An amino acid residue (other than glycine) has no symmetry elements. The general operation of conversion of one residue of a single chain into a second residue equivalent to the first is accordingly a rotation about an axis accompanied by translation along the axis. Hence the only configurations for a chain compatible with our postulate of equivalence of the residues are helical configurations.
Linus PaulingThe power to destroy the world by the use of nuclear weapons is a power that cannot be used - we cannot accept the idea of such monstrous immorality.
Linus Pauling