Mixed conformation in C alpha, alpha-disubstituted tripeptides: x-ray crystal structures of Z-Aib-Dph-Gly-OMe and Bz-Dph-Dph-Gly-OMe.

We report here the synthesis and molecular structure in the solid state of fully protected tripeptides containing C alpha, alpha-diphenylglycine (Dph), namely Z-Aib-Dph-Gly-OMe (Aib: C alpha, alpha-dimethylglycine) and Bz-Dph-Dph-Gly-OMe. The molecular conformation around the Dph residue, containing two bulky substituents, is fully extended, while the Aib residue, containing two smaller groups on the C alpha atom, adopts the typical 3(10)/alpha-helical conformation. Gly residues, without substituents on the C alpha atom, show different conformational preferences. Each residue seems to behave, from a conformational point of view, independently from the presence of the other residues, and thus mixed local conformations (folded and extended) are present in the crystals. The nonconventional peptide synthesis, using the Ugi reaction, is also reported.