Notes. Electronic spectral study of the aqua ⇌ hydroxo equilibrium of model iron(III) haems encapsulated in aqueous detergent micelles

Abstract

Electronic spectral studies have been made on the aqua ⇌ hydroxo equilibrium in model six-co-ordinated iron(III) haems, [Fe(P)(py)L](L = H₂O or OH; py = pyridine; P = meso-, proto-, or deutero-porphyrinate), encapsulated in different aqueous detergent micelles. The pK_aof the equilibrium is significantly affected by the hydrophobic interaction of the micellar cavity, and ranges from 7.1 to 10.3 depending on the porphyrin substitution and the micelle. The pK_avaries with the nature of porphyrin as meso deutero > proto, and with the micelles as sodium dodecyl sulphate > Triton X-100 > hexadecyltrimethylammonium bromide. The significance of these studies to similar acid ⇌ base equilibria in haemoproteins is discussed.