MEASUREMENT OF THERMODYNAMIC NONIDEALITY ARISING FROM VOLUME-EXCLUSION INTERACTIONS BETWEEN PROTEINS AND POLYMERS

The effective thermodynamic radii of 23 ribosomal proteins from the 50 S subunit have been determined by gel chromatography on Sephadex G-50 , thereby supporting the contention that most of the proteins of the 5 0 S ribosomal unit exhibit reasonably globular structures. To investig ate further the usefulness of modelling proteins as spheres, the secon d virial coefficient describing excluded volume interactions of some r ibosomal proteins with two inert polymers, polyethylene glycol (PEG) a nd dextran, has been determined by gel chromatography and/or sedimenta tion equilibrium techniques. Protein-polymer excluded volumes obtained with PEG 20000 and Dextran T70 as the space-filling solute are shown to conform reasonably well with a quantitative expression describing i nteraction between an impenetrable sphere and an ideal Brownian path ( K.M. Jansons and C.G. Phillips, J. Colloid Interface Sci., 137 (1990) 75).