 Abbreviations and Symbols for the Description of Conformations of Polynucleotide Chains

IUPAC-IUB Joint Commission on Biochemical Nomenclature, Recommendations1982, published in1983. Section2.3, N-Glycosidic Bond, defines chi using O4′–C1′–N9–C4 for purines and O4′–C1′–N1–C2 for pyrimidines, syn/anti regions and differences from older atom conventions. General notation and publication details: iupac.qmul.ac.uk/misc/pnuc1.html ; Pure and Applied Chemistry55,1273–1280.