Structures, Dynamics, and Biological Activities of 15 Cyclic Hexapeptide Analogs of the α-Amylase Inhibitor Tendamistat (HOE 467) in Solution

The design, synthesis, and conformational analysis of a series of 15 cyclic hexapeptides as analogs of the active sequence of the α-amylase inhibitor protein Tendamistat (HOE 467) Ser¹⁷-Trp¹⁸-Arg¹⁹-Tyr²⁰are described. A template-oriented peptide design strategy was used to expose this tetrapeptide motif to different conformational environments. Conformational analysis was carried out for each peptide in DMSO-d₆solution by means of NMR spectroscopy. For structure determination, restrained molecular dynamics (MD) simulations in vacuo and in DMSO based on experimentally derived distance and torsion constraints were performed. For eight peptides, experimental data were found to be inconsistent unless multiple fast interconverting backbone conformers were taken into account. For these peptides the NMR observables can only be described by averaging over conformational ensembles containing at least two major backbone conformations. All other compounds can be described by a single backbone conformation. Some general rules for rigidification of peptide backbone conformations can be verified by analyzing different peptide structures. It could further be shown that the use of backbone templates forces the tetrapeptide sequence to adopt its native conformation, as found in solution and crystal structures of Tendamistat. Significant biological activity as α-amylase inhibitors could be measured for these peptides. However, the suggested active tetrapeptide sequence alone is not responsible for the strong binding between Tendamistat and α-amylase, which is supported by the inspection of the preliminary solid-state structure of the Tendamistat/α-amylase complex.