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The Molecules of Life

Louis Pasteur (1822-1895) is often considered the father of bacteriology. His contributions to this science, especially in its early formative years, were enormous and are discussed in many of the succeeding chapters. Pasteur started his scientific career as a chemist, initially working in the science of crystallography. Not until he was on the Faculty of Science at Lille, France, a town with important brewing industries, did he become interested in the biological aspects of chemical problems. All of his later studies—including his work on spontaneous generation, infectious disease, and protection against infectious diseases through vaccination—employed the analytical experimental methods and thinking of a trained chemist.

He first studied two compounds, tartaric and paratartaric acids, which formed thick crusts within wine barrels. These two substances form crystals that have the same number and arrangement of atoms, yet they twist (rotate) a plane of light differently when that light passes through the crystal. Tartaric acid twists the light; paratartaric acid does not. Therefore, the two molecules must differ in some way, even though they are chemically identical. Pasteur was intrigued by these observations and set about to understand how the crystals differed. Looking at them under a microscope, he saw that the crystals of tartaric acid all looked identical but paratartaric acid consisted of two different kinds of crystals. Using tweezers, he carefully separated the two kinds into two piles and dissolved each kind in a separate flask of water. When he shone polarized light through each solution, one solution twisted the light to the left and the other twisted it to the right. When he mixed equal numbers of each kind of crystal into water and shone polarized light through the solution, the light was not twisted. Apparently, the two components of the mixture counteracted each other, and as a result, the mixture did not rotate the light. Pasteur concluded that paratartaric acid is a mixture of two compounds, each being the mirror image, or optical isomer, of the other. Optical isomers are often called stereoisomers. This mixture of two optical isomers can be viewed as a mixture of right-and left-handed molecules, represented as a right and left hand facing each other (see figure 2.15). They cannot be superimposed on each other, much as a right-handed glove cannot fit the left hand.

Pasteur thus solved a long-standing mystery in chemistry. At age 25 he had established the fundamental concept of stereochemistry. These studies proved to be far more significant in biology than Pasteur could have imagined. In later studies, he showed that molecules synthesized by living organisms have a preferred handedness, whereas molecules synthesized by chemical means are always a mixture of right- and left-handed molecules. Further, living organisms can only use one form. Indeed, an organism would starve to death if it consumed only the wrong-handed sugars, because such sugars are indigestible.

Stereoisomers of the same molecule have greatly different properties. For example, the amino acid phenylalanine, one of the key ingredients in the artificial sweetener aspartame, makes aspar-tame sweet when it is in one optical form but bitter when in the other form. As another example, researchers believe that the birth defects caused by the drug thalidomide, once used to prevent nausea in pregnant women, resulted from one, but not the other optical form in a mixture of the two forms. Unfortunately, it is difficult to separate the two forms on a large scale.

Thus, what Pasteur studied as a straightforward problem in chemistry has implications far beyond what he ever imagined. It is often difficult to predict where research will lead or the significance of interesting but seemingly unimportant observations.

—A Glimpse of History

TO UNDERSTAND HOW CELLS LIVE AND INTERACT with one another and with their environment, we must be familiar with the molecules that compose all living matter. The molecules of life make up the structure of cells and interact

18 Chapter 2 The Molecules of Life with one another with great specificity. Certain properties of molecules allow cells to carry out the myriad of functions necessary for life. What properties allow such specific interactions? What are the molecules of life composed of to give them such properties? In addition to providing answers to such questions, this chapter will define some of the vocabulary necessary to understand other chapters of the text.

For some, this information may serve as a review of material already encountered. For others, it may be a first encounter with the chemistry of biological molecules. In this case, you likely will return to this chapter frequently. The discussion proceeds from the lowest level of organization, the atoms and elements, to the highly complex associations between molecules which form large molecules, the macromolecules.

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