Continued from terms starting with R and S
Contents
Tertiary structure; threo; Topomerisation; Topomers; Torsion Angle; Torsional Stereoisomers; Torsional Strain; trans; Transannular Strain; transoid Conformation; Tub Conformation; Turnstile Rotation; Twist Form; u; Valence Isomerism; Walden Inversion; Wedge Projection; Z; Zig-zag Projection
The spatial organisation (including conformation) of an entire protein molecule or other macromolecule consisting of a single chain. See also primary structure, secondary structure, quaternary structure.
The identity reaction leading to exchange of the positions of identical ligands. The indistinguishable molecular entities involved are called topomers. For example in the reaction below the two identical ligating atoms Na and Nb are interchanged by rotation about the C-aryl bond but may be identified by n.m.r. spectroscopy. This is a degenerate isomerisation [See 'Glossary of terms used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994)].
Topomers See topomerisation.
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 proximal bond A-B is rotated in a clockwise direction through less than 180° in order that it may eclipse the distal 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 (s), those corresponding to torsion angles between ±90° and 180° anti (a). Similarly, arrangements corresponding to torsion angles between 30° and 150° or between -30° and -150° are called clinal (c) and those between 0° and ±30° or ±150° and 180° are called periplanar (p). The two types of terms can be combined so as to define four ranges of torsion angle; 0° to ±30° synperiplanar (sp); 30° to 90° and -30° to -90° synclinal (sc); 90° to 150°, and -90° to -150° anticlinal (ac); ±150° to 180° antiperiplanar (ap). 
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 (ap, sp, +sc, -sc +ac and -ac respectively). See 'Stereochemical Definitions and Notations Relating to Polymers', Pure Appl. Chem. 51, 733-752 (1981).
Stereoisomers that can be interconverted (actually or conceptually) by torsion about a bond axis. This includes E,Z-isomers of alkenes, atropisomers and rotamers.
Torsional Strain See eclipsing strain.
trans See cis, trans.
In medium-sized ring compounds, the strain due to repulsive nonbonded interactions between substituents or hydrogen atoms attached to non-adjacent ring atoms.
transoid Conformation (usage strongly discouraged)
See s-cis, s-trans. See cisoid for use of transoid as a stereodescriptor.
A conformation (of symmetry group D2d) of an eight-membered ring in which the four atoms forming one pair of diametrically opposite bonds in the ring lie in one plane and all other ring atoms lie to one side of that plane. It is analogous to the boat conformation of cyclohexane. See also crown conformation.
Turnstile Rotation See pseudo-rotation.
Twist Form See chair, boat, twist.
See 'Glossary of Terms Used in Physical Organic Chemistry', Pure Appl. Chem. 66, 1077-1184 (1994).
Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an chiral centre during a chemical reaction or transformation. It is also the configurational correlation when a chemical species Xabcd is converted into the chemical species Xabce having the same relative configuration. The configurational change when a chemical species Xabcd (where X is typically carbon), having a tetrahedral arrangement of bonds to X, is converted into the chemical species Xabce having the opposite relative configuration (or when it undergoes an identity reaction in which Xabcd of opposite configuration is produced) is called a Walden inversion or inversion of configuration. The occurrence of a Walden inversion during a chemical transformation is sometimes indicated in the chemical equation by the symbol shown below in place of a simple arrow pointing from reactants to products.
A stereochemical projection, roughly in the mean plane of the molecule, in which bonds are represented by open wedges, tapering off from the nearer atom to the farther atom. See J. Dale, Acta Chem. Scand. 27, 1115-1129 (1973). It is mainly used to illustrate the conformation of larger cycloalkanes e.g. cyclotetradecane:
A stereochemical projection for an acyclic molecule (or portion of a molecule) where the main chain is represented by a zig-zag line in the plane and the substituents are shown above or below the plane.
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