∞ generated and posted on 2016.12.22 ∞
Isomer consisting of an alternative geometrical arrangement of four different atoms or groups bonded to a single carbon atom.
Key is that the four atoms or groups that are bonded to that single carbon atom must not be chemically identical since otherwise the central carbon atom, the "chiral carbon, cannot give rise to an enantiomer. The basis of the existence of enantiomeric forms is the geometry of the arrangement of four single bonds around an individual carbon atom, which take the form of a three-sided pyramid called a tetrahedron. In particular, it is not possible to swap atoms or groups found at one vertex of the icosahedron with another vertex without breaking bonds. As a result, two distinct isomers are possible around each chiral carbon, with the bonds to the four different atoms or groups literally mirror images of each other.
A different view is that enantiomers share molecular formulae like isomers generally. Unlike structural isomers, however, enantiomers also share structures with each other. Furthermore, and unlike geometric isomers, the bonds involved are fully rotatable. Nonetheless, and similar to geometric isomers, the positions of the different atoms or groups involved cannot be swapped. Indeed, both geometric isomers and enantiomers are considered to be stereoisomers. Unlike geometric isomers, however, the different enantiomers truly are chemically identically but nevertheless sufficiently structurally distinct that enzymes easily distinguish among them. Consider, by way of example, the difficulty associated with fitting a left-hand glove on your right hand. The two gloves effectively are identical, structurally, but nonetheless are mirror images of each other, as too are your hands. Enantiomers are also known as optical isomers.
Figure legend: Two molecules with tetrahedral molecular geometries, both projecting towards you from the plane of the screen. The group 'A' is nearest to you; 'X', the chiral carbon, is found in the middle; and the triangle consisting of 'B', 'C', and 'D' is found furthest from you. Note that you cannot twirl or flip one mirror image to form the other. Instead, bonds must be broken to convert one into the other. In addition, the spatial placement of 'A', 'B', 'C', and 'D' between the two enantiomers are quite different, allowing for enzymes to easily distinguish among them.
One finds enantiomeric forms among biomolecules especially with sugars (in their ring form) and amino acids. With amino acids, one distinguishes enantiomers into L and D forms. Furthermore, the L form and only the L form is what is placed into polypeptides in the course of translation. One similarly speaks of D and L forms of glucose where, unlike with amino acids, it is the D form that is used by organisms.
The following video provides a brief introduction to the concept of enantiomers with OK though not greatly revealing graphics:
The following video is longer with better graphics and more information but still is not necessarily better overall than the previous video, though still, worth watching: