Absolute configuration
In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity (or group) that is chiral, and its resultant stereochemical description.
Absolute configuration uses a set of rules to describe the relative positions of each bond around the chiral center atom.
Absolute configurations for a chiral molecule (in pure form) are most often obtained by X-ray crystallography, although with some important limitations.
Alternative techniques include optical rotatory dispersion, vibrational circular dichroism, ultraviolet-visible spectroscopy, the use of chiral shift reagents in proton NMR and Coulomb explosion imaging.
In 1951, Johannes Martin Bijvoet for the first time used in X-ray crystallography the effect of anomalous dispersion, which is now referred to as resonant scattering, to determine absolute configuration.
The process of crystallization of the target molecules is time- and resource-intensive, and can not be applied to relevant systems of interest such as many biomolecules (some proteins are an exception) and in situ catalysts.
Just recently, novel techniques have been introduced to directly investigate the absolute configuration of single molecules in gas-phase, usually in combination with ab initio quantum mechanical theoretical calculations, therefore overcoming some of the limitations of the X-ray crystallography.
When the center is oriented so that the lowest-priority substituent of the four is pointed away from the viewer, the viewer will then see two possibilities: if the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for Latin: rectus – right); if it decreases in counterclockwise direction, it is S (for Latin: sinister – left).
The D/L system (named after Latin dexter and laevus, right and left), not to be confused with the d- and l-system, see above, does this by relating the molecule to glyceraldehyde.
Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of the smallest commonly used chiral molecules) has resulted in its use for nomenclature.
Nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and D-fructose is also referred to as levulose because it is levorotatory.
