Stability of allophycocyanin's quaternary structure.

The dissociation of allophycocyanin trimers to monomers was examined under a variety of conditions. For alkyl ureas and alcohols the dissociation increased as the straight-chain alkyls increased in length. The effect of branching chains was smaller. Tetrapropylammonium chloride was found to be a very effective agent for trimer dissociation when compared to ureas and alcohols with similar or longer alkyl chains. An explanation for these observations is that the hydrocarbons have an affinity for nonpolar regions in the contact areas between monomers in a trimeric structure. A comparison among several inorganic salts demonstrated that the chaotropic salts (NaSCN greater than NaClO4 much greater than NaNO3 greater than NaBr) fostered increased trimer dissociation, while nonchaotropes (KF, (NH4)2SO4, K phosphate, and NaCl) produced no measurable amounts of monomer. Allophycocyanin dissolved in D2O was much more stable against dissociation than when dissolved in H2O. All the above observations were consistent with hydrophobic forces being the dominant source of trimer stabilization. The equilibrium constant for the dissociation of trimers to monomers was calculated to be about 6 X 10(-16) mol2 liter-2. Calculations were made of the apparent total number of amino acids (40) in the two contact regions on each monomer. An absorption change analogous but not necessarily identical to a conversion of allophycocyanin II to III was noted when (NH4)2SO4 was present. When allophycocyanin's nonexchangeable hydrogens were replaced by deuteriums, it was more readily dissociated to monomers.