Immunoglobulins are somewhat unusual in that they are biologically active proteins capable of carrying out several different functions. Thus, they can bind specifically with antigen, activate the complement system, mediate many cytotropic reactions, and act as antigen receptors on lymphocyte membranes.

The magnetic state of iron in haem proteins has long been recognized as a convenient indicator of chemical coordination as well as more subtle biochemical properties. In the trivalent case in particular, there are three magnetically distinct configurations of the five 3d electrons, yielding spin states of S = 1/2, 3/2 and 5/2. The first and third are extremely familiar, and often lie close enough in energy so that a thermal mixture of low-spin S = 1/2 and high-spin S = 5/2 states exists in an ensemble of molecules. Selection rules for common perturbations (ΔS = ∘, ± 1 for the spin-orbit interaction and ΔS = ∘ for the electronic Zeeman interaction) ensure that quantum mixtures, in which the wave function is a true combination of S = 1/2 and S = 5/2 components, are not observed. The mid-spin, S = 3/2, state, and allowed 5/2–3/2 and 1/2–3/2 quantum mixtures including it, are much less well known. These have only rarely been invoked in recent years as an explanation for experimental haem protein magnetic data.

At even the simplest level we can expect an ATPase mechanism to comprise the following four steps: the binding of ATP, the reaction of ATP with water on the enzyme, and the release of the products ADP and P1. So at the outset techniques are needed to investigate these four processes. The range of techniques needed is soon extended once questions are asked about the role of protons and metal ions, the possibility of a multistep hydrolytic process, multistep substrate and product binding processes, and protein–lipid or protein–protein interactions. Since ATPases and ATP synthases are almost universally involved in some form of energy transduction there is a particular need in an ATPase or ATP synthase reaction to evaluate the equilibrium constants of the steps in the mechanism and to investigate the possibility of alternate reaction pathways. The nature of the coupling process by the protein of the chemical reactions of ATP to the other energetic process, be it muscle contraction, active transport, respiration or photosynthesis, is likewise of profound interest. Finally we would like to know as much as possible about the ATPase or ATP synthase mechanism during the period when the various forms of energy transduction are occurring.

The concept of processes in a living organism as of a most complex enzyme-catalysed system of coupled chemical conversions following both physical and chemical laws, opens up new possibilities of cognizance and control of vital processes.