  Radical-ion pairs, fundamental for understanding photosynthesis and the avian
magnetic compass, were recently shown to be biological open quantum systems. We
here show that the coupling of the radical-pair's spin degrees of freedom to
its decohering vibrational reservoir leads to a shift of the radical-pair's
magnetic energy levels. The Lamb shift Hamiltonian is diagonal in the
singlet-triplet basis, and results in a singlet-triplet energy splitting
physically indistinguishable from an exchange interaction. This could have
significant implications for understanding the energy level structure and the
dynamics of photosynthetic reaction centers.
