The free energy of dissociation of oligomeric structure in phycocyanin is not linear with denaturant

Using SEC HPLC and fluorescence anisotropy, absorption spectra were assigned to the specific oligomeric structures found with phycocyanin. The absorption spectra were used to quantify the population of each oligomeric form of the protein as a function of both urea concentration and temperature. Phycocyanin hexamers dissociate to trimers with equilibrium constants of 10(-6) to 10(-5). Phycocyanin trimers dissociate to monomers with equilibrium constants of 10(-15) to 10(-12). Both dissociation constants increase linearly with increasing urea concentration, and Delta G values calculated from the equilibrium constants fit best with an exponential function. Our findings appear in contrast with the commonly used linear extrapolation model, Delta G(urea)degrees = G(water)degrees + A[denaturant], in which a linear relationship exists between the free energy of protein unfolding or loss of quaternary structure and the denaturant concentration. Our data examines a smaller range of denaturant concentration than generally used, which might partially explain the inconsistency.