334 



JAMES H. C. SMITH 



ultraviolet radiation. Mr. G. C. McLeod and Miss J. Coomber, in our 

 laboratory [15], discovered that protochlorophyll holochrome irradiated 

 with various ultraviolet wavelengths between 250 and 330 myu, converted 

 only 25 to 30% of the protochlorophyll transformed at 366, 436 m^ 

 (Fig. 7), or with visible light from an electric lamp. If after ultraviolet 

 irradiation, however, the holochrome solution was placed in visible light, 

 the same degree of transformation was achieved as if no previous con- 

 version with ultraviolet had occurred. Wherefore, the ultraviolet at the 

 intensities used had no ill effect on the transformation. 



300 350 400 



Wavelength (my/) 



450 



Fig. 7. The maximum conversion of holochromatic protochlorophyll to 

 chlorophyll in the range 436 to 250 m/j. 



The conversion with ultraviolet light could not be explained by proto- 

 chlorophyll absorption else the conversion would have been augmented 

 with longer exposures. But the exposures given were two or three times 

 those necessary to achieve maximum conversion in the 250-330 mn range. 



The limited action of ultraviolet light may be reasonably explained by 

 assuming the protochlorophyll to be activated through transfer of the 

 energy absorbed by a closely associated amino acid. Only the aromatic 

 amino acids absorb appreciably throughout this range, and of these acids 

 only tyrosine has the proper pK value to correspond with the alkalinities 

 effective in the inhibition of the transformation. From these considerations 

 it is concluded that about 25 to 30% of the protochlorophvll is attached 

 to protein in the holochrome through the tyrosinyl group. 



