of nitrosoiron（ii） porphyrins with dftepr nitrosoiron（ii）卟啉与课件

1 Extracting quantitative structural Extracting quantitative structural information from EPR g-tensors with information from EPR g-tensors with density functional theory: applications to density functional theory: applications to nitrosoironnitrosoiron(II) (II) porphyrinsporphyrins S. Patchkovskii and T. Ziegler Department of Chemistry, University of Calgary, 2500 University Dr. NW, Calgary, Alberta, T2N 1N4 Canada I am on the Web: http:/www.cobalt.chem.ucalgary.ca/ps/posters/EPR-FeP/ heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 2 Introduction C o n c l u s i o n s a n d o u t l o o k I n t r o d u c t i o n . Iron-containing porphyrin complexes (hemes) serve as prosthetic groups in many vitally important enzymes1,2, such as hemoglobin. Because oxy-hemoglobin possesses a singlet electronic structure, it is not amenable to studies using electron paramagnetic resonance (EPR). Structurally similar heme nitrosyls have a spatially non- degenerate spin-doublet ground state, which is readily observable with EPR. Experimental EPR spectra of nitrosylated hemoglobins and myoglobins3,4, indicate the presence of two distinct radical species: rhombic and axial. The rhombic (“Type I“) species exhibits three distinct principal components in its EPR g tensor (g1=1.96-1.98, g2=2.00, g3=2.06-2.08). It has been assigned to a six-coordinated structure, with NO coordinated in a bent end-on orientation, and the second axial position filled by an imidazole side chain. Despite extensive investigations, the nature of the axial (“Type II“) species (g|=1.99-2.00, g=2.02- 2.03) have proven more elusive, and remains controversial. In this work, we examine the structure and EPR g-tensors of model porphyrins with density functional theory (DFT). On the basis of our calculations, we propose a new structural model for the axial species. heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 3 Theory Quasi-relativistic DFT formulation of the EPR g-tensors used in this work distinguishes between several contributions to the g-tensor5,6: free-electron g value (2.0023)diamagnetic termparamagnetic term kinetic energy correction mass-velocity correction Darwin term Relativistic corrections The paramagnetic term dominates deviation of g from the free-electron value for complexes considered here, and can be in turn separated into several contributions: frozen core contributions occupied-occupied coupling terms occupied-virtual coupling terms The occ-vir term is usually the most qualitatively important contribution. T h e o r y I heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 4 The contribution is given by (atomic units): 0.00731 the effective potential-spin current due to unit magnetic field along s=x,y, or z -spin current The form of the occupied-virtual paramagnetic contribution the the EPR g-tensor is analogous to the expression for the paramagnetic part of the NMR shielding tensor for a nucleus N, given by: The similarity between the two quantities is extremely useful both in the evaluation and in analysis of g tensor, and is unique to our DFT implementation. T h e o r y I I heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 5 The spin-current density for a spin arising due to the coupling between occupied and virtual MOs caused by the external magnetic field B0 is given by: magnetic coupling coefficient unperturbed occupied MOunperturbed virtual MO field strength in the direction s (=x,y,z) The principal contribution to the coupling coefficient u is in turn given by: unperturbed orbital energiesunperturbed MO coefficients atomic orbitals (AOs)applies to each AO T h e o r y I I I heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 6 Methods M e t h o d s heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 7 ON-Fe(P): Coordination of NO C o n c l u s i o n s a n d o u t l o o k O N - F e ( P ) : C o o r d i n a t i o n o f N O 1a: +3.9 kcal/mol 1b: +0.0 kcal/mol 1c: +0.0 kcal/mol The coordination of NO around iron gives rise to three key points on the PES of 1, namely: the C4v structure 1a with the linearly coordinated NO ligand.This is a second-order saddle point, rather than a minimum. Cs structure 1b, with bent end-on coordination of the NO ligand, pointing towards one of the meso carbon atoms of the porphyrin ligand. a second Cs structure 1c, with the NO bond eclipsing one of the equatorial Fe-Np bonds. Given the isoenergetic 1a and 1b, the NO ligand in free