Chiralpedia

Axial Chirality

Broader Concept: Chirality

Paired Concept: Chiral Center

Definition: Chirality arising from hindered rotation about an axis.
Context: Common in biaryls and allenes; impacts ligand and API design.
Example: BINAP, biaryl atropisomers.
Related Terms: Atropisomerism, Helicity.
Reference: IUPAC Gold Book.

Chiral Center (Central Chirality)

Broader Concept: Chirality

Paired Concept: Axial Chirality

Definition: A tetrahedral atom (usually carbon) bonded to four different substituents.
Context: Creates enantiomeric pairs; critical for drug selectivity and metabolism.
Example: The α-carbon of lactic acid.
Related Terms: Stereocenter, Enantiomer.
Reference: IUPAC Gold Book.

Chirality

Narrower Concepts: Central Chirality | Axial Chirality | Planar Chirality | Helical Chirality

Definition: The geometric property of an object or molecule that renders it non-superimposable on its mirror image. A chiral molecule and its mirror image exist as a pair of stereoisomers called enantiomers.
Synonyms: Molecular Handedness; Handedness (informal).
Context: Chirality is a fundamental concept in stereochemistry and plays a crucial role in chemistry, biology, pharmacology, agrochemistry, materials science, and molecular recognition. Because biological systems are inherently chiral, the two enantiomers of a chiral molecule may exhibit different pharmacological, toxicological, sensory, or environmental properties. Chirality can arise from a stereogenic center, axis, plane, helix, or other stereogenic elements.
Example: The two enantiomers of lactic acid are mirror images that cannot be superimposed on one another. Likewise, a person's left and right hands are macroscopic examples of chirality.
Related Terms: Chiral Molecule, Achiral Molecule, Enantiomer, Stereochemistry, Stereogenic Center, Axial Chirality, Planar Chirality, Helical Chirality.
Reference: International Union of Pure and Applied Chemistry. Compendium of Chemical Terminology (Gold Book): Chirality; Eliel, E. L., Wilen, S. H., & Doyle, M. P. (1994). Basic Organic Stereochemistry. John Wiley & Sons. ISBN: 978-0471374993; Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). Oxford University Press. ISBN: 978-0199270294; Morrison, R. T., & Boyd, R. N. (1992). Organic Chemistry (6th ed.). Prentice Hall, Englewood Cliffs, NJ. ISBN: 978-0136436691.
Key Distinction
Chiral: Non-superimposable on its mirror image.
Achiral: Superimposable on its mirror image.
Chirality: The property of being chiral.
Enantiomers: A pair of non-superimposable mirror-image stereoisomers resulting from chirality. Key Insight: Chirality is often described as "molecular handedness." Just as left and right hands are mirror images that cannot be perfectly overlaid, chiral molecules exist in mirror-image forms that may exhibit profoundly different behavior in biological and chemical systems. This phenomenon underpins the importance of stereochemistry in modern science and medicine.

DNA Chirality
Definition: DNA adopts right-handed helices (B-form) with chiral sugar backbone.
Context: Chiral recognition of intercalators and drugs depends on helix sense.
Example: D-sugar backbone in nucleic acids.
Related Terms: Helicity, Stereorecognition.
Reference: Watson & Crick; Voet & Voet (2011).

Exciton Chirality Method
Definition: Assigns absolute configuration from sign of ECD exciton couplets between interacting chromophores.
Context: Widely applied to biaryls and helicenes.
Example: Positive couplet → P helicity.
Related Terms: ECD, Exciton Coupling.
Reference: Harada & Nakanishi (1972).

Helical Chirality

Broader Concept: Chirality

Paired Concept: Planar Chirality

Definition: A form of stereoisomerism arising from the three-dimensional screw-like arrangement of atoms or molecular subunits, producing non-superimposable mirror-image structures distinguished by opposite helical handedness rather than a conventional stereogenic center.
Context: Helical chirality occurs when molecular architecture adopts a stable spiral or helical geometry that cannot be superimposed onto its mirror image. Unlike classical point chirality, which originates from tetrahedral stereogenic centers, helical chirality emerges from overall molecular topology and spatial organization. Helical chirality is commonly observed in: Helicenes (ortho-fused aromatic systems); Peptides and proteins (α-helices, collagen helices); DNA and RNA structures; Helical polymers; Supramolecular assemblies; Foldamers; Chiral nanomaterials Helical chirality is typically designated using: P (plus, right-handed) - clockwise screw sense; M (minus, left-handed) - counterclockwise screw sense; The stereochemical stability depends upon the barrier to helix inversion; sufficiently high inversion barriers permit isolation of distinct enantiomeric helices. Helical chirality influences: Molecular recognition; Circular dichroism (CD); Circularly polarized luminescence (CPL); Chiral catalysis; Biomolecular folding; Materials optical properties; Protein-ligand interactions
Example: DNA predominantly adopts a right-handed B-form helix, [6]Helicene exists as separable P and M enantiomeric helices, α-Helices in proteins are overwhelmingly constructed from L-amino acids, contributing to biological homochirality. Related Terms: Helicity (P/M); Axial Chirality; Topological Chirality; Homochirality; Conformational Chirality; Foldamers; Chiral Materials
Reference: IUPAC. Compendium of Chemical Terminology (IUPAC Gold Book). 2nd Edition, 1997 (updated 2019). Yashima, E.; Maeda, K.; Iida, H.; Furusho, Y.; Nagai, K. Helical Polymers: Synthesis, Structures, and Functions. Chemical Reviews, 109, 6102-6211 (2009). Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds. Wiley, New York (1994).

Memory of Chirality
Definition: Retention of stereochemical information through achiral or planar intermediates via conformational constraints.
Context: Enables net stereospecificity where racemization might be expected.
Example: Acylium ion cyclizations retaining chirality.
Related Terms: Stereomutation, Enantiospecificity.
Reference: Houk, Angew. Chem. (2001).

Planar Chirality

Broader Concept: Chirality

Paired Concept: Helical Chirality

Definition: Chirality resulting from the arrangement of substituents in a plane.
Context: Seen in ferrocene ligands and metallocenes used in asymmetric catalysis.
Example: Planar-chiral 1,2-disubstituted ferrocenes.
Related Terms: Helicity, Axial Chirality.
Reference: IUPAC Gold Book.

Prochirality
Definition: An achiral entity that can become chiral by a single desymmetrizing step.
Context: Basis for enantioface/enantioselective reactions in synthesis and enzymology.
Example: Prochiral ketones undergoing enantioselective reduction.
Related Terms: Re/Si Face, Pro-R/Pro-S.
Reference: IUPAC Gold Book.

Protein Homochirality
Definition: Proteins are composed almost exclusively of L-amino acids.
Context: Drives stereoselective binding and metabolism in biology.
Example: Enzymes discriminating D- vs L-substrates.
Related Terms: Homochirality, Stereorecognition.
Reference: Blackmond, PNAS (2004).

Stereogenic Axis (Axis of Chirality)
Definition: A linear element in a molecule that gives rise to chirality due to restricted rotation.
Context: Common in atropisomeric systems and cumulenes like allenes.
Example: Axially chiral biaryl ligands such as BINAP.
Related Terms: Atropisomerism; Axial Chirality; Planar Chirality.
Reference: IUPAC Gold Book (2019).

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.