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 an 15 N 18 O isotope shift of 8 cm −1 , and is therefore assigned to ν(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 δ 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 ν(Fe-NO) and δ 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 ~550 cm −1 had usually been associated with ν(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 coordinated NO is weak, and mostly leads to a polarization of the π/π* orbitals of Correspondence to: Nicolai Lehnert, lehnertn@umich.edu; J. Timothy Sage, jtsage@neu.edu; W. Robert Scheidt, Scheidt.1@nd.edu. Supporting Information Complete ref. 39 , DFT-calculated NRVS spectra, complete list of force constants altered in the fit of the NRVS data of 1, vibrational assignments for the adjusted BP86/TZVP result, cartesian coordinates of the DFT-optimized structures, definition of the force field of 1 used in the QCC-NCA simulation, and complete list of force constants from the QCC-NCA fit.

Spectroscopic identification of reactive porphyrin motions

“…In particular, modes of ν 53 or ν 50 character, involving stretching of the equatorial Fe-N bonds, dominate NRVS measurements on heme proteins and on model compounds and provide useful markers for the spin state of the Fe. In comparison with Fe(P), the E-symmetric modes ν 49 , ν 50 , and ν 53 in Fe(P)(Cl) mix significantly as well as shifting in frequency. In addition, Fe(P) modes involving Fe motion perpendicular to the heme plane (mainly γ 6 and γ 7 ) mix with halide translation to produce the Fe−halide stretching vibration (Fig.…”

“…The total area 0.377 accounts for the NRVS feature at 345 cm −1 (area 0.33). The NRVS peak at 391 cm −1 (area 0.17) corresponds to the DFT predicted ν 50 mode doublet of E symmetry ( Fe(P)(Cl) Fe(P) 53 (E u ) 23 (E g ) 26 (E g ) waving 50 (E u ) 49 (E u ) 22 5. Correlation between the low frequency modes of Fe(P) and modes of B 1 , E, B 2 , and A 2 C 4v symmetry classes of Fe(P)(Br).…”

“…11,[23][24][25][26][27][28][29][30] Since the use of NRVS to measure low frequency vibrations is restricted only by the experimental resolution (∼10 cm −1 ), it provides an important opportunity to characterize thermally excitable reactive modes. In particular, NRVS measurements have identified 11,[23][24][25]28 the doming mode of the heme (Fig.…”

“…In contrast, NRVS measurements on oriented single crystals definitively identify Fe motion orthogonal to the heme plane associated with low frequency modes in model compounds. 17,25,27,28,[48][49][50] In addition, NRVS represents the ultimate limit in atomic selectivity, because it specifically reveals the vibrational spectrum of the probe nucleus, even in the presence of thousands of other vibrating atoms. Fe-ligand vibrations dominate the 57 Fe NRVS signal.…”

Definition of the intermediates and mechanism of the anticancer drug bleomycin using nuclear resonance vibrational spectroscopy and related methods

“…This study found the heterolytic cleavage pathway for the ðBLMÞFe III ─ H 2 O 2 complex to be thermoneutral and therefore feasible. Because ABLM is inaccessible to vibrational characterization by resonance Raman (rR) spectroscopy due to photodecay (16), we employed nuclear resonance vibrational spectroscopy (NRVS) (17)(18)(19)(20) to obtain vibrational information and define the axial ligands of Fe III BLM and ABLM, thus distinguishing between possible reaction mechanisms.…”