Solution Properties of Phycocyanin. I. Studies of Dissociation-Association by Sedimentation Measurement

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Abstract
The dissociation-association properties of phycocyanin, isolated from the red algae, “Porphyra tenera,” have been investigated by sedimentation velocity measurements at pH 6.8 and 5.4 and at the ionic strengths of 0.1 and 0.2. The sedimentation coefficient data indicated that the predominant component in the phycocyanin solution was a trimer at pH 6.8 and a hexamer at pH 5.4. The concentration dependence of the sedimentation coefficient for various hypothetic systems of the dissociation-association equilibrium of phycocyanin was simulated by a computer model. It was reasonable to consider, from the simulation, that the dissociation-association system of phycocyanin was trimer\ightleftharpoonsmonomer at pH 6.8 and hexamer\ightleftharpoonsmonomer at pH 5.4. The dissociation constant was calculated for each system. The value of the dissociation constant was 0.15×10−4 (g/dl)2 at pH 6.8 and an ionic strength of 0.1; 0.13×10−4 (g/dl)2 at pH 6.8 and an ionic strength of 0.2; 0.5×10−13 (g/dl)5 at pH 5.4 and an ionic strength of 0.1 and 0.1×10−13 (g/dl)5 at pH 5.4 and an ionic strength of 0.2. The dissociation constants decrease with an increase in the ionic strength in solution.