COMPLEXING PROPERTIES OF DIASTEREOISOMERS OF 1-(L-METHIONYLAMINO)ETHYLPHOSPHONIC ACID

The acid-base and complexing properties of (S,S) and (S,R) diastereois omers of 1-(methionylamino)ethyl-phosphonic acid were studied pH-metri cally (25 degrees C, 0.1 mol dm(-3) KNO3). The protonation and stabili ty constants for complex formation with Co2+, Ni2+ and Zn2+ indicate c o-ordination only via the amine, carbonyl peptide and phosphonic group s and, for Cu2+, also via nitrogen of the deprotonated peptide amide g roup. The presence of the same types of complexes as in phosphonodipep tide series which do not contain sulfur in the amino acid side chain w as observed. Similarly, the differences found between diastereoisomers correspond to the interaction of the hydrophobic and/or hydrophilic p arts of the molecule. Go-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 sul fur in a wide pH region; in the alkaline region, sulfur atoms are repl aced by the peptide amide group. A 1 : 1 ratio indicates I the formati on of dimers with co-ordinated phosphonic groups. The pH-metrically de termined species were confirmed by P-31-{H-1} and H-1 NMR titrations o f the systems involving Pd-II and Pt-II.