... and in biochemistry

By selecting a limited number of atoms (20-60) from the active site of an enzyme, bonding and reaction mechanisms can be studied with quantum-chemcial methods. The surrounding protein is described with simpler methods. The model below shows a possible mechanism of how myoglobin in muscles protects itself agains carbon monoxide poisoning. Using hydrogen bonding to one of the amino acids in the protein, oxygen can overcome the competition from the dangerous carbon monoxide.