Example Of Essay On Chemistry: Molecular Models II
Cisplatin is a compound from four ligands – two chlorine atoms and two amine molecules – that are covalently bonded to a central metal ion, which is platinum. This compound has a square planar shape with two similar ligands on one side; that is, the two chloride ions are on the same side of the square. This inorganic compound is currently being used to treat cancer due to its capability to adversely affect the replication process of the cancer cells’ DNA molecules. Accordingly, under aqueous environments, the chloride ligands are readily replaced by water molecules to reduce the stearic effects exerted by the two chlorine atoms to each other. These water molecules are then replaced by some parts of the DNA and proteins. This means that one chloride ligand becomes replaced by a nucleotide; particularly the nitrogen atom at the seventh position of the guanine nucleotide, from the DNA and the other chloride ligand is replaced by the protein. The bonded protein inhibits the repair mechanism of the DNA along the destruction of the DNA’s three-dimensional structure; hence, its function. When the DNA and protein molecules are bonded together by cisplatin, cross linking takes place which induces the formation of loop structures that ultimately destroys the DNA through condensation. It should be noted that the mechanism of reaction between the DNA and the cisplatin molecule involves the nucleophilic attack of the former to the latter; hence, the cis configuration or structure of cisplatin molecule plays a crucial role in the entire reaction process. The nucleophilic attack is less hindered in the cis configuration compared to its trans isomer. Numerous studies have shown that transplatin does not promote cross linking; hence, it is not an effective drug to fight cancer.
2. Optical Isomers
The behavior of molecules in a chemical reaction is not governed solely by its chemical formula, but by the arrangement of the atoms in the molecules as well. Compounds or molecules with the same chemical formula but with different spatial configuration are pertained to as isomers. It should be noted, however that differences in configuration due to the rotation of molecules about an axis of rotation are not included in the definition of isomers. There are categories of isomers and one of these is the category called the stereoisomers. Stereoisomers are compounds with the same chemical formula and the same sequence or arrangement of bonds, but the three-dimensional orientation of these bonds or atoms differ. One of example of stereoisomers is optical isomers or enantiomers. These stereoisomers interact differently with plane polarized light. It should be noted that in most chemical reactions where the products are stereoisomers, it is rarely to find only one type of optical to be present. The usual case is that both optical isomer types are present in almost equal amounts. However, if a chemist is able to isolate one from the other and expose each to plane polarize light; one of the two types will rotate the light in the clockwise direction. This optical isomer type is designated by the positive or plus sign (+). The other optical isomer, on the other hand, will rotate the place polarized light in the counter clockwise direction and is designated by the negative or minus sign (-). A mixture with equal number of (+) and (-) optical isomers, which is termed a racemic mixture, will not affect plane polarized light. Alanine is a compound that exhibit optical isomerism.
3. Minimum Requirement for Optical Isomerism
There are some requirements for a compound so that it will be able to exhibit optical isomerism. The first is that there must be a chiral center atom. A chiral center is an atom that you cannot bisect at the center and obtain an exact replica one side to the other. This means that is a plane will divide the center atom in half, the two halves will not be perfectly equal or the same. This second requirement is that all atoms or groups that are attached to the center atom are different from each other. The third requirement is that the atoms between the two optical isomers are arranged in different spatial arrangement. Two atoms that are optical isomers are mirror image of each other. This means that you cannot upper imposed them as our two hands look the same by having five fingers each with each finger on the right hand have a corresponding finger on the left, but you cannot superimpose both hand on top of the other so that they fit perfectly. An example of a hypothetical molecule is shown below. Note how the two molecules become mirror image of each other with molecule A being the left hand and molecule B being the right hand.
Even if one of the molecules, let us say molecule B is rotated, the two molecules remain to be non-superimposable. In the figure below, molecule B was rotated by 1800. As can be seen only the green atoms and the center atoms have the same spatial orientation, while the rest of the atoms have different spatial orientation. These two molecules are therefore enantiomers or optical isomers.
Chirality, which is the main requirement for molecules to exhibit optical isomerism, is very important in making drugs for medicinal purposes. This is best shown in the case of the drug called thalidomide. Thalidomide was synthesized and marketed by Germany in 1957. It was sold to more than 40 countries in more than 30 brand names. It was recommended by doctors for pregnant women as a sedative and as a drug to induce sleep, but it was found out later that the drug has adverse effects to developing embryos, which ultimately resulted in a stricter production of thalidomide and chiral drugs, in general. This compound has a chiral center as marked by an asterisk in the figure below.
Thalidomide, when it was made was old in the market as a racemic mixture. Years after it has been in the market, there have been a significant number of incidences of birth defects among those mothers who have taken the drug. It was later found out that the (-) optical isomer is a teratogen, which means that it affects the development of fetuses. Most the said infants had underdeveloped limbs and similar defects. It was the (+) optical isomer, however, which as effective sedative and sleep inducing compound. Further studies have revealed that because protein molecules are chiral they react differently to chiral molecules such as thalidomide. Isomerism is indeed an important topic in chemistry especially when it is applied to medicine in producing drugs. The spatial orientation of atoms within a molecule matters as such orientation help determine how they will behave in a chemical reaction or on which compounds they will react to.