Electron transfer

Electron transfer is a redox process in which an electron moves from an electron donor to an electron acceptor. The donor is oxidized and the acceptor is reduced; the two events are therefore complementary parts of one reaction. Transfer may occur between separate molecules, between metal centers in a coordination compound, across a solid interface, or through a chain of cofactors in a protein.¹

The electron can move completely from donor to acceptor, as in many ionic reactions, or be shared to a measurable degree in a mixed-valence material. Electron transfer is distinct from proton transfer, although proton and electron movements frequently become coupled in proton-coupled electron transfer. In biological systems, the relevant donor and acceptor are often separated by a few nanometres, with intervening bonds and molecular groups mediating the pathway.

The direction of electron transfer is governed by free energy, while its speed depends on the activation barrier and the coupling between donor and acceptor. A transfer is thermodynamically favorable when the reaction free energy is negative, but favorable reactions can still be slow if nuclear rearrangement or weak electronic coupling creates a substantial barrier.

Marcus theory describes this barrier using the driving force, the reorganization energy, and electronic coupling. Reorganization energy includes changes in the geometry of the reacting molecules and in their surrounding solvent or lattice. The theory predicts the ordinary “normal” region, where greater driving force accelerates transfer, and the less intuitive inverted region, where an excessively favorable reaction can become slower because the nuclear configuration must reorganize more extensively.²

Electron-transfer chains convert redox energy into gradients, chemical bonds, and electrical currents. In cellular respiration, electrons pass through membrane proteins and ultimately reduce oxygen, while the associated free energy helps establish a proton gradient used to make ATP. In photosynthesis, light excites pigments and initiates charge separation, directing electrons through cofactors whose arrangement limits wasteful recombination.

Proteins control transfer by placing cofactors at suitable distances and orientations and by tuning their local electrostatic environments. The same principles support fuel cells, batteries, corrosion, molecular electronics, redox catalysis, and photochemistry. In electrode reactions, heterogeneous electron transfer couples a solution species to a conductive solid; the rate depends on electrode potential, surface structure, solvent organization, and mass transport as well as molecular chemistry.

Electron transfer can occur without a conventional covalent bond and can proceed by tunneling through a barrier rather than by an electron physically crossing the intervening space in a classical trajectory. In proteins, short-range tunneling is commonly mediated by covalent bonds, hydrogen bonds, and aromatic residues; small changes in distance can therefore produce large changes in rate.

Long-range biological transfer is not limited to soluble cofactors. Certain bacteria use extracellular electron-transfer pathways to exchange electrons with minerals or electrodes, a capability studied in microbial fuel cells and biogeochemistry. Another subtle case is electron transfer dissociation, an analytical-chemistry method in which reagent ions transfer electrons to multiply charged biomolecular ions, causing distinctive fragmentation. Electron transfer may also be coupled to bond breaking, proton motion, spin changes, or structural switching, so its observable outcome is not always simply a change in formal oxidation number.

Electron donor

A species that gives up an electron and is oxidized.

Electron acceptor

A species that receives an electron and is reduced.

Reorganization energy

The energy required to rearrange nuclear positions and the surrounding environment for electron transfer.

Electronic coupling

The quantum-mechanical interaction that permits an electron to move between donor and acceptor states.

Redox reaction

A reaction involving coupled oxidation and reduction processes.