An X-ray absorption study of chemically modified bacterial photosynthetic reaction centers

Local structures around metal atoms in Ni and Zn substituted 132-hydroxy-bacteriochlorophyll a (Ni-HOB, and Zn-HOB) with and without insertion into the accessory bacteriochlorophyll a (BChla) sites in Rhodobacter sphaeroides photosynthetic reaction centers (RC) are studied using X-ray absorption spectroscopy. The penta-coordination of Ni-HOB and Zn-HOB in the RC is similar to that of BChl a in native RC, where the fifth ligand is from a nearby histidine residue. Average NiN and ZnN distances increase by 0.08 Å in Ni-HOB and decrease by 0.04 Å in Zn-HOB-Et2O as the modified chlorophylls bind with the RC due to the interaction between the protein and the macrocycle.