en.wikipedia.org/wiki/Mirror-image_life
4 corrections found
All proteinogenic amino acids are L-α-amino acids.
This is incorrect because glycine is a proteinogenic amino acid but is achiral, so it is neither L- nor D-α-amino acid.
Full reasoning
The statement is too absolute. Glycine is one of the standard proteinogenic amino acids, but it is achiral because its α-carbon has two hydrogens attached. Since glycine has no chiral center, it cannot be classified as either L or D.
Authoritative sources explicitly identify glycine as the only achiral proteinogenic amino acid. That means it is an exception to the claim that all proteinogenic amino acids are L-α-amino acids.
A more accurate phrasing would be that nearly all chiral proteinogenic amino acids are L-α-amino acids, with glycine as the achiral exception.
2 sources
- Glycine | C2H5NO2 | CID 750 - PubChem
Glycine is the simplest (and the only achiral) proteinogenic amino acid, with a hydrogen atom as its side chain.
- 3AA-3 to 3AA-5
Note further that in the names of optically active derivatives of glycine, such as L-2-phenylglycine, the prefix must be placed before the name of the substituent as glycine itself is achiral.
proteins made by the ribosome are exclusively composed of left-handed amino acids
This overstates the case: ribosomal proteins also contain glycine, which is achiral rather than left-handed.
Full reasoning
This sentence is misleading because it says ribosomal proteins are exclusively made of left-handed amino acids. In reality, glycine is a standard proteinogenic amino acid incorporated by ribosomes, and glycine is achiral. Because glycine has no chiral center, it is neither left-handed (L) nor right-handed (D).
So while biological proteins overwhelmingly use L-amino acids for the chiral amino acids, they are not exclusively composed of left-handed amino acids: they also commonly contain glycine, the achiral exception.
A more accurate formulation would be that proteins are built from L-amino acids, except for achiral glycine.
2 sources
- Glycine | C2H5NO2 | CID 750 - PubChem
Glycine is the simplest (and the only achiral) proteinogenic amino acid, with a hydrogen atom as its side chain.
- Standard Amino Acid Structure
Glycine is the only standard AA with a symmetric Cα-carbon and, thus, it is not stereoisometric and is technically not an "L-amino acid."
Handedness refers to the direction in which polarized light skews when beamed through a pure solution of the molecule.
This misdefines molecular handedness. Chirality is about a molecule being non-superposable on its mirror image; optical rotation is a separate measured property.
Full reasoning
This parenthetical conflates chirality/handedness with optical rotation.
According to IUPAC, chirality is the geometric property of being non-superposable on a mirror image. That is what chemists mean by a molecule's handedness. By contrast, optical activity is the experimentally observed rotation of plane-polarized light, and IUPAC treats its sign using (+)/(−) (historically d/l), not as the definition of handedness itself.
So a molecule's handedness is not defined by how it rotates polarized light. The statement is especially misleading because the common biochemical D/L labels for sugars and amino acids do not, by themselves, specify the direction of optical rotation.
A more accurate gloss would be that handedness refers to whether a molecule and its mirror image can be superimposed, while optical rotation is one measurable consequence that chiral substances may exhibit.
3 sources
- IUPAC Gold Book - chirality
The geometric property of a rigid object (or spatial arrangement of points or atoms) of being non-superposable on its mirror image.
- IUPAC Gold Book - optical activity
A sample of material able to rotate the plane of polarization of a beam of transmitted plane-polarized light is said to possess optical activity... An enantiomer causing rotation in a clockwise direction ... is designated by the prefix (+)-; one causing rotation in the opposite sense ... by the prefix (−)-.
- IUPAC Gold Book - d, l, dl
Obsolete alternatives for, respectively, the prefixes (+)- and (−)- [used to designate the sign of optical rotation of enantiomers under specified condition].
Class of amino acids
This description is incorrect: a D-peptide is a peptide made from D-amino acids, not a class of amino acids.
Full reasoning
The label misdescribes what a D-peptide is. A peptide is a short chain of amino acids linked by peptide bonds, not an amino acid class. A D-peptide is therefore a peptide composed wholly or partly of D-amino acids, rather than being itself a class of amino acids.
Authoritative definitions distinguish clearly between amino acids and peptides. The National Human Genome Research Institute defines a peptide as a short chain of amino acids. Reviews of D-peptide therapeutics likewise describe D-peptides as peptides composed of D-amino acids.
A more accurate short description would be something like "peptide made of D-amino acids" or "class of peptides", not "class of amino acids."
3 sources
- Peptide
A peptide is a short chain of amino acids (typically 2 to 50) linked by chemical bonds (called peptide bonds).
- Chirality Effect on Physical and Biological Properties of Peptide-Based Hydrogels
mirror-image proteins (proteins composed exclusively of D-amino acids) have been synthesized...
- D-Peptides as Recognition Molecules and Therapeutic Agents
This review focuses on the design and application of D-peptides with biological activity.