Torsion angleIUPAC Term

In a chain of atoms A-B-C-D, the dihedral angle between the plane containing the atoms A,B,C and that containing B,C,D. In a Newman projection the torsion angle is the angle (having an absolute value between 0 and 180 ) between bonds to two specified (fiducial) groups, one from the atom nearer (proximal) to the observer and the other from the further (distal) atom. The torsion angle between groups A and D is then considered to be positive if the bond A-B is rotated in a clockwise direction through less than 180 in order that it may eclipse the bond C-D: a negative torsion angle requires rotation in the opposite sense. Stereochemical arrangements corresponding to torsion angles between 0 and 90 are called syn ( i s /i ), those corresponding to torsion angles between 90 and 180 anti ( i a /i ). Similarly, arrangements corresponding to torsion angles between 30 and 150 or between -30 and -150 are called clinal ( i c /i ) and those between 0 and 30 or 150 and 180 are called periplanar ( i p /i ). The two types of terms can be combined so as to define four ranges of torsion angle; 0 to 30 synperiplanar ( i sp /i ); 30 to 90 and -30 to -90 synclinal ( i sc /i ); 90 to 150 , and -90 to -150 anticlinal ( i ac /i ); 150 to 180 antiperiplanar ( i ap /i ). The synperiplanar conformation is also known as the syn- or cis-conformation; antiperiplanar as anti or trans and synclinal as gauche or skew. For macromolecular usage the symbols T, C, G , G-, A and A- are recommended ( i ap /i , i sp /i , i sc /i , i -sc /i , i ac /i and i -ac /i respectively).