The iron(III) binding properties of Mytilis edulis foot protein 1 (Mefp1), 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 analogues (peptide S-1 : Y-9 = DOPA; peptide S-2 : Y-5 = Y-9 = DOPA) have been investigated by polarography and spectrophotometry. A "chelate scale" was constructed by measuring the reduction potentials of iron(III) complexes with known stability constants and was used to estimate stability constants for the iron(III) interactions with peptides. A linear relationship was found to exist between the reduction potentials and the pH independent thermodynamic stability constants for iron(III) complexes spanning 20 orders of magnitude in chelate stability. Spectrophotometric data allowed the stoichiometry of the peptide-iron(III) interactions to be determined. At neutral pH all peptides favored an intermolecular bis(catecholato) coordination mode, through the DOPA nearest the C terminus. Reasons for the extra stability observed in the peptide 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 synthetic analogues. The decapeptides provide good models for iron(III) coordination by the parent protein. The biological and technological relevance of the iron(III)-DOPA interactions is discussed.