Monellin, a sweet protein, was discovered in 1969 in the fruit of the West African shrub known as serendipity berry (Dioscoreophyllum cumminsii); it was first reported as a carbohydrate. The protein was named in 1972 after the Monell Chemical Senses Center in Philadelphia, U.S.A., where it was isolated and characterized. Protein composition Monellin's molecular weight is 10.7 kDa. It has two noncovalently associated polypeptide chains: an A chain sequence with 44 amino acid residues, and a B chain with 50 residues. Monellin chain A (44 AA): Monellin chain B (50 AA): Amino acid sequence of the sweet protein monellin adapted from Swiss-Prot biological database of protein. UniProtKB/Swiss-Prot database entry #P02882 Monellin has a secondary structure consisting of five beta-strands that form an antiparallel beta-sheet and a 17-residue alpha-helix. In its natural form, monellin is composed of the two chains shown above (), but the protein is unstable at high temperatures or at extremes of pH. To enhance its stability, single-chain monellin proteins were created in which the two natural chains are joined via a Gly-Phe dipeptide linker. This modified version of the protein (MNEI) has been studied using NMR and X-ray diffraction. In addition to its secondary structure, four stably bound sulfate ions were located on the monellin protein, three on the concave face of the protein and one on the convex face of the protein. The sulfate ion on the convex face of the protein is of particular interest because it lies adjacent to a patch of positive surface potential, which may be important in electrostatic interactions with the negative T1R2-T1R3 sweet taste protein receptor. Sweetness properties Monellin is perceived as sweet by humans and some Old World primates, but is not preferred by other mammals. The relative sweetness of monellin varies from 800 to 2000 times sweeter than sucrose, depending on the sweet reference against which it is assessed. It is reported to be 1500-2000 times sweeter than a 7% sucrose solution on a weight basis and 800 times sweeter than sucrose when compared with a 5% sucrose solution on a weight basis. Monellin has a slow onset of sweetness and lingering aftertaste. Like miraculin, monellin's sweetness is pH-dependent; the protein is tasteless below pH 2 and above pH 9. Blending the sweet protein with bulk and/or intense sweeteners reduces the persistent sweetness and shows a synergistic sweet effect. Heat over 50 °C at low pH denatures monellin