Thermal unfolding of proteins is used extensively in screening of drug candidates because molecular interactions with ligands and substrates affect strongly protein stability, transition temperature, and cooperativity. We use synchrotron radiation circular dichroism to monitor the thermal evolution of secondary structure in proteins as they approach the melting point and the impact of substrate on their thermal behavior. Using Landau free energy expansion, we quantify transition strength and proximity to a critical point through the relative separation tau(+) between the transition temperature T-m and the spinodal T+, obtained from the equation of state. The weakest transition was observed in lysozyme with tau(+) = -0.0167 followed by holo albumin with tau(+) = -0.0208 with the strongest transition in monomeric apo albumin tau(+) = -0.0242. A structural transition at 45 degrees C in apo albumin leads to a noncooperative melt with tau(+) = -0.00532 and amyloidogenic increase in beta content.