(;. WEBER - 95 



SO2 



N+ 



/ \ 

 H3C CH3 



to the lowest excited state. The emission and absorption spectra 



can give inlorniation upon the properties of the enviroinnent in the 



protein-DNS conjugates. 



Excited States in the Protein 



Simple proteins: these can be divided into two groups: Class A, 

 containing phenylalanine and tyrosine; and Class B, containing 

 tryptophan as well. The contribution of phenylalanine to the total 

 absorption is very small, and since its fluorescence yield appears to 

 be also very small qualitatively, only tyrosine and tryptophan 

 fluorescence is observed. 



Class A proteins include insulin, tropomyosin, zein, and ribo- 

 nuclease, besides several trypsin inhibitors. The first four proteins 

 mentioned have quantum yields in the neighborhood of 0.03, while 

 the inhibitors have yields much smaller than 0.01 (24) . The fluores- 

 cence spectrum of this class of proteins is indistinguishable from 

 that of free tyrosine, with a maximum at 303 ni/x. The polarization 

 spectra, which were studied in 50-90% propylene glycol water mix- 

 tures, at — 70°C are essentially those of tyrosine, differing only by 

 the fact that the absolute polarization values are smaller than those 

 of tyrosine or phenol over the whole spectral region (Fig. 10) . The 

 conclusion is unavoidable that some transfer of the excited state 

 among residues of tyrosine takes place. Since the distances between 

 the tyrosine and their relative spatial orientation in these proteins 

 is not known, an accurate estimation of the extent of the transfer 

 cannot be given. 



Class B proteins, which include the majority of globular pro- 

 teins, show a fluorescence spectrum which is characteristic of trypto- 

 phan, and of tryptophan alone (24) . The maxima of emission range 

 from chymotrypsin (332 m^a) to pepsin (346 m^) (Fig. 11). From 

 these observations one could conclude that in pej«in the tryptophan 

 residues of which the fluorescence is observed are in contact with the 

 water or a medium of similar polarizability properties, while in 



