Pointgroup Symmetry of Oligomeric Macromolecules

Regular assemblies of proteins come only in certain flavours since there are only so many ways to assemble oligomers. Oligomeric assemblies obey specific symmetry rules and exhibit characteristic symmetry axes. A basic understanding of these symmetry rules may prevent one from adding (yet) another impossible oligomeric structure to the literature. For example, oligomers of a biological macromolecule cannot contain mirror symmetry planes since that would imply that the protein structure would for example contain both right-handed and left-handed α-helices. Mirror-symmetric pointgroups are possible for small metal clusters, since metal atoms themselves can have internal (mirror) symmetry. Similarly, the chain of a biopolymer in oligomeric assembly cannot cross a symmetry axis of the assembly: there necessarily must be “empty” space around every symmetry axis. Information about pointgroup symmetries of biological molecules may be found in various handbooks in X-ray crystallography.