ABSTRACT
The living state is the electronically desaturated state of protein. When life originated three and a half billion years ago, our globe was covered by a dense layer of water vapor. There was no light and no oxygen near its surface. Albert Szent Györgyi in “The living state and cancer” [317]
A. SINGLET OXYGEN 1. Reactivity of oxygen
In order to understand the reactivity of molecular oxygen we have to understand its electronic structure [318]. One of the interesting characteristics of oxygen is the fact that it has two unpaired electrons occupying two different molecular orbitals. This type of structure is called a triplet state and means that oxygen is a diradical. Radicals are usually very reactive trying to find another electron with which they can pair. So it is quite surprising that oxygen does not react rapidly with organic molecules and we do not suffer spontaneous combustion. The reason why this does not happen is the Pauli Exclusion Principle. Most organic molecules exist in the singlet state, which means that all the electrons occupying the molecular orbitals have electrons with antiparallel spin. The reaction of a singlet molecule with a triplet molecule like oxygen to give a stable singlet product is spin forbidden by the laws of quantum mechanics. This is the Pauli Exclusion Principle, which states that no more than two electrons can occupy the same orbital and that these two electrons must have opposite spin.
