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

Occupancy of the two calcium-binding sites of codfish parvalbumin by the redox-active probe ions Yb3+ and Eu3+ causes the average tryptophan (Trp) fluorescence lifetime in this protein to decrease and become non-exponential from the single exponential values found for Ca2+ and La3+ 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, Delta G degrees, for the Eu3+ and Yb3+ ET systems are different, whereas the nuclear rearrangement factor, lambda, and electron donor-acceptor coupling, H-AB, of semiclassical ET theory should be nearly the same for both ions. This allows the I-value to be determined from the measured rates (similar to 2.05 eV). Temperature dependence studies show that the rate constant for the Eu3+ system is near the activationless maximum value. Yb3+ and Eu3+ 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. (C) 1999 Elsevier Science S.A. All rights reserved.