Proton NMR investigation of Ln³⁺ complexes of thymopoietin_32-36

The pentapeptide Arg-Lys-Asp-Val-Tyr (TP5) is a biologically active fragment of thymopoietin, the thymic hormone that induces selective T-cell differentiation. The formation of lanthanide(III) complexes of TP5 is demonstrated through the observation of Tb³⁺ fluorescence enhancement. The equilibria, stoichiometry and solution conformation of the La³⁺, Pr³⁺ and Yb³⁺ complexes of TP5 have been investigated using NMR spectroscopy. In addition, the dissociation constants of two methyl ester analogs of TP5 have been studied. Evidence is presented supporting an interaction between the arginine guanidino Nε{lunate}H and the aspartate carboxylate of TP5. Binding of Ln³⁺ appears to be accompanied by a disruption (or weakening) of this interaction and a concomitant increase in the 180° rotamer population for the aspartate carboxylate group. The observed trends in the magnitudes of the dissociation constants and the rotamer populations appear to suggest that, although a significant amount of monodentate complexes may also exist, the metal ion binds predominantly to both carboxylates in a bidentate fashion.