PRESSURE-INDUCED DISSOCIATION AND DENATURATION OF ALLOPHYCOCYANIN AT SUBZERO TEMPERATURES

The thermodynamics of assembly of the allophycocyanin hexamer was exam ined employing hydrostatic pressures in the range of 1 bar to 2.4 kbar and temperatures of 20 to -12 degrees C, the latter made possible by the decrease of the freezing point of water under pressure. The existe nce of two processes, dissociation of the hexamer into dimers, (alpha beta)(3) --> 3 (alpha beta), and dissociation of the alpha beta dimers into monomers, (alpha beta)(3) --> alpha + beta have been recognized previously by changes in the absorbance and fluorescence of the tetrap yrrolic chromophores owing to added ligands. The same changes are obse rved in the absence of ligands at pressures of under 2.4 kbar and temp eratures down to -12 degrees C. On decompression from 2.4 kbar at 0 de grees C, appreciable hysteresis and a persistent loss of 50% in the ab sorbance at 653 nm is observed, It results from the conformational dri ft of the isolated subunits and is reduced to 10% when the highest pre ssure is limited to 1.6 kbar, The thermodynamic parameters of the reac tion alpha + beta --> alpha beta can be determined from pressure effec ts on perchlorate solutions of allophycocyanin, which consist of dimer s alone, Their previous knowledge permits estimation, under suitable h ypotheses, of the thermodynamic parameters of the reaction 3(alpha bet a) --> (alpha beta(3) from the overall pressure effects on the hexamer s. Both association reactions have positive enthalpy changes, and the whole hexamer assembly is made possible by the excess entropy.