Exploring electron spin-spin interactions of paramagnetic iron and radical cations of bacteriochlorophyll from oxidized LH1 in the presence of electron transfer in the frozen state.

Cation free radicals of bacteriochlorophyll (BChl(+)) are formed in the light harvesting complex 1 (LH1) of photosynthetic bacteria upon oxidation by potassium ferricyanide. Unusually narrow EPR line widths are observed for BChl(+) in the frozen state. These narrow line widths are consistent with a molecular-wire behavior where rapid electron/hole transfer occurs between the BChl constituents of the pigment array responsible for light harvesting in bacterial photosynthesis. However, in addition to electron/hole transfer, two distinct types of spin-spin exchange could contribute the EPR line width narrowing, thus obfuscating the determination of LH1 as a molecular wire. First, because excess ferricyanide ion is always present during the EPR measurements, electron spin-spin interactions between the paramagnetic ferricyanide and BChl(+) could be a major source of the EPR line width changes previously attributed solely to electron/hole hopping within the array of BChl molecules in a LH1 unit. Fixing the potential of the ferricyanide/ferrocyanide redox couple gives a constant concentration of paramagnetic iron as the amount of BChl oxidized in LH1 changes. As long as the fraction of oxidized BChl in LH1 remains the same, the EPR line width is found independent of the concentration of the ferricyanide oxidant. Additionally, the trend in EPR line width as a function of temperatures depends only on the fraction of oxidized BChl and not on the concentration of ferricyanide ion. Second, spin-spin exchange interactions between BChl(+)s within LH1 rings could also change the EPR line width. Using LH1 preparations containing at most a few BChl cations per LH1 complex also eliminates the occurrence of significant electron spin-spin exchange as a cause of the observed line width narrowing in minimally oxidized LH1. This investigation of the two types of electron spin-spin exchange interactions demonstrates (1) that electron/hole hopping can take place in oxidized LH1 without involvement of paramagnetic ferricyanide or spin-spin exchange between BChl(+)s and (2) that LH1 maintains a molecular-wire nature at cryogenic temperatures.