Iron Twin‐Coronet Porphyrins as Models of Myoglobin and Hemoglobin: Amphibious Electrostatic Effects of Overhanging Hydroxyl Groups for Successful CO/O₂ Discrimination

Abstract

Inspired by the observation of polar interactions between CO and O(2) ligands and the peptide residues at the active site of hemoglobin and myoglobin, we synthesized two kinds of superstructured porphyrins: TCP-IM, which contains a linked imidazole ligand, and TCP-PY, which contains a linked pyridine ligand, and examined the thermodynamic, kinetic, and spectroscopic (UV/Vis, IR, NMR, and resonance Raman) properties of their CO and O(2) complexes. On both sides of each porphyrin plane, bulky binaphthyl bridges form hydrophobic cavities that are suitable for the binding of small molecules. In the proximal site, an imidazole or pyridine residue is covalently fixed and coordinates axially to the central iron atom. In the distal site, two naphtholic hydroxyl groups overhang toward the center above the heme. The CO affinities of TCPs are significantly lower than those of other heme models. In contrast, TCPs have moderate O(2) binding ability. Compared with reported model hemes, the binding selectivity of O(2) over CO in TCP-IM and TCP-PY complexes is greatly improved. The high O(2) selectivity of the TCPs is mainly attributable to a low CO affinity. The comparison of k(on)(CO) values of TCPs with those of unhindered hemes indicates the absence of steric hindrance to the intrinsically linear CO coordination to Fe(II) in TCP-IM and TCP-PY. The abnormally large k(off)(CO) values are responsible for the low CO affinities. In contrast, k(off)(O(2)) of TCP-PY is smaller than those of other pyridine-coordinated model hemes. For the CO adducts of TCPs, unusually low nu(Fe-CO) and unusually high nu(C-O) frequencies are observed. These results can be ascribed to decreased back-bonding from the iron atom to the bound CO. The lone pairs of the oxygen atoms of the hydroxyl groups prevent back-bonding by exertion of a strong negative electrostatic interaction. On the other hand, high nu(Fe-O(2)) frequencies are observed for the O(2) adducts of TCPs. In the resonance Raman (RR) spectrum of oxy-TCP-IM, we observed simultaneous enhancement of the Fe-O(2) and O-O stretching modes. Furthermore, direct evidence for hydrogen bonding between the hydroxyl groups and bound dioxygen was obtained by RR and IR spectroscopy. These spectroscopic data strongly suggest that O(2) and CO binding to TCPs is controlled mainly by the two different electrostatic effects exerted by the overhanging OH groups: destabilization of CO binding by decreasing back-bonding and stabilization of O(2) binding by hydrogen bonding.

CO as a vibrational probe of heme protein active sites

“…(Similar frequencies have been reported for superstructured model porphyrins, in which naphtholic hydroxyl groups are positioned over the bound CO [40].) Also the CO adduct of guanylyl cyclase lies on the low end of the Mb correlation, but is shifted back to the non-polar region on addition of the effector molecule YC-1 [41].…”

Diatomic Ligand Discrimination by the Heme Oxygenases from Neisseria meningitidis and Pseudomonas aeruginosa

“…Specifically, the TCP⅐O 2 complex was stabilized by hydrogen bonding with two overhanging hydroxyl groups, whereas the TCP⅐CO complex was destabilized because of suppression of backbonding from the iron atom to bound CO. The decrease in backbonding was caused by strong negative electrostatic interactions of bound CO with the lone pairs of the hydroxyl groups in the distal pocket, which resulted in an increase in the dissociation rate of CO from TCP compared with chelated protoheme (41). These model heme studies thus support the view that the reason nmHO exhibits a slower CO k off than HO-1 is due to less favorable electrostatic stabilization of the CO complex compared with HO-1.…”

“…2, A and C, respectively. The second-order association rate constants for the binding of CO and O 2 by (41). nmHO and paHO were obtained from the slopes of the linear plots of the observed rate constants (k obs ) versus ligand concentration (Fig.…”

“…However, electrostatic control of CO binding is supported by model heme studies. One comprehensive study on CO-and O 2 -bound twin-coronet porphyrins (TCPs) established that CO/O 2 ligand discrimination may be controlled by the polar effects exerted by overhanging hydroxyl groups (41). Specifically, the TCP⅐O 2 complex was stabilized by hydrogen bonding with two overhanging hydroxyl groups, whereas the TCP⅐CO complex was destabilized because of suppression of backbonding from the iron atom to bound CO.…”

Vibrational analysis of Ni(II) complex with 4,12-ditolyl-16,24-diphenyl-3-thiaporphyrin (SDTDPPNi(II)Cl) and its isotopic labeled (61Ni(II), −d6, and −d10) derivatives

“…Iron complexes of porphyrins are widely used as model compounds of prosthetic groups of many biological molecules such as cytochromes, hemoglobin, myoglobin, catalases, etc. [5][6][7]. The modification of the porphyrin's core by replacement of the NH fragment of the pyrrole ring(s) by isoelectronic heteroatoms from the 16 th group of the periodic table, e.g.…”