Spectrometric Investigation of Thaumatin I and II, Two Sweet‐Tasting Proteins from Thaumatococcus daniellii Benth

The sweet‐tasting proteins, thaumatin I and II, isolated from the fruits of Thaumatococcus daniellii Benth, both lose their sweetness on heating. Circular dichroism and proton magnetic resonance measurements indicate that the proteins undergo reversible conformational changes as the temperature is increased. Then at a certain temperature, which varies with pH, irreversible heat denaturation occurs. At pH 5.0, the native conformation is most stable and the conformational change is reversible up to 75 °C even on prolonged heating. We concluded from the spectroscopic data that the proteins are flexible molecules and that tyrosine residues and at least one disulphide chromophore are involved in the conformational change. The results justify the assumption that the irreversible heat‐induced conformational transition is responsible for the loss of sweetness of the proteins.