AN X-RAY-ABSORPTION STUDY OF CHEMICALLY-MODIFIED BACTERIAL PHOTOSYNTHETIC REACTION CENTERS

Local structures around metal atoms in Ni and Zn substituted 13(2)-hyd roxy-bacteriochlorophyll a (Ni-HOB, and Zn-HOB) with and without inser tion into the accessory bacteriochlorophyll a (BChla) sites in Rhodoba cter sphaeroides photosynthetic reaction centers (RC) are studied usin g 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 Angstrom in Ni-HOB and decrease by 0.04 An gstrom in Zn-HOB-Et(2)O as the modified chlorophylls bind with the RC due to the interaction between the protein and the macrocycle.