226 H. K. SCHACHMAN AND R. C. WILLIAMS 



molecular, or particle, weiglit. The use of the term "molecular weight" as 

 applied to viruses is not acceptable to all, but m those cases where a high 

 degree of physical homogeneity has been demonstrated in a virus suspension 

 the use of the term is a convenience. At least two kinds of molecular weight 

 can be distinguished; an anhydrous weight, and a weight that includes 

 bound water. But ambiguity exists as to how "bound" water is defined, and 

 with some techniques there is uncertainty as to how its presence might 

 affect the determination of a molecular weight. Hydration (and its hydro- 

 dynamic effects) is one of the physical properties of viruses that is still the 

 subject of vigorous investigation and discussion. 



Discussed only briefly is a consideration of the movement of particles m 

 a solution subjected to an electric field. Despite the great utility of electro- 

 phoresis m both the isolation of viruses and in the analysis of preparations 

 of purified viruses, little is known about the relationship between the rate 

 of migration of charged macromolecules and the number, nature, and loca- 

 tion of ionizing groups on their surface. 



Despite the somber implications of the above remarks we can form some 

 sort of picture of a virus particle in suspension. It is an object that generally 

 has a symmetrical shape, frequently spherical or nearly so. Within it there 

 are polypeptide chains and nucleic acid polymers in, presumably, some regular 

 spatial array. Tliroughout the whole particle, in the interstices between the 

 chains, there are water molecules and some ions which are influenced in their 

 distribution by the charged groups on the polypeptides and nucleic acids. 

 The central region of the particle is probably filled only with fluid. The outer 

 surface is certainly not smooth, and may consist of deep groovings and 

 knobs. The surface has many charge groupings, and the water and ions of the 

 environment are locally bound with differing concentrations, depending 

 upon the type and nature of the groups. Outside the particle as a whole there 

 is an ionic atmosphere that is radially nonuniform, extending for a distance 

 perhaps as large as the radius of the particle and depending upon the ionic 

 strength of the solution. This is the sort of object whose physical properties 

 we are going to consider. 



The last ten years ha,ve witnessed revolutions in many of the disciplines 

 used in the characterization of large molecules, with the result that precision 

 not previously contemplated is now commonplace and measurements hardly 

 visualized ten years ago are routine. Thus many of the results on many types 

 of macromolecules are obsolete, and new data will rapidly supercede those 

 compiled with great labor over many years. Simultaneous with these advances 

 in theory and technique have been improvements in preparative procedures. 

 The methods employed now for the isolation and purification of viruses are 

 often milder and faster than those used years ago, with the result that the 

 deleterious effects of various chemical reagents and cellular enzymes are 



