Low-spin iron(III) porphyrins encapsulated in aqueous detergent micelles: proton- and nitrogen-15 nuclear magnetic resonance studies

Abstract

Variable temperature ¹H and ¹⁵N n.m.r. studies have been made on low-spin iron(III) protoporphyrin IX (3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropionic acid) complexes, [FeL(CN)₂]^– and [FeL(py)(CN)], encapsulated in aqueous detergent micelles. The haem is shown to be monodispersed in the micellar solution, which ensures the paramagnetic shift is free from the effect of aggregation. The effects of the hydrophobic interactions of the micellar cavity on the linewidth, and the magnitude and spread of the chemical shift, are discussed and compared with those in absence of micelles and in haemoproteins. The temperature dependence of the paramagnetic shift of the haem methyl proton shows deviation from Curie's law, which is discussed in terms of the electronic structure of haem. The effect of the size and hydrophobicity of the micelles is further shown in the ¹⁵N n.m.r. signal of the bound CN^–, where the downfield shift follows the trend: absence of detergent < sodium dodecyl sulphate < Triton X-100 < hexadecyltrimethylammonium bromide.