Observed chemical shifts of 50µM 3p-FB in the presence of Fe(env5.0)3 : EDTA-C29-e5.0 in ratios of 6;6, 9:9, 12:12, 20:20 and 25:25. Data were obtained in the absence (plotted against fb, open symbols and dashed lines) and in the presence of Co2+ (plotted against fb·fb’, solid symbols and solid lines). Experiments were performed at 800MHz.