102 



I. M. KLOTZ 



bound to the albumins disappears and the spectrum reverts to that of the 

 unbound form. On the other hand, in contrast to the situation with the immune 

 globulins, on change of the anionic group from sulfonic to carboxylic to phos- 

 phonic to arsonic, all the compounds behave in the same way. With any one 

 of these dyes and serum albumin you get the upper-left spectrum at pH 6 and 

 the lower-left spectrum at pH 9. Hence, the nature of the interaction in this 

 case is certainly identical in quality for all three; there are slight differences in 

 the extent of binding and the extent of displacement, but the displacement is 

 always in the same direction. It is cjuite clear that there is not the same type 

 of steric fitting phenomenon here that is involved in other cases of steric rela- 

 tionships, such as with the gamma-globulins. 



We also found that if we changed the sulfonic acid group from the para 

 position to the meta, it made no difference. The meta compound also behaved 

 in the same way, showing the shift with pH; but if we went to the ortho posi- 

 tion, then we did get a marked difference. At pH 9 the spectrum was no longer 

 shifted upward but shifted to the left, just as it is in the upper left of Fig. 11. 

 Evidently then, there is some critical steric effect which occurs when the sul- 

 fonic acid is at this particular point. 



pH 6 



(CH,\^-(^-N^U-(^-SO, 



(-^3\^-(^-^-^-(^-^0, 



WAVELENGTH 



i 



jH 9 



(cHj)^ N-< ^ '^ -N = N-< ^ y -POjH- 



(C»3\ ^-{ ^ -N=N -<( ^ - A503H " 



WAVELENGTH 



Fig. 11. Spectra of some organic anions bound, or not bound, to human serum 

 albumin. 



