Solid-phase synthesis of crystalline [Ser41] B-chain monellin, an analogue of the sweet protein monellin.

The sweet protein monellin consists of two noncovalently associated polypeptide chains, the A chain of 44 amino acid residues and the B chain of 50 residues. Since blocking the free SH group of Cys41 in the B chain or treating the adjacent Met42 with CNBr removed its sweetness, this part of the molecule has been suggested to be essential for the sweetness. The [Ser41] B chain, an analogue of the B chain, was synthesized by the stepwise Fmoc solid-phase method in an overall yield of 1.9%. The synthetic B chain analogue was combined with the A chain, which was left over from a previous work, to give [Ser41] B-chain monellin in a yield of 31.0%. This synthetic monellin analogue was 2000 times as sweet as sucrose. Changing the Cys41 residue to the Ser residue significantly decreased the sweetness potency and stability of the molecule in solution. Crystallization was carried out by a vapor diffusion method.