Some Physical and Chemical Properties 

 of the Protochlorophyll Holochrome 



James H. C. Smith 



Carnegie Institution of Washington, Department of Plant Biology, 

 Stanford, Calif., U.S.A. 



Protochlorophyll, the chlorophyll precursor of dark-grown seedlings, 

 has been isolated in active form. Even though this material is separated 

 from the plant it is transformed by light to chlorophyll. Since the active 

 material is proteinaceous, it can be extracted and fractionated by the pro- 

 cedures of protein chemistry. The active material is particulate and shows 

 a distinct sedimentation peak in the ultracentrifuge diagram [i] with sedi- 

 mentation constant of about i6 to 17 Svedberg units. If it is assumed to 

 have the same density, i • 2^2>^ ^^ many other proteins with the same sedi- 

 mentation constant, its molecular weight would be about 400 000. Later 

 experiments have shown the particle to have a density in solution of 

 approximately i • 16. Based on this density the molecular weight would be 

 about 700 000 [2]. This agrees fairly well with a molecular weight of 

 900 000 calculated from the ratio of protochlorophyll to protein obtained 

 by analysis [2] when a molecular ratio of one, for pigment to protein, was 

 assumed. Conversely, the agreement of the results obtained bv analysis 

 and by centrifugation supports the assumption that the pigment-protein 

 complex contains only one protochlorophyll component. 



Carotenoids in protochlorophyll holochrome 



Although the centrifugation pattern indicates a fairly homogeneous 

 molecular species in respect to molecular weight, the carotenoid content 

 of the isolated material shows that all the particles cannot be of exactlv the 

 same composition. The caroienoid-protochlorophvH ratio varies from 

 preparation to preparation. Furthermore, on occasion, the molecular ratio 

 of total carotenoid to protochlorophyll is about one-half [3]. If each holo- 

 chrome particle contains one protochlorophyll, then some particles lack 

 carotenoids entirely. But the carotenoid fraction is made up of several 

 carotenoids, as reference to Fig. i shows, and this complicates the 

 stoicheiometry of the holochrome still further. 



The ratios of carotenoid to protochlorophyll imply either of two 



