Helicalversus Planar Conformation of Homooligopeptides Prepared from Diethylglycine (=2-Amino-2-ethylbutanoic Acid)

Homooligopeptides containing α,α-diethylgycine (=2-amino-2-ethylbutanoic acid), were synthesized by conventional solution methods. An ethyl or methyl ester was used as protecting group at the C-terminus and a trifluoroacetyl group as protecting group at the N-terminus of the peptides. The conformations of such tri-, penta-, and hexapeptides in the solid state were studied using X-ray crystallographic analysis, and were shown to be a bent planar C5-conformation in the case of tripeptide 8a, and a 310-helical structure in the case of pentapeptide 10 and hexapeptide 11. IR and 1H-NMR spectra revealed that the dominant conformation of hexapeptide 11 in CDCl3 solution was not the 310-helical structure shown in the solid state, but a fully planar C5 structure.