Three-electron two-centred bonds and the stabilisation of cationic sulfur radicals

Electronic communication in biological systems is fundamental to understanding protein signalling and electron hopping pathways. Frequently studied examples are cationic radical methionine and its functional derivatives. These systems are understood to be stabilised by a direct 'three-electron two-centred' bond. We demonstrate for methionine and a series of cationic radical methionine analogues that long-range multi-centred indirect stabilisation occurs, which cannot be attributed to three-electron two-centred interactions. A revised description of the radical stabilisation process is presented, which includes contributions from all atoms with accessible p-orbitals, independent of the distance to the sulfur radical.