MOLECULES

Atoms attract each other to form molecules, like the water molecule H₂0 sketched in Figure (1). It was from x-ray studies of ice, the crystalline form of water, that we know the distance from the center of the oxygen atom to the center of a hydrogen atom is .95 8 × 10^–8 cm and that the hydrogen atoms are spread out at an angle of 104.5 degrees as shown.
X-ray studies of large biological molecules began in the late 1950s. For example, myoglobin is a substance found in muscle tissue. The myoglobin molecule contains over 2500 atoms, mostly carbon, hydrogen, oxygen, nitrogen, and one iron atom. For determining the precise structure of the myoglobin molecule from x-rays of crystals of myoglobin, John Kendrew and Max Peritz received the 1963 Nobel Prize in chemistry. Their model of the molecule is shown in Figure (2). Recent advances in computer modeling now provide detailed views of numerous kinds of molecules. An example is Figure (3) showing the cholera toxin Bsubunit.

Figure 1: The water molecule H₂O. We know the precise location of the centers of the three atoms.

Figure 2: Model of the myoglobin molecule.

Figure 3: Computer model of the cholera toxin B-subunit. (Courtesy of Argonne National Laboratory.)