Solution conformation of c-[Gln-Trp-Phe-Gly-Leu-Met], a NK-2 tachykinin antagonist

The conformation of cyclo-[Gln-Trp-Phe-Gly-Leu-Met], a potent tachykinin antagonist selective for the NK-2 receptor, has been studied by 1H NMR spectroscopy in DMSO-d6 and in a DMSO-d6/H2O cryoprotective mixture in the temperature range 280-320 K. The NMR data cannot be interpreted on the basis of a single ordered conformation. An exhaustive search, based mainly on missing NOEs among skeleton protons, yields a description of the conformational state in solution consisting of a few interconverting structures that can explain all observed NMR parameters. The relative position of the side chains of key residues may be interpreted in terms of bioactive conformations.