Chiralpedia

Hashed (Wedge-Dash Representation)
Definition: A visual notation in molecular drawings where hashed bonds represent bonds going behind the plane of the paper.
Context: Used to depict stereochemistry in three-dimensional projections.
Example: Hashed bond to indicate the backward substituent in Fischer or wedge projections.
Related Terms: Wedged; Fischer Projection; Newman Projection.
Reference: Morrison & Boyd. Organic Chemistry. Prentice Hall, 1992.

Helical 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).

Helicity (P/M)
Definition: Screw sense of helical molecules designated P (plus) or M (minus).
Context: Describes helicenes and protein helices relevant to binding.
Example: P-helicene vs M-helicene.
Related Terms: Axial Chirality, Planar Chirality.
Reference: IUPAC Gold Book.

Heterochiral

Paired Concept: Homochiral

Definition: Describing a molecular system, assembly, crystal, material, or mixture containing both enantiomeric forms, or chiral components of opposite handedness.
Context: Heterochirality represents the coexistence of opposite stereochemical forms within the same system. In chemistry, heterochiral arrangements may exhibit distinct thermodynamic stability, crystal packing, supramolecular organization, and biological behavior compared with corresponding homochiral systems. In supramolecular chemistry and crystallography, heterochiral assemblies often compete with homochiral assemblies, and the balance between them can strongly influence material properties. In biological systems, heterochiral combinations are generally uncommon but can occur in specialized circumstances, such as D-amino acids in bacterial cell walls or synthetic peptide systems.
Example: A racemic crystal containing both R- and S-enantiomers in the same crystal lattice is a heterochiral assembly, A peptide containing both L- and D-amino acids is a heterochiral peptide.
Related Terms: Homochiral; Racemate; Scalemic Mixture; Mirror-Image Biology; Chiral Materials
Reference: Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds. Wiley, New York (1994).

Homochiral

Paired Concept: Heterochiral

Definition: Describing a molecular system, material, assembly, or population composed exclusively or predominantly of a single enantiomeric form, such that all constituent chiral units possess the same handedness.
Context: Homochirality is one of the most fundamental characteristics of terrestrial life. Nearly all naturally occurring proteins are constructed from L-amino acids, while nucleic acids contain D-ribose or D-deoxyribose sugars. This remarkable stereochemical uniformity enables highly specific molecular recognition, enzyme catalysis, self-assembly, and biological information transfer. Homochirality may occur at multiple levels: Molecular homochirality, Supramolecular homochirality, Polymer homochirality, Crystal homochirality, Biological homochirality. The origin of biological homochirality remains one of the major unresolved questions in chemistry and origins-of-life research.
Example: Natural proteins are homochiral because they are composed almost entirely of L-amino acids, DNA is homochiral because its sugar backbone consists of D-sugars.
Related Terms: Homochirality; Heterochiral; Mirror-Image Biology; Chiral Recognition; Protein Homochirality
Reference: Blackmond, D. G. The Origin of Biological Homochirality. Cold Spring Harbor Perspectives in Biology, 2(5): a002147 (2010); Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds. Wiley, New York (1994); Chen Y, Ma W. The origin of biological homochirality along with the origin of life. PLoS Comput Biol. 2020 Jan 8;16(1):e1007592. doi: 10.1371/journal.pcbi.1007592; Gal J (1998). "Problems of stereochemical nomenclature and terminology. The homochiral controversy. Its nature, origins, and a proposed solution". Enantiomer. 3: 263-273; Gal J (1998). "On the meaning and use of homochiral". Journal of Chromatography A. 829 (1-2): 417-418. doi:10.1016/s0021-9673(98)00845-0.

Historical Controversies and Terminological Notes
The term "homochiral" has not always been used consistently. Historically, several definitions appeared in the literature:
Classical Usage: Homochiral commonly referred to systems composed entirely of one enantiomer.
Example: Pure L-alanine, Pure R-BINAP, A crystal containing only one enantiomorphic form
Expanded Usage: Later researchers extended the term to include: Homochiral assemblies, Homochiral crystal packing, Homochiral supramolecular structures, Homochiral polymers and biomacromolecules. In these cases, homochirality refers to collective stereochemical organization, not merely molecular composition.
Origins-of-Life Debate: A major controversy concerns whether biological homochirality arose through: Chance fluctuation followed by amplification, Asymmetric autocatalysis (e.g., Soai reaction), Chiral crystallization phenomena, Circularly polarized light, Weak-force parity violation, Extraterrestrial delivery of enantiomerically enriched molecules. No consensus mechanism has yet been universally accepted.
Homochiral vs Enantiopure: These terms are often confused. Enantiopure: Refers to a single molecular species consisting of one enantiomer. Homochiral: Refers to an entire system sharing the same handedness.
For example: Pure L-alanine -> both enantiopure and homochiral. A protein composed of many different L-amino acids -> homochiral but not a single enantiopure molecular species. A crystal of pure R-BINAP -> homochiral. A racemic crystal -> heterochiral.

ChiralPedia Insight
A useful mental model: Enantiopure describes a molecule. Homochiral describes a system. A vial of pure L-alanine is enantiopure. A living cell built from millions of L-amino acids is homochiral.
One is a chemical composition. The other is an architectural principle of life itself. And that distinction turns out to be one of the deepest mysteries in chemistry: nature did not merely choose chirality-it chose a side and then never looked back.

Horeau Method
Definition: Indirect determination of ee via formation of meso/dl diesters.
Context: Historical approach for ee estimation.
Example: Horeau’s diacid anhydrides with alcohols.
Related Terms: Optical Purity, CDA.
Reference: Horeau, Tetrahedron (1961).