Long-range electron transfer in metalloproteins

Abstract

Studies of the dependence of the rate of electron transfer (ET) on donor-acceptor separation and reaction driving force ( − †G°) in ruthenated derivatives of myoglobin (Mb) and cytochrome c (cyt c) have been performed. Measurements of MP* → Ru3+ ET (MP* is an electronically excited metalloporphyrin) at four fixed distances (His48, 81, 116, 12) in modified sperm whale Mb have defined limits for long-range heme: Ru electronic couplings. The lnkET values decrease exponentially with the donor to acceptor edge-edge distance; from a plot of InkET vs. d, a β of ∼ 0.9 Å−1 is obtained. The 3ZnP* → Ru3+ and Ru2+ → ZnP+ rates in Ru(His33)Zncyt c derivatives have been analyzed in terms of λ ∼ 1.2 eV and electronic couplings of roughly 0.1 cm−1. The electronic couplings for Ru(His39)Zncyt c reactions (∼ 0.2 cm−1) are twice as large as those for Ru(His33)Zncyt c, but the experimentally derived electronic coupling in Ru(His62)Zncyt c ( ∼ 0.01 cm−1) is much smaller than the couplings in either Ru(His33)Zncyt c or Ru(His39)Zncyt c. The donor-acceptor electronic coupling order, His39 > His33 ≪ His62, accords well with the effective lengths of calculated ET pathways for these derivatives. The evidence suggests that shortcuts in ET pathways that involve hydrogen bonds lead to stronger couplings than those requiring through-space jumps.