280 X-ray Analyses of Proteins and Nucleic Acids /1 5 : 4 



cells is believed to be coded in DNA. This has only four monomers; 

 these are nucleotides containing respectively A, T, C, and G. If it 

 seems surprising that this alphabet is sufficient, it should be remembered 

 that any English sentence can be written in Morse code which has three 

 basic letters, a dot, a dash, and a pause. 



4. X-ray Diffraction 



The physical behavior of molecules found in biological structures can be 

 investigated from different points of view. One of the most fruitful of 

 these has been an analysis of the atomic architecture as determined by 

 X-ray diffraction patterns. The term "X ray" is used to refer to a 

 beam of photons (electromagnetic radiation) formed by bombarding a 

 metal target with electrons. These X rays are shorter in wavelength 

 than other electromagnetic radiation referred to as visible and ultra- 

 violet light. (For a more complete discussion of the electromagnetic 

 spectrum, refer to Chapter 26.) 



The method of X-ray diffraction is a relatively new one in physical 

 chemistry. X rays were discovered by Roentgen just before the start 

 of this century. Quite a bit of simple X-ray crystallography was done 

 between 1912 and 1920. However, accurate measurements of X-ray 

 wavelengths and the corresponding studies of crystal structure have 

 been possible only since about 1920. These studies profoundly affected 

 scientists' ideas of the physical world at many different levels. The 

 form of the periodic table, the exact values of the electronic charge e 

 and of Avogadro's number N, and the arrangement of atoms in crystals 

 and electrons within atoms, all have been based on X-ray measurements. 



Although the diffraction of X rays by simple crystals, as NaCl, had 

 been well studied for many years, the present interpretations of X-ray 

 diffraction patterns of biologically interesting molecules were formulated 

 since World War II. These were made possible by the same factor 

 which is basic to so much of biophysics, namely the development of 

 suitable electronic techniques. The detailed interpretation of X-ray 

 diffraction data from complex molecules is possible only with the use of 

 electronic analog computers and of high speed, digital electronic 

 computers. 



These studies of the diffraction of X-ray beams by biologically 

 interesting molecules have influenced current ideas of the structure and 

 action of almost all forms of biological compounds. The arrangement 

 of the atoms within small molecules such as amino acids, purines, and 

 sugars have been (or are being) determined. The chemical structural 

 formula of certain antihistamines and the various isomers of vitamin A 



