732 



SCIENCE 



[Vol. LVI, No. 14(31 



the details we observe are significant and vfhich 

 are negligible. 



II 

 Living matter is essentially colloidal in cbar- 

 acter and we can not well conceive of an organ- 

 ism consisting exclusively of crystalloidal 

 matter. This fact suggests that life phenomena 

 depend upon or are intrinsically linked with 

 certain characteristics of colloidal behavior. It 

 is, therefore, natural that a systematic study 

 of the nature of special life phenomena should 

 be preceded !by a scientific theory of colloidal 

 behavior. By a scientific theory, however, we 

 do not understand speculations or guesses Ibuilt 

 on qualitative experiments or no experiments 

 at all, but the derivation of the results from a 

 ratianalistic, mathematical formula w^hich per- 

 mits us to calculate, with an adequate degree 

 of accuracy, the quantitative measurements of 

 colloidal 'behavior. 



Proteins are amphoteric electrolytes which 

 are capable of fonning salts with either alka- 

 lies or aeids. With alkalies they form salts 

 like Na proteinate, Ca proteinate, etc., and 

 with acids they form salts like protein chloride, 

 protein sulfate, etc. Whether they do the one 

 or the other deijends on the hydrogen ion con- 

 centration of the protein solution. There is 

 one definite hj^drogen ion concentration at 

 which a protein can combine practically with 

 neither acid nor alkali, and this hydrogen ion 

 concentration, which may be different for dif- 

 ferent proteins, is called the isoelectric point. 

 The isoelectric point is (in terms of Sorensen's 

 logarithmic symbol) for gelatin and casein at 

 Pji 4.7; for crystalline egg albumin at Pn 4.8. 

 Gelatin can combine with acid ooily or prac- 

 tically only when the pg is less than 4.7 and 

 with alkali only or practically only when tlie 

 Ph is higher than 4.7. Or in other words, 

 when a salt, e. g., NiCh, is added to gela- 

 tin solutions of different pjj, Ni gelatinate can 

 only be formed when the p^ is greater than 

 4.7; and when K4Fe(CN)6 is added gelatin- 

 Pe(CN)6 can only be formed when the pj, is 

 less than 4.7. This can ibe shown by methods 

 discussed in a recent book.^ 



2 Loeb, J. : " Proteins and the Theory of Col- 

 loidal Behavior," New York and London, 1922. 



The proof that proteins corribine stoichiomet- 

 rically with acids and alkalies can be fur- 

 nished by titration curves. For this purpose 

 (and perhaps for work with proteins in gen- 

 eral) it is necessary to use as standard mate- 

 rial protein of the p„ of the isoelectric point. 

 We have seen that proteins combine with acids 

 only at a pjj below that of the isoelectiic point, 

 which for gelatin or casein is about pn 4.7 

 and for crystalline egg albumin 4.8. It hap- 

 pens that at a pn helow 4.7 most of the weak 

 dibasic and tribasic acids dissociate as mono- 

 basic acids. Thus II3PO4 dissociates into H+ 

 and the monovalent anion H.,PO- . Hence if 

 acids combine stoichiometricalfy with isoelectric 

 protein, it should require exactly three times 

 as many ce. of 0.1 N H3PO4 to bring a 1 per 

 cent, solution of am isoelectric protein, e. g., 

 gelatin or crystalline egg albumin or casein, to 

 the same hydrogen ion concentration, e. g., pg 

 3.0, as it requires of 0.1 N HCl or HNO3. 

 Titration experiments show that this is the 

 case. Furthei-more, since H2SO4 is a strong 

 acid, splitting off both hydrogen ions even at a 

 Ph below 4.7, the same number of cc. of 0.1 

 N H2SO4 as of HCl should be required to bring 

 1 gim. of isoelectric pix)tein in 100 cc. of solu- 

 tion to the same p^, e. g., 3.0, and this was 

 found also to be true. 



Fig. 1 gives the titration curves for crystal- 

 line eigg albumin for four acids, HCl, H2SO4, 

 H3PO4, and oxalic acid. One gram of isoelec- 

 tric albumin was in 100 cc. H2O containing 



S » 



pH 25 22 24 2S 2B 30 i2 M 38 as . 



Fig. 1 



