Heme: From quantum spin crossover to oxygen manager of life

The review discusses how the electronic structure of heme explains its central importance to oxygen-based life on Earth. Emphasis is on the chemical bonding of heme, its spin crossover, reversible O2 binding, and O-O bond activation, put in relation to its physiological functions. The review discusses the spectroscopic and computational data that have helped to elucidate the nature of this remarkable molecular system, how it works, and how it is tuned by a range of molecular strategies. This tuning enables heme to carry out the two essential functions required for oxygen management of life, i.e. reversible oxygen binding for transport and storage, and oxygen activation for use in catalytic processes.