VOLTAMMETRIC ESTIMATION OF IRON(III) THERMODYNAMIC STABILITY-CONSTANTS FOR CATECHOLATE SIDEROPHORES ISOLATED FROM MARINE-BACTERIA AND CYANOBACTERIA

Thermodynamic stability constants have been estimated for the complexa tion of iron(III) with catecholate-type siderophores isolated from the marine bacterium Alteromonas luteoviolacea and from the marine cyanob acterium Synechococcus sp. PCC 7002. Stability constants were determin ed utilizing the ''chelate scale'' of Taylor et al. (1994). The scale is based upon a linear relationship between the reduction potentials a nd the pH-independent thermodynamic stability constants for known iron (III) complexes. Log K values for the alterobactin B ferric iron compl ex are 43.6 +/- 1.5 at pH 8.2 and 37.6 +/- 1.2 at pH 6, consistent wit h a shift from bis-catecholate to monosalicylate/monocatecholate iron coordination with decreasing pH. Synechococcus isolates PCC 7002 Nos. 1 and 3 formed iron(III) complexes with stability constants of approxi mately 38.1 +/- 1.2 and 42.3 +/- 1.5, respectively. The binding streng ths of the iron(III) complexes examined in this study are quite high, suggesting that catecholate siderophores may play a role in the solubi lization and biological uptake of iron in the marine environment.