Threo-
Definition: A relative stereochemical descriptor for diastereomers with two adjacent chiral centers having substituents on opposite sides in a Fischer projection.
Context: The opposite of erythro-; used in carbohydrate and amino acid stereochemistry.
Example: Threo-2,3-butanediol has hydroxyl groups on opposite sides.
Related Terms: Erythro-; Diastereomer; Relative Configuration.
Reference: Eliel & Wilen (1994).
Topicity
Definition: Relationship of identical groups/sites: homotopic, enantiotopic, diastereotopic.
Context: Guides selectivity predictions in synthesis and NMR.
Example: Diastereotopic methylene hydrogens in chiral environments.
Related Terms: Enantiotopic, Diastereotopic.
Reference: Eliel & Wilen (1994).
Topological Chirality
Definition: Chirality arising from molecular topology (e.g., knots, catenanes) rather than stereocenters.
Context: Inspires novel drug-like architectures and materials.
Example: Molecular trefoil knots.
Related Terms: Axial Chirality, Helicity.
Reference: IUPAC Gold Book.
Turbo Chirality
Definition: A higher-order chirality phenomenon in amino acid and peptide derivatives in which planar amide and carboxyl groups arrange as propeller-like blades around an α-carbon, amplifying chiral expression beyond classical point chirality.
Context: Conventional descriptors such as R/S or L/D do not fully capture this higher-order stereochemical behavior in certain amino acid and peptide derivatives. Analysis of X-ray structures of N-acetyl amino acids and the peptide biphalin suggests that surrounding functional groups organize into a directional, propeller-like motif, which they present as a distinct chirality phenomenon referred to as turbo chirality.
Example: An amino acid derivative in which the amide and carboxylic acid substituents around the α-carbon are not merely attached to a chiral center, but are spatially arranged as coordinated "propeller blades," producing a stronger, more distributed chiral signature than point chirality alone. This behavior is demonstrated by the opioid peptide biphalin.
Related Terms: Absolute Configuration; Homochirality; Peptide Chirality; Stereogenic Center; Chiral Fidelity; Conformational Chirality.
Reference: Yuan, Q.; Pandey, A.; Liu, H.; Bouley, B.; Li, Z.; Zhu, H.; Liang, R.; Li, G. A New Chirality Phenomenon in Amino Acid and Peptide Derivatives. ChemRxiv (2026). DOI: 10.26434/chemrxiv-2026-kncjf.
Twist-Boat Conformation
Definition: A less strained intermediate form of cyclohexane between the boat and chair conformations.
Context: Has lower energy than pure boat form; relevant in conformational equilibria.
Example: Cyclohexane adopts twist-boat at high energy transition states.
Related Terms: Boat Conformation; Chair Conformation; Conformational Isomer.
Reference: Eliel & Wilen (1994).