Oriented single-crystal nuclear resonance vibrational spectroscopy of [Fe(TPP)(MI)(NO)]: quantitative assessment of the trans effect of NO

Abstract

This paper presents oriented single-crystal Nuclear Resonance Vibrational Spectroscopy (NRVS) data for the six-coordinate (6C) ferrous heme-nitrosyl model complex [(57)Fe(TPP)(MI)(NO)] (1; TPP(2-) = tetraphenylporphyrin dianion; MI = 1-methylimidazole). The availability of these data enables for the first time the detailed simulation of the complete NRVS data, including the porphyrin-based vibrations, of a 6C ferrous heme-nitrosyl, using our quantum chemistry centered normal coordinate analysis (QCC-NCA). Importantly, the Fe-NO stretch is split by interaction with a porphyrin-based vibration into two features, observed at 437 and 472 cm(-1). The 437 cm(-1) feature is strongly out-of-plane (oop) polarized and shows a (15)N(18)O isotope shift of 8 cm(-1) and is therefore assigned to nu(Fe-NO). The admixture of Fe-N-O bending character is small. Main contributions to the Fe-N-O bend are observed in the 520-580 cm(-1) region, distributed over a number of in-plane (ip) polarized porphyrin-based vibrations. The main component, assigned to delta(ip)(Fe-N-O), is identified with the feature at 563 cm(-1). The Fe-N-O bend also shows strong mixing with the Fe-NO stretching internal coordinate, as evidenced by the oop NRVS intensity in the 520-580 cm(-1) region. Very accurate normal mode descriptions of nu(Fe-NO) and delta(ip)(Fe-N-O) have been obtained in this study. These results contradict previous interpretations of the vibrational spectra of 6C ferrous heme-nitrosyls where the higher energy feature at approximately 550 cm(-1) had usually been associated with nu(Fe-NO). Furthermore, these results provide key insight into NO binding to ferrous heme active sites in globins and other heme proteins, in particular with respect to (a) the effect of hydrogen bonding to the coordinated NO and (b) changes in heme dynamics upon NO coordination. [Fe(TPP)(MI)(NO)] constitutes an excellent model system for ferrous NO adducts of myoglobin (Mb) mutants where the distal histidine (His64) has been removed. Comparison to the reported vibrational data for wild-type (wt) Mb-NO then shows that the effect of H bonding to the coordinated NO is weak and mostly leads to a polarization of the pi/pi* orbitals of bound NO. In addition, the observation that delta(ip)(Fe-N-O) does not correlate well with nu(N-O) can be traced back to the very mixed nature of this mode. The Fe-N(imidazole) stretching frequency is observed at 149 cm(-1) in [Fe(TPP)(MI)(NO)], and spectral changes upon NO binding to five-coordinate ferrous heme active sites are discussed. The obtained high-quality force constants for the Fe-NO and N-O bonds of 2.57 and 11.55 mdyn/A can further be compared to those of corresponding 5C species, which allows for a quantitative analysis of the sigma trans interaction between the proximal imidazole (His) ligand and NO. This is key for the activation of the NO sensor soluble guanylate cyclase. Finally, DFT methods are calibrated against the experimentally determined vibrational properties of the Fe-N-O subunit in 1. DFT is in fact incapable of reproducing the vibrational energies and normal mode descriptions of the Fe-N-O unit well, and thus, DFT-based predictions of changes in vibrational properties upon heme modification or other perturbations of these 6C complexes have to be treated with caution.

Figure 1

NRVS VDOS spectra of [⁵⁷Fe(TPP)(MI)(NO)] (1). Black: powder spectrum; blue and red: normalized single-crystal in-plane (blue) and out-of-plane (red) polarized spectra; green: predicted powder spectrum calculated by adding the in-plane and out-of-plane polarized contributions (total NRVS VDOS: D(ν̃) = D(ν̃)_oop + 2 D(ν̃)_ip).

Figure 2

Top: fully optimized structure of complex [Fe(TPP)(MI)(NO)] (1) using BP86/TZVP in side view. Bottom: porphyrin core diagram that indicates oop displacements of the atoms of the porphyrin ring, negative displacements are towards NO. The predicted distortion corresponds to ruffling. The angle between the Fe-N-O and the imidazole plane in the obtained structure is 26°, which is underestimated compared to experiment.

