POLAROGRAPHIC AND SPECTROPHOTOMETRIC INVESTIGATION OF IRON(III) COMPLEXATION TO 3,4-DIHYDROXYPHENYLALANINE-CONTAINING PEPTIDES AND PROTEINSFROM MYTILUS-EDULIS

The iron(III) binding properties of Mytilis edulis foot protein 1 (Mef p1), its component peptides hexapeptide A (A(1)K(2)P(3)T4Y5K6; Y-5 = 3 ,4-dihydroxyphenylalanine (DOPA)) and decapeptides B to E (A(1)K(2)P(3 )S4Y5P6P7T8Y9K(10); peptides B to E: P-6 = 3-hydroxyproline (3HYP), P- 7 = 4HYP, Y-9 = DOPA; peptide B: P-3 = 4HYP, Y-5 = DOPA; peptide C: P- 3 = 4HYP; peptide D: Y-5 = DOPA), and their synthetic decapeptide anal ogues (peptide S-1: Y-9 = DOPA; peptide S-2: Y-5 = Y-9 = DOPA) have be en investigated by polarography and spectrophotometry. A ''chelate sca le'' was constructed by measuring the reduction potentials of iron(III ) complexes with known stability constants and was used to estimate st ability constants for the iron(III) interactions with peptides. A line ar relationship was found to exist between the reduction potentials an d the pH independent thermodynamic stability constants for iron(III) c omplexes spanning 20 orders of magnitude in chelate stability. Spectro photometric data allowed the stoichiometry of the peptide-iron(III) in teractions to be determined. At neutral pH all peptides favored an int ermolecular bis(catecholato) coordination mode, through the DOPA neare st the C terminus. Reasons for the extra stability observed in the pep tide complexes compared with simple catecholates are offered, with the possibility of hydroxyproline involvement eliminated on the basis of comparisons of behavior of the natural peptides with that of their syn thetic analogues. The decapeptides provide good models for iron(III) c oordination by the parent protein. The biological and technological re levance of the iron(III)-DOPA interactions is discussed.