Abstract

The photosynthetic reaction center (RC) from purple photosynthetic bacteria is a membrane-bound protein-pigment complex that serves as an excellent model for studying biological energy conversion. This energy conversion takes place by electron transfer reactions, which occur within the protein and are often coupled to conformational changes. In order to study these conformational changes, recovery of the oxidized bacteriochlorophyll dimer, from the RC of the purple photosynthetic bacterium Rhodobacter sphaeroides , to its original state was measured by light-minus-dark optical difference spectroscopy. Laser flash excitation generated an electron transfer that takes place across the membrane; creating the primary charge-separated state with a lifetime of 100 ms. Prolonged illumination induced subsequent conformational rearrangements in the RC protein complex which result in lifetimes of the same charge-separated state that are significantly different from that measured after flash excitation. The structural details of the conformational rearrangements on the molecular level will be discussed. The conformational changes were sensitive to duration and wavelength of illumination, pH, temperature, hydrophobic environment (liposome or detergent), head-group charge of the detergent, and presence of a bound metal ion. By systematically varying these parameters, we were able to extend the lifetime of the charge-separated state up to 21 mins. Based on these results, our goal is to utilize the bacterial RC protein complex as a biocapacitor, since the positive and negative charges are separated by a hydrophobic core of the protein with a low dielectric constant. This biocapacitor can be discharged rapidly by inducing pH jump.