Solution conformation of a retro-D analogue of tocinamide

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Abstract
The solution conformation of a retro-D analogue of tocinamide .**GRAPHIC**. was examined using PMR and circular dichroism spectroscopy. The observations support major contributions to the conformational distribution from structures with a type I .beta. turn in the sequence D-Asp-D-Gln-D-alle-D-Tyr. This is topologically similar to the .beta. turn proposed for oxytocin, L-Tyr-L-Ile-L-Gln-L-Asn, but with the polarity of the CONH groups reversed along the chain; the peptide is hormonally inert. In conjunction with NMR resonance data, the circular dichroisim spectra are interpreted to indicate that the region of the peptide ring near the disulfide occurs in at least 2 different conformations. One of the side-chain carboxamides, probably that of asparagine, appears to be intramolecularly associated rather than freely exposed to solvent.