528 Magnetic Measurements /28 : 3 



current loops. According to the principle of Le Chatelier, 1 the net 

 change is in such a fashion as to oppose the magnetic field applied. 



All atoms possess this basic diamagnetism. Paramagnetism, when it is 

 present, tends to be a larger effect. In enzymes, one is interested in the 

 excess of k over the basic diamagnetism rather than in the actual value. 

 In large molecules, the individual electrons cannot change the orienta- 

 tion of their orbital momentum to line up with the magnetic field. The 

 quantity of interest is the paramagnetic contribution to k, which is due 

 to unpaired electronic spins. 



A striking example is the reaction of reduced hemoglobin Hb and 

 oxygen 2 to form oxyhemoglobin HbO a ; that is 



Hb + O z ^ Hb0 2 Reactants 

 4 2 Unpaired Electrons 



where Hb represents a heme iron. The numbers below the reactants 

 are the unpaired electrons. Thus, hemoglobin and oxygen, both 

 paramagnetic, react to form diamagnetic Hb0 2 . Because both Hb 

 and Hb • 2 have an even number of unpaired electrons, it is concluded 

 that the iron has lost an even number of electrons ; that is, it is in the 

 ferrous state in both Hb and Hb • 2 . 



The susceptibility changes of enzymes are measured in the presence of 

 an excess of water. For 10 fxM. ferric iron, the maximum susceptibility 

 difference from pure water is, at room temperature 



A/c = 1.4 x 10- 10 emu 



If this is to be measured with 5 per cent accuracy, one must be able to 

 detect 



A/c = 7 x 10- 12 emu = -10" 5 x /c Hz0 



Thus, the total susceptibility must be measured with an accuracy of one 

 part in 10 5 . (If one tried to work with the permeability /x., one would 

 need an accuracy of one part in 10 12 !) This iron concentration, 

 10 juM, is high for enzyme studies. 



3. Static Measurement Techniques 



Several different techniques have been used to measure magnetic sus- 

 ceptibility in biological materials; three methods are discussed in this 

 chapter. The Gouy balance is a static susceptibility measuring device. 

 At one time, it was the most widely used type of apparatus for measuring 



1 This principle is usually referred to as Lenz's law when describing magnetically 

 induced currents. 



