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

✍ Scribed by Ronald M. Supkowski; James P. Bolender; Wendy D. Smith; Lewis E.L. Reynolds; William DeW. Horrocks Jr

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 151 KB
• Volume: 185-186
• Category: Article
• ISSN: 0010-8545
• DOI: 10.1016/s0010-8545(98)00276-8

✦ Synopsis

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, DG°, for the Eu 3 + and Yb 3 + ET systems are different, whereas the nuclear rearrangement factor, l, and electron donoracceptor coupling, H AB , of semiclassical ET theory should be nearly the same for both ions. This allows the u-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