.44ps for trimers, while APC monomers lacked any corresponding very fast lifetime.The long-range pigment-protein interactions for (PCBs) in Phycocyanin were approximately taken into account by using polarizable continuum model (PCM) at TDDFT level to estimate the influence of protein environment on the preceding calculated physical quantities.The influence of the short-range interaction (which dependent mainly on interchromophore overlap) caused by aspartate residue nearby PCBs was examined as well. Only when the protonation of PCBs and its long-and short-range interactions were properly taken into account, the calculated energy transfer rates (1/K) in the framework of Forster model at TDDFT/B3LYP/6-31+G*level were in good agreement with the experimental results of C-PC monomer and trimer. Our calculated results suggested that the energy transfer pathway in C-PC monomer is predominant from β-155to β-84(1/K=13.4ps), however, from α-84of one monomer to β-84(1/K=0.3-0.4ps) in a neighbor monomer in C-PC trimer. In C-PC hexamer, an additional energy flow was predicted to be from β-155(or a-84) in top trimer to adjacent b-155(or a-84)(1/K=0.5-2.7ps) in bottom trimer.In this study, we illustrated the main features of photophysical process in antenna systems, by summarizing how electronic excited states develop on different timescale along the entire network of the chromophores in terms of exciton relaxation for each state of the chromophore, localization of this excited energy, and determining the energy transfer to another chromophore, in hence to reach the reaction center. In addition, we have studied the influence of ASP residues and their contribution to the electronic coupling, we have computationally and schematically specified their effect on excited energy localization in these systems which contribute and collaborate mainly to the electronic coupling which play the major and the main role in electronic energy transfer (EET) calculations.