Anti Conformation
 
 Anti is used as a prefix to describe the relative orientation of two substituents on adjacent atoms of a molecule when their torsion angle is about 180° (1). In addition to its formal usage, anti has become broadly used to describe the orientation of the nucleic acid bases relative to the ribose ring when describing the torsion angle about the glycosidic bond (2). The glycosidic bond torsion is defined by the relative positions of the four atoms surrounding this bond, O4′—C1—N9  (1—C4(2)) for the purine (pyrimidine) nucleosides (3). When C4 is not over the ribose ring, the conformation is anti, corresponding to torsion angles of –80 to –180 and 100 to 180°. Conformations where the purine ring is over the sugar are syn and correspond to torsion angles of –80 to 100°. These conformations for adenine are shown in Figure 1. 

Figure 1. The anti and syn conformations of dAMP are shown. The bonds defining the glycosidic torsion angle are darkened for emphasis in both conformers. In the anti conformer, the torsion is 110° , and in the syn conformer the torsion angle is +60.
Reference
1. W. Klyne and V. Prelog (1960) Experientia 16, 521. 
2. A. E. V. Haschemeyer and A. Rich (1967) J. Mol. Biol. 27, 369–384. 
3. IUPAC-IUB Commission on Biochemical Nomenclature (1983) Eur. J. Biochem. 17, 193–201.