IUPAC's Nomenclature of Organic Chemistry, 1979 edition, like earlier editions and like Beilstein's Handbuch der organischen Chemie, uses parentheses in formulae for side chains and square brackets for condensed chains. We try to follow this in our representations, e.g.
H2N-[CH2]3-CH(NH2)-COOH
(CH3)3N+-[CH2]2-OH = Me3N+-CH2-CH2OH
(HO)2P(O)- = H2O3P- = P-
A further example of this is given below in the representation of oligosaccharide chains.
Glcα1-[4Glcα1-]24(Xylpβ1-2)Glcα1-4Glc
or
1. IUPAC-IUB Joint Commission on Biochemical Nomenclature (JCBN), and Nomenclature Commission of IUB (NC-IUB),.Newsletter 1981, Arch. Biochem. Biophys., 1981, 206, 458-462; Eur. J. Biochem., 1981, 114, 1-4; Hoppe-Seyler's Z. Physiol. Chem., 1981, 362, I-IV; J. Biol. Chem., 1981, 256, 12-14.