H. GUTFREUND 



1 Solute-solvent interaction (solvation) 



2 Solute interaction (van der Waals and Coulomb forces) 



3 Change of entropy of the solution due to presence of large mole- 

 cules (this effect becomes very marked for asymmetric particles). 



Equation 2 is only valid at zero concentration and becomes 



P = mRTg ....(3) 



at finite concentrations ; g is the osmotic coefficient and is dependent 

 upon the protein concentration, other conditions (solvent composition 

 and temperature) being kept constant. 



Table I 



Table II 



The Osmotic Pressure of Solutions 

 of Horse Haemoglobin in 0-2M 

 Phosphate 



The Osmotic Pressure of Solutions 

 of Human Haemoglobin in 0-2M 

 Phosphate 



To obtain correct values for M from equation 1 the usual procedure 

 is to determine the osmotic pressure over a series of protein con- 

 centrations and plot P/C against C and use the value for P/C obtained 

 from extrapolation to zero concentration for the computation of M. 

 The value for g due to any of the three causes given above tends to 

 unity as C approaches zero. For haemoglobin therefore the plot of 

 P/C against C should extrapolate to P/C = 3-5. Figures la and lb, 



200 



