Complexing properties of diastereoisomers of 1-(L-methionylamino)ethylphosphonic acid

Abstract

The acid–base and complexing properties of (S,S) and (S,R) diastereoisomers of 1-(methionylamino)ethylphosphonic acid were studied pH-metrically (25 °C, 0.1 mol dm ^-3 KNO ₃ ). The protonation and stability constants for complex formation with Co ²⁺ , Ni ²⁺ and Zn ²⁺ indicate co-ordination only via the amine, carbonyl peptide and phosphonic groups and, for Cu ²⁺ , also via nitrogen of the deprotonated peptide amide group. The presence of the same types of complexes as in phosphonodipeptide series which do not contain sulfur in the amino acid side chain was observed. Similarly, the differences found between diastereoisomers correspond to the interaction of the hydrophobic and/or hydrophilic parts of the molecule. Co-ordination via sulfur has not been observed. The species determined by ‘out of cell’ titrations of the platinum(II) and palladium(II) systems depend on the initial metal∶phosphonic acid molar ratio. A 1∶2 ratio indicates the co-ordination of amine and sulfur in a wide pH region; in the alkaline region, sulfur atoms are replaced by the peptide amide group. A 1∶1 ratio indicates the formation of dimers with co-ordinated phosphonic groups. The pH-metrically determined species were confirmed by ³¹ P-{ ¹ H} and ¹ H NMR titrations of the systems involving Pd ^II and Pt ^II .