Alkali Cation Complexation and Transport Properties of Synthetic Bicyclic Decapeptides: Structural, Thermodynamic, and Kinetic Analysis

Alkali cation complexation and bilayer transport by the bicyclic decapeptides S,S′‐bis‐cyclo‐glycyl‐L‐hemicystyl‐glycyl‐glycyl‐L‐prolyl (1), S, S′‐bis‐cyclo‐glycyl‐L‐hemicystyl‐glycyl‐glycyl‐D‐prolyl (2), S, S′‐bis‐cyclo‐glycyl‐L‐ hemicystyl‐sarcosyl‐sarcosyl‐L‐prolyl (3), and S, S′‐bis‐cyclo‐glycyl‐L‐hemicystyl‐sarcosyl‐sarcosyl‐D‐prolyl (4) were analyzed according to structural, thermodynamic and kinetic criteria; valinomycin was used as a reference ionophoretic system. Structural analysis of peptide 3 with spectroscopic methods showed different conformational arrangements in the bicyclic system depending on its state of complexation. Circular dichroism indicated the presence of a multitude of conformations with differing helicities around the disulfide bridge in both free and complexed states. Thermodynamic analysis by microcalorimetry demonstrated a far lower cation selectivity among the synthetic peptides than displayed by valinomycin. Peptide 3 shows cation affinities of about two orders of magnitude higher than peptide 4, but still much lower than found for the complexes of valinomycin with K+ and Rb+. In contrast to the latter case, the complexation reactions of peptides 3 and 4 are driven by both enthalpy and entropy contributions. Neither peptide 1 and 2 nor the cyclic partial structures of all four peptides displayed significant cation complexation. A kinetic analysis of the K+‐complexation by peptide 3 based on the microcalorimetry experiments showed far lower rates of cation exchange for the synthetic peptide than those reported for valinomycin. Transport studies with peptide 3 using artificial lipid bilayer membranes gave negative results. The apparent lack of ionophoretic properties of these synthetic peptides despite their considerable ability to form complexes with cations is discussed in terms of structural parameters.