Ultracentrifuge study of C-phycocyanin aggregation

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Abstract
The MW of C-phycocyanin [from Phormidium luridum] was determined as a function of protein concentration in a sodium acetate buffer solution of pH 4.8 at 21.2.degree. C by the Y phantis method. The higher aggregate, the dodecamer (19 S), is first removed by dissociation simply by dialysis against pH 3.9 acetate buffer solution. The MW data are best interpreted by the simultaneous presence of monomers, trimers and hexamers. Assuming a monomer .**GRAPHIC**. trimer .**GRAPHIC**. hexamer equilibrium system, the equilibrium constants were calculated, and the values are K13 = 1.4 .times. 1012 (l/mol)2 and K36 = 6.1 .times. 105 l/mol. The corresponding free energies for each step were calculated to be -16.3 kcal/mol (.DELTA.G.degree.13) and -7.76 kcal/mol (.DELTA.G.degree.36). With these values of K13 and K36, the weight-average as a function of concentration is calculated. Satisfactory agreement was obtained between the calculated curve and the experimental data. Sedimentation velocity studies performed with a band-forming centerpiece, on the same protein solutions used for the sedimentation equilibrium studies, confirm the presence of 3 species: monomers, trimers and hexamers.

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