Thermodynamics of the alkaline transition in phytocyanins

The thermodynamics of the alkaline transitionwhich influences the spectral and redox properties of thetype 1 copper center in phytocyanins has been determinedspectroscopically. The proteins investigated include Rhusvernicifera stellacyanin, cucumber basic protein and itsMet89Gln variant, and umecyanin, the stellacyanin fromhorseradish roots, along with its Gln95Met variant. Thechanges in reaction enthalpy and entropy within the proteinseries show partial compensatory behavior. Thus, thereaction free energy change (hence the pKa value) is rathervariable. This indicates that species-dependent differencesin reaction thermodynamics, although containing animportant contribution from changes in the hydrogenbondingnetwork of water molecules in the hydrationsphere of the protein (which feature enthalpy–entropycompensation), are to a large extent protein-based. Thedata for axial ligand variants are consistent with thehypothesis of a copper-binding His as the deprotonatingresidue responsible for this transition.