Total synthesis of cofacial chlorin dimers of two different symmetries - Models of photosynthetic reaction centers

In photosynthetic reaction centers, special pair (SP) functions as the primary and significant part of the electron transfer. The function of SP is resulted from the particular structure with two nearly two-folded bacterio(chlorophyll). When invoking models that mimic the reaction center and the SP, numerous systems based on porphyrin were performed. However they indicated the limited potential of electron and energy transferto compare to models based on natural chlorin pigments.

In this account, to achieve better understanding of the mechanism of electron and energy transfer in the reaction center, we focus on the basic principles underlying the design of artificial special pair. Toward this goal, two molecules were designed of having different symmetries and arranged in a cofacial fashion mimicking the special pair chlorophyll of naturally occurring photosynthetic reaction center. We employed chlorins as building blocks and they were connected at different positions to the diethynylbiphenylen spacer affording cis or trans orientations.

The two chlorins with iodine substituents were successfully synthesized based on the condensation of 4 heterocyclic building blocks. The diethynylbiphenylen spacer linked two chlorin subunits via shonogashira coupling affording the cofacial chlorin dimer. This construced model in trans symmetry mimicking the special pair of photosynthetic reaction center. The total synthesis of the cofacial chlorin dimer in cis orientation is still in progress.