Lanthanide ions as redox probes of long-range electron transfer in proteins

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Occupancy of the two calcium-binding sites of codfish parvalbumin by the redox-active probe ions Yb 3+ and Eu 3+ causes the average tryptophan (Trp) fluorescence lifetime in this protein to decrease and become non-exponential from the single exponential values found for Ca 2+ and La 3+ of 4.45 ns. These observations are interpreted in terms of an electron transfer (ET) deexcitation mechanism wherein excited singlet state Trp transfers an electron to the Ln 3+ ion, reducing it to the +2 oxidation state and producing a Trp cation radical. Back ET reestablishes the initial system. The driving forces, Δ G°, for the Eu 3+ and Yb 3+ ET systems are different, whereas the nuclear rearrangement factor, λ, and electron donor-acceptor coupling, H AB, of semiclassical ET theory should be nearly the same for both ions. This allows the λ-value to be determined from the measured rates (∼2.05 eV). Temperature dependence studies show that the rate constant for the Eu 3+ system is near the activationless maximum value. Yb 3+ and Eu 3+ are established as redox probes of long-range ET in proteins making Ln 3+-substituted calcium-binding proteins convenient model systems for studying the distance-dependence of ET.