(S)-C alpha-ethyl, C alpha-benzylglycine [(S)-(alpha Et)Phe] peptides fold in left-handed helical structures

The first x-ray diffraction structure analysis of a C alpha-ethyl, C alpha-benzylglycine [(alpha Et)Phe]-containing peptide, N alpha-benzyloxycarbonyl-alpha-aminoisobutyryl-alpha-amino-isobutyr yl-(S)- C alpha-benzylglycyl-alpha-aminoisobutyric acid (methanol solvate), has been performed. In the crystal state the N alpha-protected tetrapeptide is folded in an incipient, left-handed 3(10)-helical structure. This finding confirms that the relationship between (alpha Et)Phe alpha-carbon chirality and screw sense of the helix that is formed is opposite to that exhibited by protein amino acids, including Phe.