Harnessing D-amino acids for peptide motif designs. Synthesis and solution conformation of Boc-D-Glu-Ala-Gly-Lys-NHMe and Boc-L-Glu-Ala-Gly-Lys-NHMe.

In examining the use of D-amino acids in designing specific peptide folding motifs, the tetrapeptide Boc-D-Glu-Ala-Gly-Lys-NHMe 1 and its analog 2 featuring L-Glu were synthesized for a comparison of their solution conformations by NMR spectroscopy. The temperature coefficients of amide proton resonances, NOE data, side-chain CH2 anisotropies and salt titration results suggest a weak type II reverse-turn conformation for peptide 2, and a tandem II' turn-3(10)-helix conformation of appreciable conformational stability for peptide 1 in apolar solvents. The latter is of potential interest as the N-terminal helix cap that could support the design of longer 3(10) helices. Possible origins of appreciable difference in the conformational stabilities of the diastereomers are discussed.