Thermal unfolding studies of a phytocyanin

The thermal stability of umecyanin, a stellacyanin from horseradish roots, has been investigated by differential scanning calorimetry, optical absorption and fluorescence spectroscopy at neutral and alkaline pH. Above pH 9 the Cu(II) protein experiences a blue shift of the main visible absorption band at ~600 nm and changes colour from blue to violet. The thermal transition of the protein is irreversible and occurs between 61.4 and 68.8 °C at pH 7.5 and between 50.7 and 57.4 °C at pH 9.8. The calorimetric data indicates that at both pH values the thermally induced transition of the protein between the native and denaturated states can be described in terms of the classical Lumry-Eyring unfolding model Native↔Unfolded→Final. The analysis of the reversible step in the unfolding pathway demonstrates a significant reduction in conformational stability (G) of the alkaline form of the protein. Such a reduction is consistent with an enhanced flexibility of UMC at high pH and has mainly entropic character.