14] TOBACCO MOSAIC VIRUS 255 



actions between tyrosine and undissociated carboxyl groups have been ob- 

 served also in model experiments of Laskowski,-*' and there are indications 

 that they may produce much smaller spectral shifts than does the carboxylate- 

 tyrosine interaction. However, such an effective masking of protein carboxyl 

 groups has not previously been demonstrated and will have to be supported 

 by much more experimental evidence than is available at present. 



15 



10 



91 mg TMVr BiaM subunits 



Constant pH = 7,5 



T=41° 



+ SDS 



10 



15 



5 



MINUTES 



Fig. 3. Alkali consumption during degradation of TMV in 1 % sodium dodecyl sulfate 

 at a constant pH (7-4). 



A recent series of observations appears to both confirm and extend the 

 information concerning particular inter-subunit carboxyl binding sites in the 

 virus. The X-ray diffraction data of Caspar-'' have shown that lead is bound 

 by the virus stoichiometrically, and in two distinct localizations. Unpub- 

 lished observations from this laboratory have indicated that both these sites 

 differ from the SH group which is not substituted by lead. It has also been 

 observed that the addition of lead acetate (pH 6) to virus at pH 6 produces 

 a sharp drop in pH which is compensated by an equivalent amount of 

 alkali. This could be regarded as evidence that lead can replace the two non- 

 dissociated carboxyl hydrogen atoms of each subunit. Support for this con- 

 cept comes from the finding that no alkali is consumed upon the subsequent 

 addition of SDS, or at pH 9 (40°). The supposed replacement of the hydrogen 

 bond by a lead atom does not weaken, but rather strengthens, the structure 

 of TMV as indicated by the extent of splitting of protein and nucleic acid 

 under standard conditions of degradation. Other divalent metals giving stable 

 acetates (pH 5 -5-6 -5) such as zinc and magnesium, react similarly to lead, 

 although with decreasing affinity. This was indicated by slower and less 

 stoichiometric interaction (approaching only one site with Mg++), and less 

 stabilization upon splitting. Also subsequently added lead produced a further 

 pH drop, while other metal salts added after lead acetate had no such effect. 

 The particular affinity of lead for carboxyl groups is in accord with the 



