CONFORMATIONAL-ANALYSIS OF THE DIPEPTIDE SWEETENER ALITAME AND 2 STEREOISOMERS BY PROTON NMR, COMPUTER-SIMULATIONS, AND X-RAY CRYSTALLOGRAPHY

The conformational preferences of the dipeptide sweetener alitame (L-aspartyl-D-alanine 2,2,4,4-tetramethyl-thietanylamide) and the related L,L and D,D stereoisomers of alitame were investigated by using high field proton NMR and computer simulations. In addition, the crystal structure of the L,D stereoisomer (alitame) was determined with a final R index of 0.079. The preferred conformations in DMSO of the intensely sweet L,D stereoisomer (alitame) can be described as possessing an ''L shape'', with the A-H and B containing aspartyl moiety as the stem of the L and the hydrophobic four-membered thietane ring as the base of the L, coplanar with but nearly perpendicular to the zwitterionic ring. These conformations were in agreement with those observed in the crystal structure, with only one backbone dihedral angle of the dipeptide differing somewhat (D-Ala-psi). For the L,L stereoisomer of alitame in solution, the thietane ring system projects beyond the plane of the zwitterionic ring, behind the stem of the ''L'' shaped peptide backbone. In contrast, the D,D stereoisomer of alitame shows considerable projection of the thietane ring system in a direction opposite of the enantiomeric L,L stereoisomer (i.e., in front of the L). The tastes of these molecules (i.e., the L,D, L,L, and D,D stereoisomers; sweet, bitter, and tasteless, respectively) are correctly predicted by our model for sweet taste developed with previous aspartyl-based peptide sweeteners. This work supports the notion that, in the case of aspartyl-based taste ligands, conformations in solution and in the crystal are closely related to each other.