526 Magnetic Measurements /28 : 2 



align with the magnetic field, thereby reinforcing the field. This 

 property of paramagnetism can be used to determine molecular elec- 

 tronic structure as well as reaction kinetics. The effects are all extremely 

 small; to observe them, the paramagnetic substance usually must be 

 highly concentrated compared to its naturally occurring state. 



Paramagnetic changes have been studied particularly during enzyme- 

 catalyzed reactions. The nature of enzyme reactions has been discussed 

 in detail in Chapters 17, 18, and 22. For the present chapter, it is 

 sufficient to know that enzymes are proteins which catalyze most of the 

 reactions in living systems. Enzyme reactions are usually observed by 

 the spectropho tome trie methods discussed in Chapter 26. Sometimes 

 thermal changes are studied, and sometimes direct chemical analyses 

 are performed. For many of the enzyme-catalyzed reactions, chemical 

 analysis is too slow to reveal much information about intermediates. 

 Paramagnetic changes offer an alternate means of studying rapid 

 enzyme reactions. It is possible to use paramagnetic rate constants to 

 confirm spectrophotometric ones and also to detect new intermediates. 



The paramagnetic measurements are directly related to the electronic- 

 energy levels within the molecule, discussed in the last chapter. Most 

 organic molecules are normally diamagnetic; that is, they possess no 

 permanent magnetic dipdle moment. Those containing metal atoms or 

 free radicals, in contrast, often possess a net dipole moment. The 

 magnetic susceptibility, which quantitatively measures the para- (or 

 dia-) magnetism, can be used to determine the state of the metal atoms 

 or the presence of free radicals. 



2. Paramagnetism and Diamagnetism 



If one holds a bar magnet near a piece of iron, the iron is strongly 

 attracted to the ends of the bar magnet. These reactions are easy to 

 observe. Materials such as iron which are strongly attracted by a 

 magnet are called ferromagnetic or sometimes just magnetic. No living 

 systems are known to contain ferromagnetic substances. 



Although it is more difficult to demonstrate, some nonferromagnetic 

 substances are attracted by the bar magnet and others are repelled by it. 

 Those attracted are called paramagnetic and those repelled are called 

 diamagnetic. The diamagnetic are the most common for biologically 

 important molecules. 



To put this on a quantitative basis, it is convenient to define a few 

 terms used in describing magnetism. If a current flows through a loop 



of wire, it gives rise to a magnetic field H. If a loop of wire carrying a 

 current is brought into the magnetic field, it will be acted on by a force 



