PHYSICAL AXD CHEMICAL PROPERTIES — PROTOCHLOROPHYLL HOLOCHROME 333 



From the changes in absorption with changes in pH it is possible to 

 calculate pK values for the components undergoing change. The equation 

 for this calculation is 



pK = pH-log.4 + log(.4o-^) 

 Here pK and pH have their usual meaning, and A is the absorbance 

 change at a particular pH while ^o ^^ the maximum change in absorbance 

 produced by increase of pH. The change in absorbance is measured from 

 a reference absorbance which is constant over a considerable pH range at 

 the lower pH values. In order to make the calculations consistent among 

 themselves, the changes in absorption were always related to the maximum 

 absorption of the corresponding curves. From manv ultraviolet absorption 



Wavelength {m/.i) 



Fig. 6. The variation in the ultraviolet absorption spectrvim of protochlorophyll 

 holochronie with pH. 



curves, such as those presented in Fig. 6, pK values were calculated from 

 changes in the absorbancies at ^oo m/x and at the minimum near 2^0 m/Lt 

 prominent in the left-hand cur\e. The two values obtained were 11 -o and 

 10-4, respectivelv. 



The changes in absorption at ^00 m/t and the pK \alue obtained 

 certainly implicate tyrosine as one of the amino acids undergoing ionization 

 in the holochrome. The changes at the absorption minimum could possibly 

 be ascribed to cysteine [14] although this is by no means certain. The pK 

 value of 10-4 ajiproximates to that reported for cysteine, 9- 1 to 10 -8 [12]. 



ULTRAVIOLET IRRADIATION AND PROTOCHLOROPHYLL TRANSFORMATION 



A further reason for assuming that protochlorophyll is attached to 

 more than one amino acid is the effect of exposing the holochrome to 



