|A functional_group is a simple molecular fragment that has a distinct elemental composition and a distinct mode of attachment to the rest of the molecule. Functional groups typically have less than 10 atoms and are frequently represented by the uppercase letter R. Examples are the methyl group, where R = -Me = 0CH3 and the acetyl group, where R = -Ac = -COCH3.
N.B. The curent version of GlycO does not suport the specification of intramolecular_link between a functional_group and another part of the molecule. That is, a functional group is merely a way of specifying that a residue, etc. contains a certain grouping of atoms. As an example, GlcNAc has an N-acetyl functional_group, but there is no need to specify an intramolecular link from the acetyl group to the nitrogen, as this is implied by the residue name GlcNAc.|