ABSTRACT
Chemistry is a discipline that studies how molecules convert into one another. A chemical reaction can involve several different molecules. For example, a molecule of sodium chloride can fall apart (i.e., dissociate) into the sodium and chlorine atoms. The simplest possible chemical reaction, however, involves conversion of some molecule A into a different form B. To describe this reaction, we write the following equation:
A = B. (3.1) By convention, whatever molecules appear on the left hand side of the equation are
called the “reactants” and the molecules appearing on the right are the “products.” So, in our example, A is the reactant and B is the product. Since no other molecules are involved in this process, A and B must consist of the same atoms. That is, they must be described by the same chemical formula. The only difference between A and B can then be in the manner in which those atoms are arranged in the molecule. Figure 3.1 shows two examples: In the first one, we have the molecule of 1,2-dichloroethylene, which is described by the chemical formula C2H2Cl2. This molecule can exist in two possible conformations (called isomers), which are our molecules A and B. The conversion between A and B in this case is what chemists refer to as isomerization. The second example involves a much more complicated process, in which a disordered molecular chain A undergoes a rearrangement, whereby it forms a well-defined structure, the protein molecule B. This process is commonly referred to as protein folding, where A represents the unfolded protein state and B is the folded one. Biological molecules, such as proteins and RNA, have to fold in order to function, and so the processes of this type are of great interest to biophysicists and biochemists.