five-coordinated heme nitrosyl should undergo free intramolecular rotation around the Z axis. A closer inspection of the optimised structures reveals that the rotation of the NO ligand is coupled to the distortion of the porphyrin ligand. As a consequence, hindering the distortion increases the barrier for NO rotation increases to about 1.2 kcal/mol. Z heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 8 ON-Fe(P)-Im: Coordination of imidazole C o n c l u s i o n s a n d o u t l o o k O N - F e ( P ) - I m : C o o r d i n a t i o n o f i m i d a z o l e 1.0 2.0 3.0 2.02.8 E, kcal/mol RFe-N(Im), Dissociation of the imidazole was examined by gradually increasing the iron-imidazole distance, and optimizing the rest of the structure. The resulting energy profile is exceptionally flat, with variations in the Fe-N(Im) bond length in the 2.05-2.50 range changing energy by less than 1 kcal/mol. As a consequence, experimental bond lengths are likely to be influenced by substitution and environment effects. Although the potential energy profile for the dissociation of imidazole appears to indicate a presence of a local minimum at 2.4, g-tensors show no qualitative changes in the vicinity of this structure. exp heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 9 C o n c l u s i o n s a n d o u t l o o k O N - F e ( P ) - I m : r o t a t i o n o f t h e a x i a l l i g a n d s 32 2a2b2c2d2e 0.00 0.20 0.40 0.60 050100150200250300350 NO, degree Im= 0 Im=45 energy, kcal/mol In the six-coordinated complex, both axial ligands preferentially appear in staggered orientation (2a-2c). The local minima appear within 0.3 kcal/mol of the global minimum (2c), and will be substantially populated at non-zero temperature. The orientations of NO and imidazole in condensed phases are likely to be determined by substituent effects and intermolecular interactions. Free rotation of both axial substituents may also be expected at room temperature. heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 10 Origin of the g tensors: C4v structure C o n c l u s i o n s a n d o u t l o o k O r i g i n o f g i n C 4 v s t r u c t u r e . +3.0 +2.0 +1.0 0.0 -1.0 -2.0 -3.0 -4.0 eV +14 +20 -36 1* 2 1 n1 SOMO -spin -spin Contributions to g are in parts per thousand (ppt) Mulliken d-population on iron in 1 (6.7) is consistent with d7 electron count, formally making 1 a complex of FeI and NO+. The unpaired electron is mostly on irons dz2 AO. The SOMO vanishes upon action of the Mz operator, so that all spin-restricted terms in the parallel component g| also vanish. If the magnetic field is in the XY plane, the field- induced coupling with -spin * MOs (dxz, dyz-like), localised on the Fe-N=O fragment, results in a contribution of -36 ppt to g. This contribution is almost identically cancelled by two -spin terms, (+14 and +20 ppt) heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 11 C o n c l u s i o n s a n d o u t l o o k O r i g i n o f g i n m e s o - C s s t r u c t u r e Origin of the g tensors: Cs structures +3.0 +2.0 +1.0 0.0 -1.0 -2.0 -3.0 -4.0 eV +14 +20 -36 1* 2 1 n1 +37 +7 +32 -15-13 linearbent In the bent 1b, irons dz2 AOs overlap with the * orbitals of the NO ligand, tilting the dz2- like contribution to the SOMO towards the direction perpendicular to the N=O bond. Such orientation allows SOMO to interact with occupied non-bonding dx2-y2 orbital n1. The SOMO and SOMOn terms exhibit a qualitatively different dependence on the relative orientation of the NO ligand and the porphyrin core, so that g-tensor in bent 1 depends on the orientation of NO. heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 12 Origin of the g tensors: ON-Fe(P)-Im C o n c l u s i o n s a n d o u t l o o k O r i g i n o f g i n O N - F e ( P ) - I m 1b: SOMO 2b: SOMO Upon coordination of the imidazole, a weak * interaction between an imidazole lone pair, and the dz2-like lobe of the SOMO, destabilises the SOMO by 0.9 eV. The Mulliken spin population on iron is also reduced, from 0.9 (1b) to 0.5 (2b). decrease in the d character of the SOMO reduces all spin- orbit coupling matrix elements, decreasing all gs. a large NO * contribution to the SOMO increases the magnitude of matrix elements of the M operator in -SOMO - * terms. destabilisation of the SOMO increases -SOMO - * terms, while decreasing -SOMO - contributions. heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 13 g1g2g3 1a2.0082.0032.003 1b 1.9942.0052.063 1c1.9972.0242.033 ON-Fe(TPP)2.0102.0642.102 