128 



THE CELL AND PROTOPLASM 



form and the retention of constant prop- 

 erties following repeated crystallization 

 have been used as criteria of purity with 

 such great success in the chemistry of in- 

 organic and simple organic compounds that 

 the layman has tended to place undue con- 

 fidence in them. Henderson Smith has 

 stated that there was "attached a kind of 

 sanctity to the word 'crystal' " and that 

 crystallinity "was a sort of a guarantee of 

 purity. ' ' On the other hand, chemists, and 

 especially protein chemists, who are famil- 

 iar with the great tendency of proteins to 

 form solid solutions and to carry along 

 J impurities on crystallization, have long rec- 

 ognized that while crystallinity and the re- 

 tention of constant properties on repeated 

 crystallization are very useful tests in the 

 determination of the purity of a material, 

 they are not infallible tests; they are less 

 well suited for purity tests in the case of 

 proteins, and most certainly they are not 

 an absolute guarantee of purity. Neverthe- 

 less, following the announcement in 1935 of 

 the "isolation of a crystalline protein pos- 

 sessing the properties of tobacco-mosaic 

 virus," the fact that the material could be 

 obtained in crystalline form was more in- 

 strumental than any other in securing the 

 acceptance of the material as pure and as 

 the virus, although to me and to many 

 others it was perhaps the least convincing 

 of the experimental evidence. The fact 

 that the protein could be repeatedly crystal- 

 lized with retention of constant properties 

 was a more significant bit of evidence. The 

 crystallinity of the protein also caused 

 many individuals to decide that the virus 

 was non-living because they considered it 

 impossible for living organisms to take on 

 a crystalline structure. However, crystal- 

 linity is simply a structural regularity and 

 actually there need be no incompatibility 

 between the living and the crystalline 

 states. Nevertheless, if the protein had not 

 been obtained in crystalline form, it is very 

 probable that it would have been generally 

 considered as a minute living organism 

 somewhat similar to ordinary bacteria, be- 

 cause of the general conception of viruses 

 as living organisms. The crystallinity was 



useful in that it focused attention on the 

 other physical and chemical properties of 

 the protein, and although some workers 

 doubted that the virus activity was a prop- 

 erty of the protein, the material was gener- 

 ally accepted as being composed essentially 

 of large protein molecules. It should be 

 emphasized that crystallinity, of itself, of- 

 fers no evidences as to the living or non- 

 living nature of a material and is no abso- 

 lute guarantee of purity, and that retention 

 of constant properties following repeated 

 crystallization should be regarded as 

 merely one of a great many tests used 

 to determine purity, any one of which may 

 be fallible and all of which should be used 

 before a decision regarding purity is 

 reached. It has not been found possible 

 to crystallize some of the unstable virus 

 proteins, and it is likely that more viruses 

 will be isolated which cannot be crystal- 

 lized. In the cases of unstable viruses, the 

 continual breakdown may provide sufficient 

 impurity to prevent crystallization, and in 

 other cases it is possible that pure prepara- 

 tions may be obtained which will fail to 

 crystallize; hence failure to obtain a virus 

 in crystalline form should not be considered 

 as a definite indication of an impurity. 



Tobacco-mosaic and its strains, aucuba, 

 masked, and enation-mosaic viruses, the 

 closely related cucumber-mosaic-3 and -4 

 viruses, tobacco-necrosis virus, and bushy- 

 stunt-of-tomato virus have all been obtained 

 in the form of crystals readily visible under 

 the microscope. All of the first-named 

 viruses were obtained in the form of long, 

 thin, pointed needles, while the bushy-stunt 

 virus was obtained in the form of rhombic 

 dodecahedra, and the tobacco-necrosis virus 

 as thin plates. Latent-mosaic-of -potato vi- 

 rus has not been obtained in the form of 

 distinct crystals, but the pellet obtained on 

 high-speed centrifugation was found to be 

 doubly refracting. When 1 to 2 per cent 

 solutions of latent-mosaic virus or of to- 

 bacco-mosaic virus or its strains are allowed 

 to stand, they gradually separate out into 

 two distinct layers, the lower of which is 

 liquid crystalline. No purified viruses 

 other than those mentioned above have been 



