Conformational transitions of monellin, an intensely sweet protein.

Conformational transitions of monellin, an intensely sweet protein from the berries of Dioscoreophyllum cumminsii, were studied by the circular dichroism (CD) probe. According to the CD spectra, monellin has a low content of the helical structure and a significant amount of the pleated sheet (beta) conformation. The native conformation was found to be sensitive to alkali, sodium dodecyl sulfate, and guanidine-HC1, but it was stable in acid (e.g. pH 2.4) as shown by CD and persistence or the disappearance of sweet taste. The main chain conformation of the alkali-denatured monellin (pH 10.9) was restored upon acidification (pH 3.3) of the alkaline solutions. The tertiary structure, however, was not completely restroed, as indicated by CD in the 230-300 nm spectral zone, although the sweet taste reappeared. If the pH of a neutral solution was raised to 9.6, the CD in the near ultraviolet was significantly altered, though the sweet taste persisted. This indicates that a slight conformational change did not interfere with the effects on the taste buds. While sodium dodecyl sulfate readily disorganized the tertiary structure, the main chain was reconstructed by this reagent into a new form of higher helix content than in the native macromolecule. Reconstruction into a modified conformation of higher helix content was achieved also with 50% ethanol. The main chain conformation was not affected by 25% ethanol which produced slight changes in the CD at 230-260 nm zone and did not abolish the sweet taste.