The mathematical term gauche means not lying in a plane. In stereochemistry, specific conformations that are related by rotation about a single covalent bond between two tetrahedral atoms have been called gauche when none of the substituent atoms lie in the same plane (see Torsion Angle). Conformers where the substituents are staggered, and the torsion angle between the largest substituents on two neighboring carbons is about 60°, are termed gauche (1). These conformers are typically syn or anti clinal in the more rigorous designation of torsional conformers (see Conformation) (2). The alternative conformations are anti and eclipsed. The two gauche conformers and one anti conformer about the C4′—C5′ ribose bond of AMP are shown in Figure 1.
Figure 1. The conformations of the three rotameric conformers of 5′—AMP. The first two are gauche because the torsion angle defined by 05′—C5′—C4′—C3′ is between 0 and ±90°. The third conformer is anti because this torsion angle is 180°.
