MOLECULAR AND MACROMOLECUL AR STRUCTURE 195 



The theoretical basis for the interpretation of the measurements is still 

 developing (Leach, 1959) but with the increase in data sound, partly- 

 empirical methods now exist. The specific rotation [a] is defined as: 



[a]i ~ CpL 



where a is the angle of rotation of the plane of polarized light, C the 

 concentration in grammes of solute in 100 milliequivalents of solvent, p 

 the density, and L the path length. The change of [c 

 to as dispersion and is described by the classical equation of Drude : 



l-ja - A 2 _ A 2 c 



(k and A c constants of the system). 



The Drude equation is followed at high wavelengths in solutions of pro- 

 teins; at low wavelengths the dispersion becomes " anomalous " and the 

 anomalous contribution is thought to be supplied by the portion of the 

 molecular chains folded into helices. 



Anomalous dispersion is usually described by a modification of the 

 Drude equation due to MofRtt (1956): 



/100 n* + 2 \ \ a Q \l 



(A 2 -A*) 5 



M is the residue weight, a ,b and A are constants. The second term in 

 square brackets represents the anomalous contribution due to the helical 

 structure. It is customary to plot l/[a] against A 2 to test the fit to the 

 Drude equation which is usually good for protein solutions at longer wave- 

 lengths and allows an estimate of A c and k. Various methods exist for 

 estimating the amount of folded material. In general globular proteins in 

 aqueous solution have specific rotations (Na Z)-line) of the order of from 

 —25° to —80° which increases to —80° to —120° when unfolded by urea. 

 Assuming that the change in [a]^ on unfolding is a linear function of the 

 number of residues unfolded, an estimate of the percentage of folded form 

 is possible if the values of [<x] D in the fully helical and fully unfolded 

 conditions are known. These values have been obtained from polymers 

 and certain proteins where independent means of knowing the helical 

 content exist. 



The estimated helical content of most globular proteins is only 15-50%. 

 For certain a-type fibrous-muscle proteins (Review by Leach, 1959) the 

 figure is higher (50-90%). This high figure is thought to be due to their 

 low content of proline residues and disulphide bonds which restrict the 

 formation of the a-helix. 



