Proton-Assisted Electron Transfer

Abstract

The case for highly selective long range “proton assisted” electron transfer in biomolecules (PA-ET), involving the hopping of protons and hydrogen atoms along H-bond chains connecting two redox sites, is discussed and analyzed on systems closely resembling typical biochemical sequences. These systems consist of an electron acceptor, an H-bond/covalent-bridge chain and an electron donor, and monohydroparabenzoquinone as the electron acceptor and a xanthine-like molecule as the electron donor and acceptor species held together by one or more peptide bridges. It is shown that, in biochemical structures, despite the involvement of the imidol (oximine) form of the peptide link, (a) PA-ET is energetically efficient and (b) the rate constants for proton-transfer, which is arguably the rate-controlling step, are reasonably high, the transfer times being on the order of hundreds of picoseconds.

Electron Transfer between Quinones in Photosynthetic Reaction Centers

“…ET from Q B can then occur by shifting two hydrogen atoms, as in the long-range proton-assisted electron-transfer (PAET) mechanism. 31,32 An attractive feature of the PAET mechanism is that the first proton transfer step could be the chemical process that stabilizes the charge-separated state, increasing its lifetime. Experimental evidence has in fact shown that, in carotenoid-porphyrin-quinone triads exhibiting photoinduced ET, the addition of a carboxylic group, in a position in which it can form an intramolecular H-bond with a quinone oxygen, significantly stabilizes the charge-separated state, probably by transferring a proton from the carboxylic group to the semiquinone anion.…”

Electron transfer rates and Franck–Condon factors: an application to the early electron transfer steps in photosynthetic reaction centers

“…Semiempirical MNDO/PM3 as well as unrestricted Hartee-Fock ab initio computations (with the standard 6-31G basis set and polarization functions on the atoms engaged in Hbonds), carried out on a model system of Zn substituted RC consisting of the two quinones and the metal ion with its full coordination sphere, confirm that ET from Q A to Q B can occur via the proton assisted electron transfer (PAET) mechanism schematized subsequently [42][43][44]: Upon the arrival of an additional electron on Q − A , the latter takes up the proton of the H-bonded HisM219, localizing the negative charge of the iron-histidine bridge, step (1). Since the two quinones are chemically equivalent and the driving force for ET is small, being provided by the different environments of Q A and Q B [45,46], the nuclear configuration formed after PT is nearly degenerate with that in which Q B is protonated and the negative charge is localized on HisL190, the iron histidine ligand opposite to HisM219, forming H-bond with Q B .…”

Elementary electron transfer reactions: from basic concepts to recent computational advances

“…The experimental Δ E of the three processes are also reported ,. The case of the pair ubiquinone/ubiquinone anion is intriguing for the huge difference with respect to the other two long‐range ET processes, which would suggest that different mechanisms must probably be invoked …”

“…92,93 The case of the pair ubiquinone/ubiquinone anion is intriguing for the huge difference with respect to the other two long-range ET processes, which would suggest that different mechanisms must probably be invoked. [94][95][96][97][98][99][100][101][102] In a discrete state approach to ET dynamics, such as that outlined before, the selection of the vibronic states to be used in the time evolution is probably the most important problem to deal with. In fact, as shown in Figure 5, the density of the vibrational states increases very rapidly as the internal energy increases, see Figure 5.…”