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Correlated circular dichroism, magnetic circular dichroism and high performance liquid chromatography studies of "palladium (II) — thioether peptides" complexes

A conformational analysis of palladium (II)-thioether (side chains of free amino-acids and of residues) complexes was carried out by circular dichroism and magnetic circular dichroism. Two types of thioethers were studied : those in linear side chains as in L-methionine and those in cyclic side chains as in L-thiazolidine-4-carboxylic acid). In the complexes the carboxylic group of amino-acids Is free of Interaction with palladium at acid pH, as shown by C.D. titration. For the diketoplperazine of the cyclic thioether e.g. cycloglycyl-L-thiazolidine 4-carboxylic acid complexed with Pd (II) a possible correlation appears to exist between the variation of the Cotton effect at 230 nm and a change of conformation (S y — exo to S y — endo) of the thiazolidine ring, this occurring by increasing pH from acid to neutral. Magnetic circular dichroism revealed charge transfer (CT) bands at 230 nm having varied Intensities. For the complexes of Pd (II) with amino-acids the smaller the chelate ring the larger is the Faraday Effect. A peptide containing two methionyl residues showed about double the Faraday Effect than a peptide having a single methionine residue.On the other hand, Ion-pair reversed phase chromatography was performed to separate the palladium (II) complexes. By using cetyltrimethylammonium as a counter ion it has been possible to resolve the complexes according to a dynamic ion exchange mechanism.Finally, the optical and chromatographic properties of palladium (II)-thloether peptides complexes seem to be correlated : the larger the magnetic ellipticity, the longer is the retention time.