Editing Optical rotation (section)

===Chirality prefixes===
{{Main|Chirality (chemistry)}}
{{See also|Absolute configuration|Aldose}}

A compound may be labeled as dextrorotary by using the "(+)-" or "''d''-" prefix. Likewise, a levorotary compound may be labeled using the "(−)-" or  "''l''-" prefix. The [[International Union of Pure and Applied Chemistry]], the authority on chemical nomenclature, strongly discourages use of the "''d''-" and "''l''-" prefixes.<ref>{{cite journal |last1=Moss |first1=G. P. |title=Basic terminology of stereochemistry (IUPAC Recommendations 1996) |journal=Pure and Applied Chemistry |date=1 January 1996 |volume=68 |issue=12 |pages=2193–2222 |doi=10.1351/pac199668122193 |s2cid=98272391 |url=http://iupac.org/publications/pac/68/12/2193/pdf/ |access-date=6 February 2025 |issn=1365-3075|doi-access=free }}</ref> The lowercase "''d''-" and "''l''-" prefixes are distinct from the <small>[[Small caps|SMALL CAPS]]</small> "<small>D</small>-" and "<small>L</small>-" prefixes. The "<small>D</small>-" and "<small>L</small>-" prefixes are used to specify the enantiomer of chiral [[organic compounds]] in [[biochemistry]] and are based on the compound's [[absolute configuration]] relative to (+)-[[glyceraldehyde]], which is the <small>D</small>-form by definition.

The prefix used to indicate absolute configuration is not directly related to the (+) or (−) prefix used to indicate optical rotation in the same molecule. For example, nine of the nineteen <small>L</small>-[[amino acid]]s naturally occurring in proteins are, despite the <small>L</small>- prefix, actually dextrorotary (at a wavelength of 589&nbsp;nm), and <small>D</small>-[[fructose]] is sometimes called "levulose" because it is levorotary. The two naming systems can be combined to indicate both absolute configuration and optical rotation, as in <small>D</small>-(+)-glyceraldehyde.

The <small>D</small>- and <small>L</small>- prefixes describe the molecule as a whole, as do the (+) and (−) prefixes for optical rotation. In contrast, the (''R'')- and (''S'')- prefixes from the [[Cahn–Ingold–Prelog priority rules]] characterize the [[absolute configuration]] of each specific chiral [[stereocenter]] with the molecule, rather than a property of the molecule as a whole. A molecule having exactly one chiral stereocenter (usually an [[asymmetric carbon]] atom) can be labeled (''R'') or (''S''), but a molecule having multiple stereocenters needs more than one label. For example, the essential amino acid [[Threonine|<small>L</small>-threonine]] contains two chiral stereocenters and is written (2''S'',3''S'')-threonine. There is no strict relationship between the R/S, the <small>D</small>/<small>L</small>, and (+)/(−) designations, although some correlations exist. For example, of the naturally occurring amino acids, all are <small>L</small>, and most are (''S''). For some molecules the (''R'')-enantiomer is the dextrorotary (+) enantiomer, and in other cases it is the levorotary (−) enantiomer. The relationship must be determined on a case-by-case basis with experimental measurements or detailed computer modeling.<ref name=Stephens>See, for example, {{cite journal | doi = 10.1002/chir.10270 | title = Determination of absolute configuration using calculation of optical rotation | year = 2003 | last1 = Stephens | first1 = P. J. | last2 = Devlin | first2 = F. J. | last3 = Cheeseman | first3 = J. R. | last4 = Frisch | first4 = M. J. | last5 = Bortolini | first5 = O. | last6 = Besse | first6 = P. | journal = Chirality | volume = 15 | pages = S57–64 | pmid = 12884375}}</ref>