1H NMR study of interaction of probe molecules with the phosphate ion on the iron core of horse spleen ferritin.

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The surface of the iron core of ferritin was investigated by 1H NMR using small probe molecules which have different sizes and charges. It was shown that the parameter k, which is the linewidth for each NMR probe normalized by the total concentration of iron in ferritin, is dependent on the content of phosphorus in ferritin. At pH 7.4, the k values are 1–10 for positively charged probes, 2×10−2–8×10−2 for negatively charged probes and 0.1–0.2 for neutral probes. Above pH 7, the k values for positively charged probes increase with an increase in pH, whereas those for negatively charged probes decrease. By contrast, the k values for neutral probes remain constant over the entire pH range. It was concluded that the k values reflect the mode of interactions of small molecules with the phosphate which exists on the surface on the iron core of ferritin, and that these interactions are determined primarily by the electrostatic force between the small molecules and the negative charge of the phosphate of ferritin. Furthermore, it was shown by using t-butylamine as a probe that K+ ion binds strongly to ferritin and that chelating agents interact with ferritin in a wide concentration range.
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