Optical isomerism is a form of isomerism (specifically stereoisomerism) where the different two isomers are the same in every way except being non-superposable mirror images1 of each other. Optical isomers are known as chiral molecules (prounounced ki-rall) .
The (-)-form of an optical isomer rotates the plane of polarization of a beam of polarized light that passes through a quantity of the material in solution counterclockwise , the (+)-form clockwise. It is due to this property that it was discovered and from which it derives the name optical. The property was first observed by Louis Pasteur in 1848 in racemic acid.
The study of optical isomerism is now called stereochemistry. Optical isomers are often called stereoisomers (in fact, stereoisomers constitute a more general group, since stereoisomerism needn't necessarily imply optical activity).
This form of isomerism can arise when an atom (usually carbon) is surrounded by four different functional groups. Swapping two of the groups can arise in two different molecules - mirror images of each other.
Systems of naming optical isomers
By optical activity: (+)- and (-)-
An optical isomer can be named by the direction in which it rotates the plane of polarized light. If an isomer rotates the plane clockwise as seen by a viewer towards whom the light is traveling, that isomer is labeled (+). Its counterpart is labeled (-).
The (+) and (-) isomers have also been termed d- and l-, respectively (for dextrorotatory and levorotatory). This labeling is easy to confuse with D- and L-.
By configuration: D- and L-
An optical isomer can be named by the spatial configuration of its atoms. The D/L system does this by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are labeled D and L. Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration. For example, it can be converted to the amino acid glycine: glycine has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from.
The D/L labeling is unrelated to (+)/(-); it does not indicate which enantiomer is dextrorotatory and which is levorotatory. Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde. 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.
The dextrorotatory isomer of glyceraldehyde is in fact the D isomer, but this was a lucky guess. At the time this system was established, there was no way to tell which configuration was dextrorotatory. (If the guess had turned out wrong, the labeling situation would now be even more confusing.)
A rule of thumb for determining the D/L isomeric form of an amino acid is the "CORN" rule. The groups:
COOH, R, NH2 and H (where R is an unnamed carbon chain) -- - -
are arranged around the chiral center carbon atom. If these groups are arranged clockwise around the carbon atom, then it is the D-form. If anti-clockwise, it is the L-form.
By configuration: R- and S-
The R/S system is another way to name an optical isomer by its configuration, without involving a reference molecule such as glyceraldehyde. It labels each chiral center R or S according to a system by which its ligands are each assigned a priority, according to the Cahn Ingold Prelog priority rules, based on atomic number. If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: a clockwise traversal of the remaining three may hit them in decreasing order, or in increasing order. In the first case, the center is labeled R; in the second, it is S.
This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). It thus has greater generality than the D/L system, and can label, for example, an (R,R) isomer versus an (R,S) — diastereomers.
The R/S system has no fixed relation to the (+)/(-) system. An R isomer can be either dextrorotatory or levorotatory, depending on its exact ligands.
The R/S system also has no fixed relation to the D/L system. For example, one of glyceraldehyde's ligands is a hydroxy group, -OH. If a thiol group, -SH, were swapped in for it, the D/L labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's R/S labeling, due to the fact that sulfur's atomic number is higher than carbon's, whereas oxygen's is lower.
For this reason, the D/L system remains in common use in certain areas, such as amino acid and carbohydrate chemistry. It is convenient to have all of the common amino acids of higher organisms labeled the same way. In D/L, they are all L. In R/S, they are not, conversely, all S — most are, but cysteine, for example, is R, again because of sulfur's higher atomic number.
Properties of optical isomers
They are identical with respect to ordinary chemical reactions, but differences arise when they are in the presence of other chiral molecules. For example, spearmint leaves and caraway seeds respectively contain L-carvone and D-carvone - enantiomers of carvone. These smell different to most people because our taste receptors also contain chiral molecules which behave differently in the presence of different enantiomers.
D-form Amino acids tend to taste sweet, whereas L-forms are usually tasteless. This is again due to our chiral taste molecules. The smells of oranges and lemons are examples of the D and L enantiomers.
Penicillin's activity is stereoselective. The antibiotic only works on peptide links of D-alanine which occur in the cell walls of bacteria - but not in humans. The antibiotic can kill only the bacteria, and not us, because we don't have these D-amino acids.
The anti-nausea drug Thalidomide was widely prescribed to pregnant women until it was linked to birth defects. It was suspected, and has been widely reported, that one enantiomer was responsible for the teratogenic effects, but this turned out not to be the case.
Photons in plane-polarized light all oscillate in a geometric plane as opposed to the random oscillations they present normally. That plane is bisected by an axis determined by the photon's direction of travel. In other words: optically active isomers rotate the plane that the photons oscillate in. The polarized light is actually rotated in a racemic mixture as well, but it is rotated to the left by one of the two enantiomers, and to the right by the other, which cancel out to zero net rotation.
Note 1: The term non-superposable distinguishes mirror images which are superposable, such as the mirror image of the letter "A", on the original, from those that aren't. The classic example of this are human hands. The left hand is a non-superposable mirror image of the right hand: No matter how the two hands are oriented relative to one another, one cannot line up all the major features of one hand with the other, whereas such an operation is trivial for a non-chiral mirror image (e.g., the letter "A").
- IUPAC nomenclature for amino acid configurations.