On the oxy analogues to the 3 leads to 1 intramolecularly hydrogen-bonded peptide conformations.

The possible occurrence of the oxy analogues to the 3 leads to 1 intramolecularly hydrogen-bonded peptide conformations (main feature of the gamma turn), recently proposed in solution by several authors, has been investigated in a number of N-tert-butyloxycarbonyl-alpha-amino acids by x-ray diffraction, infrared absorption, proton magnetic resonance, and circular dichroism techniques. These folded conformations are absent in the solid state in all cases so far examined; however, they seem to be present in solution, the extent of the population of these forms in the conformational equilibrium mixtures being solvent, temperature, and structure dependent. In the solid state N-tert-butyloxycarbonyl-D-valine has the urethane-CONH-group in the cis configuration; this is the first time such a configuration has been found in the solid-state for a secondary amide group in a linear compound.