Resonance Raman studies of sterically hindered cyanomet "strapped" hemes. Effects of ligand distortion and base tension on iron-carbon bond

Abstract

We report resonance Raman studies of the iron-carbon bond stretching vibrations, nu(Fe-CN), in sterically hindered and unhindered heme (FeIII)-CN- complexes. The sterically hindred "strapped hemes" are equipped with a covalently linked 13-, 14-, or 15-atom hydrocarbon chain across one face of the heme; these are called FeSP-13, FeSP-14, and FeSP-15, respectively. These straps would presumably exert a sideway shearing strain to force the linear ligands (e.g., CN- and CO) to be tilted and/or bent. The shorter the chain length, the weaker the ligand binding affinity because of a greater steric hindrance. This study reveals that the nu(Fe-CN) frequency decreases as the chain length is decreased, in contrast with the CO complexes, where the nu(Fe-CO) frequency increases as the chain length is decreased. For the heme-CN- complexes (with N-methylimidazole as a base), the nu(Fe-CN) frequencies are: heme 5 (unhindered), 451 cm-1; FeSP-15, 447 cm-1; FeSP-14, 447 cm-1; FeSP-13, 445 cm-1. For the heme-CO complexes (with N-methylimidazole as a base), the nu(Fe-CO) frequencies are: heme 5, 495 cm-1; FeSP-15, 509 cm-1; FeSP-14, 512 cm-1; FeSP-13, 514 cm-1 (Yu, N.-T., E. A. Kerr, B. Ward, and C. K. Chang, 1983, Biochemistry, 22:4534-4540). We have also studied the cyanide complexes with three different bases (pyridine, N-methylimidazole and 1,2-dimethylimidazole), and found that the trans-effect of cyanide complex is different from that of CO complexes. The tension on Fe"'-base bond weakens the Fe"'-CN- bond, whereas the tension on Fe"-base bond strengthens the Fe"-CO bond. The origin of these differences may be attributed to different extents of the d7r(Fe)- wr*(ligand) back bonding between the CN- and CO heme complexes. The Fe-C-N bending vibrations in these cyanomet strapped hemes are not resonance-enhanced, although this bending mode has been detected at -410 cm-' via Soret excitation in cyanometinsect hemoglobins. It is suggested that the orientation of the tilted Fe-C-N unit may be important in determining the overlap between CN and porphyrin 7r* orbitals, which provide coupling of the Fe-C-N bending mode with the resonant Soret (r-7r*) transition.

Distinguishing Active Site Characteristics of Chlorite Dismutases with Their Cyanide Complexes

“…Increases in these ν Fe−C frequencies have been attributed to removal of constraints on the FeCN unit based on "strapped" heme cyanide model complexes with (CH 2 ) n chains over the heme, where lengthening or eliminating the strap hindering the bound CN − caused the ν Fe−C to shift to higher frequencies (n = 13− 15, 445−447 cm −1 ; no strap, 451 cm −1 ). 78 A similar trend is observed in heme protein cyanide complexes. For example, the crystal structure of Chlamydomonas Hb−CN − (PDB entry 1DLY) revealed that its FeCN angle is 130°due to a Hbonding network involving a Tyr and two Gln residues in its distal pocket.…”

“…Interestingly, several heme proteins having a distal Gln residue exhibit ν Fe–C frequencies comparable to those of the three Cld–CN – complexes reported here (Table ). Increases in these ν Fe–C frequencies have been attributed to removal of constraints on the FeCN unit based on “strapped” heme cyanide model complexes with (CH 2 ) n chains over the heme, where lengthening or eliminating the strap hindering the bound CN – caused the ν Fe–C to shift to higher frequencies ( n = 13–15, 445–447 cm –1 ; no strap, 451 cm –1 ) . A similar trend is observed in heme protein cyanide complexes.…”

Probing Heme Active Sites of Hemoglobin in Functional Red Blood Cells Using Resonance Raman Spectroscopy

“…The additional smaller difference patterns observed in this region arise probably from the vibrational coupling of the δ(Fe–CN) bending mode with the in-plane porphyrin skeletal modes of E u geometry, as reported previously. 12 , 15 …”

“… 12 However, in heme proteins, the CN – ligand interacts with the active site amino acid residues, and the presence of steric or polar interactions can induce a conformational change of the Fe–C≡N fragment. 14 , 15 The Fe–C≡N linkage can adopt tilted linear (also called “essentially linear”) or bent geometries, depending on the nature of steric and/or H-bonding interactions. 14 − 16 The high sensitivity and responsiveness of the Fe–C≡N linkage to the heme pocket environment make cyanide ligands an excellent probe of the active site structure.…”

Resonance Raman enhancement of the Mn-N-O bending mode in nitrosyl manganese "strapped" and "open" heme complexes

“…Previously, it was found that the stronger the trans ligand, the weaker the Fe"-CO bond (2). However, the relation of this trans effect is reversed in cyanomet Fe"' heme systems, i.e., the v(Fe"'-CN) frequency increases the increasing donor strength of the trans ligand (17). The origin of these effects might be due to different extents of the dj(M) X 7r* (XO) backbonding.…”