2a1.9551.9952.034 2d1.9672.0002.012 ON-Fe(TPP)-Im1.9702.0032.072 Mb(NO)1.9792.0022.076 -Hb(NO)1.9742.0062.081 -Hb(NO)1.9782.0082.057 Numerical results: Principal components C o n c l u s i o n s a n d o u t l o o k N u m e r i c a l r e s u l t s : P r i n c i p a l c o m p o n e n t s *: TPP = tetraphenylporphyrinato2-; Mb = myoglobin; Hb = hemoglobin As expected from the qualitative analysis, calculated principal g-tensor components in ON-Fe(P) (1) are sensitive to both the Fe-N=O bond angle, and to the orientation of the NO relative to the porphyrin (1a- 1c). The values are only in a broad qualitative agreement with experiment, and appear to indicate an orientation of the NO ligand intermediate between 1b and 1c. Part of the error may be due to the neglect of the environment effects. In ON-Fe(P)-Im (2), g-tensor is again sensitive to the orientation of NO, but is left unchanged by imidazole rotation. Calculated magnitudes of the principal components appear to be in a broad qualitative agreement with experiment. The characteristic g1 g|. The parallel component increases the rate of 19 ppt/. g grows at 35 ppt/, so that the separation between the rotationally averaged components increases with dissociation of imidazole. g tensor components g tensor components E, kcal/mol 1.0 2.0 3.0 2.02.8 RFe-N(Im), 1.96 2.00 2.04 2.08 g1 g2 g3 g g|. heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 16 Biological ON-Fe(P)-Im: Models galore C C o o n n c c l l u u s s i i o o n n s s a a n n d d o o u u t t l l o o o o k k B B i i o o l l o o g g i i c c a a l l O O N N - - F F e e ( ( P P ) ) - - I I m m : : MM o o d d e e l l s s g g a a l l o o r r e e bound-bound- dissociateddissociated AxialRhombicSource linear- bent Model up- down inclined- straight 3 13 15 16 P.E.S. g-tensor Axial structure is not a minimum; axial g-tensor components are wrong Rhombic structure is not a minimum; “Axial” structure has rhombic g-tensor. “Axial” structure has rhombic g-tensor. “Axial” structure is not a minimum, and has rhombic g-tensor heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 17 Biological ON-Fe(P)-Im: Rhombic species C o n c l u s i o n s a n d o u t l o o k B i o l o g i c a l O N - F e ( P ) - I m : r h o m b i c s p e c i e s The rhombic signal (“State I“) exhibits a characteristic pattern of gmingfreegmax. This pattern appears to correspond to the six-coordinated complex 2 with the internal rotation of the NO ligand frozen on the EPR time scale. Calculated orientations of the principal components are in an excellent agreement with the experimental result for nitrosylated myoglobin (MbNO) single crystals. At low temperatures, the direction of the g2 component in MbNO deviates by 30 from the average normal to the porphyrin plane. The value calculated at the theoretical gas-phase geometry of the model complex 2 is 32, suggesting that the ON-Fe(P) fragment in MbNO is not noticeably distorted by interactions with the protein environment. g11g22g33 Calculated1.962.002.03 -Hb(NO)1.972.012.08 -Hb(NO)1.982.012.06 heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 18 Biological ON-Fe(P)-Im: Axial species C o n c l u s i o n s a n d o u t l o o k B i o l o g i c a l O N - F e ( P ) - I m : a x i a l s p e c i e s The axial (“Type II”) spectra are usually found at higher temperatures. In the absence of a satisfactory static model, it appears reasonable to consider possible dynamical interpretations. If the rotation of NO is unfreezed, averaging of the g components leads to an axial spectrum. In the model complex, imidazole has no additional chemical bonds to the ON-Fe(P) fragment, and will dissociate completely. In heme proteins, it is provided by a histidine residue, and is tethered to the backbone. The tethering may prevent a complete dissociation, leading to smaller values of g and g|. It also allows protein to switch between Type I and Type II spectra at the same temperature: A conformational change in the backbone can pull the imidazole from the rest of the complex, g|g Calculated2.002.03 -Hb(NO)2.002.02 -Hb(NO)2.002.03 effectively causing its dissociation. This would, in turn, lower the barrier for NO rotation, and produce the change in the spectrum. heme nitrosyls from EPR g-tensors with DFT CSC83, 2000 19 Summary C o n c l u s i o n s a n d o u t l o o k S u m m a r y Structure of ON-Fe(P)-Im: In both five- and six-coordinated complexes, NO is preferably coordinated end-on, with a Fe-N=O bond angle of approximately 140. In