Solid-Phase Synthesis and NMR Structural Studies of the Marine Antibacterial Cyclic Tetrapeptide: Cyclo(GSPE)

Twelve-membered head-to-tail cyclic tetrapeptides (CTPs) are rigid molecules found in nature and possess a diverse range of biological activities. A possible reason may be due to their ability to adopt rigid conformations in solution mimicking reverse-turns. Reverse-turns are common structural motifs which serve as molecular recognition sites in many protein-receptor interactions. In this paper, we describe the solid-phase synthesis of the antibacterial cyclic tetrapeptide cyclo[Gly-Ser-Pro-Glu] (cyclo[GSPE]), first isolated from the Ruegeria strain of marine bacteria by Mitova et al. (J Nat Prod 67:1178–1181, 2004). Our NMR experiments in H2O:D2O:DMSO (18:1:1) revealed that it possessed three conformations in an approximate ratio of 4:2:1 based on NMR amide peak intensities. 2D NMR studies and computer calculations revealed that the major conformer adopted a reverse-turn conformation and have ω torsion angles twisted by up to 2°, with two transoid amide bonds between Gly-Ser, Pro-Glu and two cisoid amide bonds between Ser-Pro, Glu-Gly in a cis–trans-cis–trans (ctct) pattern. This supports previous reports that majority of CTPs adopt a ctct pattern when dissolved in hydrogen-bond disrupting solvents (Che and Marshall in J Med Chem 49:111–124, 2006 and references cited therein). An ensemble of ten lowest-energy-minimised 3D structures generated using XPLOR-NIH software revealed that cyclo[GSPE] possessed a rigid backbone ring scaffold. The remaining two minor conformers were present in quantities too low for NMR structural studies.