Figure 3

Calculated NRVS VDOS spectra of [Fe(TPP)(MI)(NO)] (1). Top: BP86/TZVP (blue) and BP86/TZVP adjusted (red, using the calculated BP86/TZVP force field plus the QCC-NCA force constants for the Fe-N-O unit from ref. 7c). In the latter case, the vibrational energies of the Fe-NO stretch at 437 cm⁻¹ and the Fe-N-O bend at 563 cm⁻¹ are in surprisingly good agreement with experiment (black), but the experimental intensities of these modes are not reproduced well. Bottom: out-of-plane (z) and in-plane (xy) NRVS VDOS for the adjusted BP86/TZVP case.

Figure 4

QCC-NCA fit of the NRVS VDOS data of complex [⁵⁷Fe(TPP)(MI)(NO)] (1) based on the BP86/TZVP result. Top: experimental powder data (black) and in-plane (blue) and out-of-plane (red) intensities form the QCC-NCA fit. Bottom: experimental powder data (black) and total NRVS VDOS intensity from the QCC-NCA fit (orange).

Figure 5

QCC-NCA fit of the NRVS VDOS data of complex [⁵⁷Fe(TPP)(MI)(NO)] (1) based on the BP86/TZVP result. Top: single-crystal out-of-plane polarized data (black) and QCC-NCA calculated out-of-plane intensity (red). Bottom: single-crystal in-plane polarized data (black) and QCC-NCA calculated in-plane intensity (blue).

Figure 6

NRVS VDOS powder data of [⁵⁷Fe(TPP)(MI)(NO)] (1, black, top) and of the corresponding ¹⁵N¹⁸O labeled complex (red, bottom) in the energy region of the δ_ip(Fe-N-O) bending mode, together with fits of these data (blue: 1, orange: ¹⁵N¹⁸O-labeled complex).

Figure 7

Atomic displacement (‘arrow’) plots for the QCC-NCA results for [Fe(TPP)(MI)(NO)] (1) based on the porphine approximation (left, from ref. 7c) and using the full TPP²⁻ ligand (right, this work) for the Fe-NO stretch at 437 cm⁻¹ (bottom) and the in-plane Fe-N-O bend at ~563 cm⁻¹ (top).

Figure 8

NRVS VDOS of [⁵⁷Fe(TPP)(MI)(NO)] (1) measured as a powder (black) and in frozen THF solution (red).

Scheme 1

Scheme 2

Effective internal coordinates for the Fe-NO stretch and the Fe-N-O bend in a [Fe(Porphyrin)] complex. These are taken from a BP86/TZVP calculation on [Fe(P)(NO)] where mixing of these internal coordinates is minimal. These effective internal coordinates are somewhat different from corresponding coordinates in a Fe-N-O trinuclear unit, because they account for intrinsic couplings of the isolated (pure) Fe-NO stretching and Fe-N-O bending coordinates of the triatomic with other internal coordinates in the actual complex. For example, in the bent Fe-N-O geometry, the Fe-N-O bending internal coordinate is always strongly mixed with (Pyr)N-Fe-N(O) octahedral bends (Pyr = pyrrole). Hence, the effective internal coordinates shown here are a better basis to understand the resulting Fe-NO stretching and Fe-N-O bending normal modes in complex 1. In comparison with Figure 7, one recognizes that the higher energy mode in 1 is always the ‘in-phase’ combination of these coordinates, whereas the lower energy mode is the corresponding out-of-phase combination, keeping in mind that (a) in the latter case, the contribution of the Fe-N-O bending internal coordinate is rather small, and (b) mixing with nearby porphyrin-based vibrations will influence the motions of the Fe-N-O unit.

Scheme 3

Correlation of δ_ip(Fe-N-O) and ν(N-O) in human myoglobin (Mb) wild-type (wt) and mutants. Importantly, in the His64 mutants the distal His that is able to form a hydrogen bond with a bound diatomic has been removed. The data are taken from ref. . The data points in the upper right box correspond to mutants where the amino acid replacing His64 has only aliphatic side chains and no capability to form hydrogen bonds. The vibrational properties of these mutants, in particular H64L, correspond closely to the model complex [Fe(TPP)(MI)(NO)]. The dashed line is a linear fit of the data of wt and H64X mutants only, showing a direct correlation of δ_ip(Fe-N-O) and ν(N-O). However, note that a correlation diagram that includes a larger variety of heme proteins and mutants does not show any significant correlation as discussed in ref..