Ion-binding and pharmacological properties of Tyr(6) and Tyr(9) antamanideanalogs

In order to investigate the antiproliferative properties of antamanide, we have synthesized and studied two antamanide analogs where the phenylalanine residue in positions 6 or 9 is substituted by tyrosine, their correspondin g linear forms and the cyclic and linear des Phe(5),Phe(6)-Tyr(9)-analogs. Antamanide and its biologically active synthetic analogs are able to form h ighly stable complexes with metal ions, particularly Na+, K+ and Ca2+. We s tudied the ion-binding properties of the Tyr-antamanide analogs by CD and T b3+-mediated fluorescence in acetonitrile. In this medium the far- and near -UV CD spectra of the neat Tyr(6)-antamanide analog are very similar to tha t of the parent cyclic decapeptide. Substantial differences occur on the co ntrary in the CD spectra of the neat Tyr(9)-antamanide, particularly in the regions at 220 nm and 270-290 nm. In acetonitrile, as already found for an tamanide, the interaction with the above-mentioned metal ions always produc es evident changes in the far- and near-UV CD spectra of both analogs. On t he contrary, the CD spectra of the linear deca- and octa- and of the cyclic octa-analogs are affected by the presence of metal ions only in the near-U V region. In the same solvent the Tb3+-mediated fluorescence spectra of all the synthetic peptides are remarkably affected by the addition of ions. On the basis of the spectral total changes, by using either or both the spect roscopic techniques, if. has been possible to determine the ion binding con stants for all the linear and cyclic Tyr-antamanide analogs and to compare them with that of the parent peptide. The antitoxic and antiproliferative a ctivities of these antamanide analogs have been tentatively correlated to t heir ion-binding properties. A preliminary account of this work was given i n (1).