On the oxy analogues to the 4 leads to 1 intramolecularly hydrogen-bonded peptide conformations.

The occurrence of the oxy analogue to the type II' 4 leads to 1 intramolecularly hydrogen-bonded nonhelical peptide conformation, recently proposed for t-BOC-Gly-L-Pro-OH in the solid state by Deber on the basis of infrared absorption evidence, has been disproved by x-ray diffraction analysis. This type of folding is also absent in solvents of moderate or high polarity. The latter conclusion is in agreement with Deber's results. However, in solvents of low polarity this intramolecularly hydrogen-bonded form could account for the strong negative Cotton effect near 230 nm observed in the circular dichroism spectrum.