X-ray structures of new dipeptide taste ligands.

The molecular basis of sweet taste was investigated by carrying out the crystal state conformational analysis by X-ray diffraction of the following dipeptide taste ligands: N-3,3-dimethylbutyl-aspartylphenylalanine methyl ester, I (N-DMB-Asp-Phe-OMe), its sodium salt (N-DMB-Asp-Phe-ONa), II, aspartyl-D-2-aminobutyric acid-(S)-alpha-ethylbenzylamide, III (Asp-D-Abu-(S)-alpha-ethylbenzylamide), aspartyl-N'-((2,2,5,5-tetramethylcyclopentanyl)-carbonyl)-(R)- 1,1-diamino-ethane, IV (Asp-(R)-gAla-TMCP), and aspartyl-D-valine-(R)-alpha-methoxymethylbenzyl amide, V (Asp-D-Val-(R)-alpha-methoxymethylbenzylamide). With the exception of the sodium salt II, all compounds are sweet-tasting, showing in some cases considerable potency enhancement with respect to sucrose. The results of this study confirm the earlier model that an 'L-shape' molecular array is essential for eliciting sweet taste for dipeptide-like ligands. In addition, it was established that (i) substitution of the N-terminal group does not inhibit sweet taste, if its zwitterionic character is maintained; (ii) a hydrophobic group located between the stem and the base of the L-shape could be responsible for sweetness potency enhancement, as found in I, III and IV; in fact, the extraordinary potency of the N-alkylated analogue I would support a model with an additional hydrophobic binding domain above the base of the 'L'; (iii) removal of the methyl ester at the C-terminus of compound I with the salt formation gives rise to the tasteless compound II; (iv) for the first time all possible side-chain conformers (g-, g+ and t) for the N-substituted aspartyl residue were observed; and (v) a retro-inverso modification, incorporated at position 2 of the dipeptide chain, confers greater flexibility to the molecule, as demonstrated by the contemporary presence of six conformationally distinct independent molecules in the unit cell and yet sweet taste properties are maintained, as found in IV.