398 S. GARD AND O. MAAL0E 



therefore be interpreted as the result of changes in the net cliarge of the 

 surface of the virus particle. No doubt the virus in this condition has to be 

 regarded as attenuated, although the property is not hereditary. 



The basic mechanisms regulating the penetration of a chemical agent into 

 the interior of the virus particle seem to be accessible to direct chemical 

 studies. At least, a very promising technique was recently described by 

 Meriwether and Rosenblum (1957). These authors measured the incorporation 

 of C^*-labeled formaldehyde in TMV. They concluded from the shape of the 

 curves that a diffusion process was involved and applied diffusion mathe- 

 matics to the analysis of the resuJts. The brief preliminary communication 

 cannot serve as a basis for a detailed discussion; it seems highly probable, 

 however, that further research along these lines will prove rewarding. 



In the following sections individual chemical agents and groups of agents 

 will be reviewed. Until the nature of the reactions involved is better under- 

 stood a truly rational classification is hardly feasible. The one here adopted 

 is somewhat arbitrarily chosen and far from rational in the sense that over- 

 lappmg frequently occurs. Actually, it is based mainly upon the amount of 

 information available rather than upon attempts at analysis of the nature 

 of the pertinent reactions. 



B. Formaldehyde 



Formaldehyde reacts readily with proteins and in many different ways 

 (for reviews of formaldehyde-protein reactions see French and Edsall, 1945; 

 Walker 1953). Keactive sites are amino and imino groups; peptide linkages; 

 amido, sulfhydryl, and hydroxy groups; and several ring structures. The 

 reactions are often initially reversible, later become irreversible. Formalde- 

 hyde seems to react mainly in the hydrated state, as methylene glycol; the 

 second step of the reaction usually includes formation of methylene bridges 

 between two reactive sites, giving rise to new ring structures, or tying side 

 chains or adjacent molecules firmly together (Fraenkel-Conrat and Olcott, 

 1948; Fraenkel-Conrat and Mecham, 1949). The over-all effect is that of a 

 tanning agent: the structure becomes denser and less permeable, charge and 

 solubility decrease, and the protein becomes chemically more inert. 



The reactions between formaldehyde and nucleic acids are less extensively 

 studied. Zamenhof et al. (1953) found no measurable change in biological 

 activity or physicochemical properties after exposure of the DNA of the 

 Hemophilus transforming principle to 0-33 M formaldehyde for 5 hours. In 

 the presence of 4 M formaldehyde, however, a, gradual decrease in viscosity 

 and a rapid loss of activity was observed, presumably attributable to sub- 

 stitutions in the amino groups and breakage of H bonds. Fraenkel-Conrat 

 (1954) found an increase of about 20 % in UV absorption and a shift of the 



