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



A great variety of data has now been amassed on the physical properties 

 of TMV. Most of the data are obsolete either because they were gathered 

 before the techniques had attained a degree of reliability worthy of considera- 

 tion today or because the virus preparations worked with were of doubtful 

 quality. The assessment of the quality of a preparation requires the subjec- 

 tive judgment of the investigator, and different attitudes are to be expected. 

 For the detailed presentation of a view considered extreme by most workers 

 in the field, the reader should consult Pirie (1957). Other positions are 

 expressed in reviews by Lauffer et al. (1949), Bawden (1950), Markham 

 (1953), Markham and Smith (1954), and Schramm (1954a,b). 



b. Molecular Weight. As long ago as 1938 Lauffer (1938a) combined data 

 from sedimentation velocity and viscosity measurements to make a com- 

 prehensive analysis of the size, shape, and molecular weight of TMV. He 

 concluded that the virus particles were rodlike with an axial ratio of about 

 30 : 1 and that the molecular weight was 43 X 10^. His calculated values of 

 the molecular weight, length, and thickness have suffered many changes in 

 the ensuing twenty years, as more precise physical chemical parameters have 

 been obtained and the quality of the virus preparations improved. These 

 changes, although by no means negligible, do not detract from the original 

 contribution which represents one of the important triumphs of physical 

 chemistry. For it was not until a few years after Lauffer's prediction of the 

 size and shape of the virus particles that their direct visualization was achieved 

 with the development of the electron microscope. To review in an integrated 

 fashion at this time the existing data that have been obtained in many 

 laboratories is difficult because only limited studies were made in most cases 

 and most preparations doubtless contained some particles that were partially 

 degraded and others that were aggregated. Despite the recent combined efforts 

 of Boedtker and Simmons (1958) and of Hall (1958), a definitive study of the 

 physical properties of TMV still remains to be done, one in which a single 

 preparation is examined critically by ultracentrifugation, diffusion, vis- 

 cometry, streaming and electric birefringence, light scattering, electron 

 microscopy, and X-ray analysis. 



Of the various sedimentation velocity studies that have been made in the 

 past twenty years, none have been performed with a degree of precision 

 that can be considered by present standards to be adequate. The various 

 sedimentation coefficients reported, for infinitely dilute solutions, range from 

 185 (Lauffer, 1944a) to 198 S (Schramm and Bergold, 1947). This spread is 

 much larger than would be expected with current techniques, but there is 

 no evidence that the most recent values, as a matter of fact, have been 

 obtained with any greater precision than was attained in those earlier deter- 

 minations. Owing to the variation of sedimentation coefficient with con- 

 centration each experiment must be evaluated in a way which accounts for 



