Simple Heme Dimers with Strongly Cooperative Ligand Binding

Abstract

We report that the simple stable heme dimers (1-Fe) 2 and (2-Fe) 2 display strongly cooperative ligand binding. For example, (1-Fe) 2 binds a second isocyanide molecule with an approximately 50-fold higher affinity than the first, or with an allosteric interaction energy of DG a = 13 AE 3 kJ mol À1 . The degree of cooperativity is comparable to that of hemoglobin, a prototypical cooperative receptor for small molecules, whereas the mechanism appears to be distinct.With these results, we aim to help address the unmet need for simple, robust, and easily accessible synthetic receptors that bind small molecules (O 2 , NO, CO) cooperatively. Such receptors would be of great value for a number of applications (noncryogenic air separation, [1] chemical O 2 delivery/storage, [2] chemical sensors [3]

Simple dimer containing dissociatively stable mono-imidazole ligated ferrohemes

“…2). The affinity of Fe II in (1Fe) 2 to i PrNC is 10 4 -fold lower than is typical for a 5-coordinate imidazole-ligated ferroheme 13,14 and 10 2 -fold lower than the affinity of 4-coordinate Fe II (por), such as Fe(tpp), 3Fe or The dissociation constant of the bisadduct, (1Fe) 2 ( i PrNC) 2 , K PrNC d (eqn 3), was 0.5 AE 0.4 mM, i.e. 10 4 -fold higher than that of (1Fe) 2 .…”

Metalloporphyrin Catalysts of Oxygen Reduction

“…18.17); when adsorbed on a graphite electrode, it manifests catalytic behavior comparable to that of series 2 metalloporphyrins. (b) The simplest structural motif that ensures the axial imidazole ligation of the Fe II site [Khvostichenko et al, 2007;Yang et al, 2008]. † Making the Fe III/II potential more oxidizing-even at the expense of lowering the O 2 affinity.…”

Supramolecular coordination chemistry

“…The stabilization of the C 3 -symmetric [(L 26 Zn) 3 ] arises through the formation of a benzene-trimer complex, which calculations suggest contribute 11 kJ mol À1 enthalpically per interaction. Monoimidazole substituted Fe porphyrins L 27 Fe and L 28 Fe form dimeric species [(L 27 Fe) 2 ] and [(L 28 Fe) 2 ] in solution through binding of the imidazole group to the metal centre. 28 These metallocycles also show cooperative binding of various ligands, with an allosteric interaction energy of DG a = 13.3 kJ mol À1 , a value similar to that observed in hemoglobin.…”

“…Monoimidazole substituted Fe porphyrins L 27 Fe and L 28 Fe form dimeric species [(L 27 Fe) 2 ] and [(L 28 Fe) 2 ] in solution through binding of the imidazole group to the metal centre. 28 These metallocycles also show cooperative binding of various ligands, with an allosteric interaction energy of DG a = 13.3 kJ mol À1 , a value similar to that observed in hemoglobin. In an analogous fashion to the metalloporphyrin assemblies, the salen ligand N,N 0 -3-pyridylenebis(salicylideneimine) H 2 L 29 has been shown to form a tetranuclear zinc metallocycle [(L 29 Zn) 4 ] through interactions of the exo-pyridine group with adjacent N 2 O 2 -bound zinc(II) centres.…”