PROTEINS 477 



(C758Hi203Oi95N2i8FeS3) coiitains but one atom of iron. This 

 and other low estimates are now regarded as inexact, the vahie 

 68,000 being the at present accepted weight of hemoglobin. 

 Three investigators, using quite different methods, have found 

 the molecular weight of hemoglobin to be 68,000, Svedberg 

 using the ultracentrifuge ; Adair, on the basis of osmotic pressure; 

 and Northrop, from diffusion rate. The molecular weight of 

 egg albumin was first (by S0rensen) put at 14,000, on the basis of 

 freezing-point determinations of osmotic value. But again the 

 first estimate was too low. The value now generally accepted 

 for egg albumin is 34,500, determined by Svedberg. It is prob- 

 able that few if any proteins have a molecular weight of less than 

 10,000 (possibly 34,500), as compared with 18 for water and 342 

 for cane sugar. 



The method of Svedberg for determining the molecular weight 

 of substances with heavy molecules is one of the most recent 

 and most ingenious. By means of a centrifuge, Svedberg has 

 been able to cause sedimentation in protein solutions — in itself a 

 startling fact, as heretofore it has not been supposed that mole- 

 cules would settle out under centrifugal force. Two factors are 

 involved — equilibrium of sedimentation (gravity) and velocity 

 of sedimentation — from either of which the molecular weight can 

 be calculated. The maximum force attained is two hundred 

 thousand times that of gravity. Hemoglobin, with a molecular 

 weight of 68,000, requires a centrifugal force of about fifty 

 thousand times gravity to clear the top layer. Egg albumin, 

 with a weight of 34,500, is one of the lightest of protein molecules. 

 Svedberg believes that proteins of higher molecular weights are 

 exact multiples of this value. Maximum values are reached in 

 the naturally occurring pigments of plants, such as phycocyanin 

 and phycoerythrin from algae. The molecular weight of both of 

 these proteins is 208,000. The highest molecular weight deter- 

 mined by Svedberg is that of the blood pigment hemocyanin 

 from the snail. It has the enormous value of 5,000,000. Whether 

 or not the values obtained are of molecules or of molecular 

 aggregates (colloidal particles) cannot be said with certainty, nor 

 is 34,000 necessarily a minimum value. Svedberg speaks of 

 low-molecular " proproteins " occurring in the living organism 

 and brought into the shape of a "real" protein of higher weight 

 by a "purification" process. 



