Abstract
1. The chlorophyll-protein compound of the spinach leaf has been studied in the air-driven ultracentrifuge using the Svedberg light-absorption method, and a direct-reading refractive index method. 2. When the untreated extracts are centrifuged at low speeds, the green protein sediments with a purely random spread of particle sizes confirming the fact that the protein is not in true solution. 3. In the presence of digitonin, bile salts, and sodium desoxycholate, the extracts are clarified. These detergents split the chlorophyll from the protein and the protein itself shows a sedimentation constant of 13.5 x 10–13 equivalent to a molecular weight of at least 265,000 as calculated from Stokes' law. This probably represents the minimum size of the protein in native form. 4. Sodium dodecyl sulfate, a detergent which also clarifies the leaf extracts, shows a different behavior. The prosthetic group remains attached to the protein but the protein is split into smaller units. In 0.25 per cent SDS, S 20 is 2.6 x 10–13 over a pH range of 5 to 9, although at the acid pH chlorophyll is converted to phaeophytin. In 2.5 per cent SDS, S 20 is 1.7 x 10–13 suggesting a further splitting of the protein. 5. No differences in behavior were found for the various chloroplast pigments.
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