PROTEIN STRUCTURE 



bonding is the salicylate ion, for which the pK. value of the hy- 

 droxyl group is displaced from its usual value of approximately 10 

 to 13.4 at 18° C, obviously because of the strong hydrogen 

 bonding to the — GOO~ group in the ortho position (1,59). 

 The m- and p- hydroxy benzoic acids show no such effect (Table 

 I). The behavior of ovalbumin, as described in the classical 



TABLE I 



pK. Values for Hydroxybenzoig Acids and Their Ethyl Esters" 



PK; (18°) pK^ (18°) /)K' ester (25°) 



Salicylic acid 2.99 13.4 10.5 



ff?-Hydroxybenzoic acid 4.06 9.99 9.1 



/)-Hydroxybenzoic acid 4.54 9.4 8.3 



" From Larsson, E., Z. anorg. Allgem. Chem., 183, 30 (1929). 



Abichandani, C. T., and Jatkar, S. K. K., J. Indian Inst. Set. A27, 417 (1938) 

 report pK.2 for salicylic acid at 30° as 12.4; the decrease in pK.2 with increase of 

 temperature is in the expected direction, but the uncertainties in the data for such 

 alkaline solutions are too great to warrant calculating a heat of ionization from 

 these data. 



study of Crammer and Neuberger (28), is in some respects 

 closely akin to that of the salicylate ion. The ultraviolet absorp- 

 tion spectrum of ovalbumin shows that the phenolic hydroxyls 

 remain unionized up to/?H values of 12 or above, until a critical 

 pH is reached at which ionization of all the groups occurs more 

 or less simultaneously. The effect is irreversible; the groups 

 remain ionized on returning to pH values between 11 and 12, 

 and the protein is found to be denatured. Obviously a system 

 of internal hydrogen bonds has been ruptured. Although the 

 other groups linked to the tyrosine hydroxyls have not been 

 identified, it is reasonable to suspect that carboxyl groups from 

 neighboring dicarboxylic amino acid side chains are involved : 



\ /° 



CH— CH2CH2G . 



^ \o - ho/ NgH2— CH 



^ \ 



How much strength these bonds actually contribute to the 

 native structure of the protein is still uncertain. It is quite 



391 



