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

In photosynthetic reaction center, the 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). In this account, 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 dimmer in cis orientation is still in progress.