Potential energy calculations on phenylalanine rotamers in different boat forms of proline-containing cyclic dipeptides.

Previous potential energy calculations have always indicated that the folded conformation is the most stable one for cyclic dipeptides containing benzylic side chains. We have carried out intramolecular van der Waals potential energy calculations on cyclo-(D-Phe-L-Pro), N-(N-phenylacetyl-L-alanyl)-cyclo-(L-Phe-D-Pro) and N-(pyruvoyl)-cyclo-(L-Phe-D-Pro) adopting the geometries found in their crystal structures. The calculations made on these compounds, having the peptide skeleton made of the same aminoacid residues but differing in the degree of buckling of the boat, show that there is a dependence between diketopiperazine ring and phenylalanine side-chain conformations. The folded conformer is preferred as long as it is allowed by the degree of buckling of the boat. When the boat becomes highly buckled, as in the case of the pyruvoyl derivative, the folded rotamer is no longer favoured. This is the first energy calculation to demonstrate that the folded rotamer is not always the most stable one.