When you reflect your left hand in a mirror, your right hand appears.
A mirror reflects an object placed in front of it, showing it as it is but flipped from left to right. While this may not matter for symmetrical shapes like squares or soccer balls, it becomes significant for asymmetrical objects. When you superimpose the shapes of an object and its mirror image, they do not perfectly overlap. For instance, when you place your left hand in front of a mirror, the reflection appears as your right hand. Although they seem to face each other and create the illusion of being identical, they are, in fact, not the same. I am very well aware of this. It seems like a laser beam is coming out of my right hand, because I make holes in my right kitchen rubber glove very often. Given the situation, I have ended up with a lot of unused left rubber gloves. I attempted to recycle a left glove for my right hand, but that didn’t work out at all. I just had to buy a new one. If both hands were identical, I could have saved a fortune(?) on glove costs.
Now, let’s think about letters. Ambulances and fire engines display their names in mirror writing so that drivers can read them quickly through their rearview mirrors in emergencies. This allows for swift action to clear the way. The example below illustrates the concept of mirror images well. The letter 'B' is asymmetric, and when reflected, it flips left to right. In mirror writing, the first letter 'A,' which should be on the far left, appears on the far right instead. This is because, in a mirror image, the left and right directions are reversed.
The Property of Hands: Handedness
Fundamentally, every object has a mirror image when reflected in a mirror. If two images facing each other do not overlap when superimposed, it indicates that they possess asymmetry, and we say that these objects have the property of handedness. The terms "left hand" and "right hand" serve as the simplest examples of this property.
This characteristic is particularly important in biochemistry related to living organisms. The basic building blocks that compose life, as well as crucial biomolecules directly involved in life processes, all exhibit this handedness. They exist as stereoisomers with mirror-image forms that cannot be superimposed, resulting from different three-dimensional spatial arrangements within the molecules.
Constitutional isomers and stereoisomers
Isomers can be broadly classified into two categories: constitutional isomers (structural isomers) and stereoisomers. Isomers are compounds that have the same molecular formula but differ in their structure or arrangement, resulting in different physical and chemical properties. The molecular formula represents the components of a molecule using chemical symbols, indicating the types and numbers of atoms (e.g., CH₄), while the structural formula shows how the atoms are combined. Constitutional isomers have the same molecular formula but differ in their structural formulas. In contrast, stereoisomers share the same molecular formula and structural formula as well as the sequence of bonded atoms, but differ only in their three-dimensional arrangement.
Among stereoisomers, a particularly important category is enantiomers, which are pairs of stereoisomers that are non-superimposable mirror images of each other. Enantiomers play a significant role in biochemistry, especially in the study of biomolecules. The term "enantiomer" comes from the Greek word meaning "opposite." Even if the term "mirror-image isomers" is unfamiliar, you may have encountered the terms L-glucose and D-glucose or L-amino acids and D-amino acids. These names are used to distinguish between specific enantiomers.
Chirality: non-superimposable mirror image
When an object or molecule has an asymmetric mirror relationship such that its two mirror images cannot be superimposed, we say that it possesses chirality. The term is derived from the Greek word "χειρ" (cheir), meaning "hand." By recalling the relationship between the left hand and the right hand, it becomes easy to understand why this term is used. To superimpose the two hands does not mean to bring them together as in a prayer position; rather, it refers to placing the back of one hand over the palm of the other, and when they are positioned this way, the shapes of the two hands are different. Objects like golf clubs, socks, scissors, spiral screws, and corkscrews also exhibit this spatial property of chirality. In other words, a chiral molecule will always have a pair of enantiomers, which are mirror-image isomers.
We must once again focus on carbon. One of the unique features of carbon, the central molecule that makes up proteins, carbohydrates, and nucleic acids, is its ability to bond with four atoms. In previous articles about hybrid orbitals, we noted that the most common type of hybridization that occurs in the formation of hybrid orbitals is sp³ hybridization, which allows for the formation of four covalent bonds (single sigma bonds) with different atoms or substituents. Stereochemical asymmetry is closely related to sp³ hybridized carbon. In a tetrahedral structure, if the central carbon is bonded to four different atoms, it can form the mirror-image asymmetric stereoisomers. Of course, there are other elements (such as nitrogen, sulfur, and phosphorus) that also undergo sp³ hybridization. However, when comparing their significance or scale in the formation of organic compounds, carbon stands out uniquely.
In the diagram below, the carbon bonded to four different substituents forms an asymmetric chiral molecule, and this carbon is referred to as a "chiral center" or "chiral carbon." When this molecule is flipped 180॰, some atoms' three-dimensional spatial arrangements change, and when superimposed, they do not overlap. Remember that in representing three-dimensional space on a two-dimensional plane, a wedge indicates the side closer to the observer, while a dashed line represents the side further away, or at the back. As shown in the diagram below, the same atoms can form two independent molecules with different three-dimensional arrangements. Many biomolecules in living organisms exist as pairs of mirror-image stereoisomers exhibiting this chirality.
The importance of carbon's sp³ hybridization lies in its ability to form "four different bonds." When these four bonds are made with different entities, the central carbon becomes chiral, resulting in the generation of two chiral molecules. Most chiral molecules have an asymmetric carbon atom at their center. However, if there are two identical entities, chirality cannot be achieved, as explained in the diagram below.
* For more details on sp³ hybridization, please refer to the previous articles.
Hybridization 1: Hybrid orbital theory and carbon's sp³, sp², and sp hybridization
We have examined what chirality is and under what conditions it is formed. Before discussing the significance of chirality for living organisms and its impact on us, let’s take a closer look at chirality. In the next article, we will explore the methods used to distinguish between pairs of mirror-image isomers and the peculiar phenomena that occur in racemic mixtures, where both forms are present together.
