Molecular Structure in Relation to Biology and Medicine

THE molecules that compose the body of a human being may be conveniently divided into two classes: small molecules and large molecules. Small molecules are molecules containing I0 or 20 or perhaps IOO atoms; examples are glucose, acetylcholine, glycine and other amino acids, thiamine and other vitamins. Large molecules are molecules containing hundreds or thousands or tens of thousands of atoms; examples are the proteins and the nucleic acids. To understand the human body in health and in disease we need to know the structure of the small molecules and the large molecules. The large molecules are especially important because it is they that carry biological specificity: their structural differences determine the differences between species of living organisms and also the differences between individuals of the same species. During the past 40 years great progress has been made in the determination of the precise structures of small molecules. The work has been done by the use of several methods, of which the study of gas molecules by electron diffraction and of crystals by X-ray diffraction are the most important; much information is also now being provided by microwave spectroscopy and nuclear-spin and electron-spin magnetic resonance. Outstanding among the X-ray studies are the astounding achievements of Mrs. Dorothy Hodgkin and her co-workers, in determining the structures of penicillin and vitamin Bra. During the recent decades there has also been developed a