Biochimica et Biophysica Acta, 11! 9 (1992) 169-174 © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-4838/92/$05.00

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BBAPRO 34108

X-ray absorption spectroscopy of carbonyl basket handle Fe(II) porphyrins: the distortion of the tetrapyrrolic macrocycle Christophe Cartier 1, Michel Momenteau 2, Elisabeth Dartyge 3, Alain Fontaine 3, G6rard Tourillon 3, Antonio Bianconi 4 and Michel Verdaguer 5 t Laboratoire de Chimie lnorganique, U.A.C.N.R.S. 420, Unil'ersit~ de Paris Sud, Orsay (France) and Laboratoire d'Utilisation du Rayonnement Electromagn~tique (M.E.N., C.N.R.S., C.E.A.), Unh'ersit~ de Paris Sud, Orsay (France), -" lnstitut Curie, Section Biologie, Unh'ersit~ de Paris Sud, Orsay (France), "~Laboratoire d'Utilisation du Rayonnement Electromagn~tique (M.E.N., C.N.R.S., C.E.A.), Unirersit~ de Paris Sud, Orsay (France), 4 Unh'ersit?z dell'Aquila, L 'Aquila (Italy) and Diparthnento di Fisica, Unirersit~ di Roma "La Sapienza" Roma (Italy) and ~ Laboratoire de chim& des m~tmt~ de transition, U.A.C.N.R.S. 419, UnicersitC P. et M. Curie, Paris (France)

(Received 5 June 1991)

Key words: XANES spectroscopy; Porphyrin; Heme structure

The distortion of the tetrapyrrolic macrocycle, the Fe-C-O bond angle and the Fe local electronic structure of carbonyl basket-handle Fe z+ porphyrins as a function of the basket-handle chain length have been studied by X-ray absorption near edge structure (XANES) spectroscopy, both in the solid state and in toluene solution. The Fe-C-O bond angle has been found to be linear in all compounds while the increasing distortion of the macrocycle with shortening of the chain length is indicated by the multiple scattering resonance in the heme plane appearing at 22 eV in the XANES spectrum.

Introduction

Among the porphyrins [1-4] known to reversibly bind carbon monoxide and oxygen without oxidation, like in hemoglobin and myoglobin, the so-called basket-handle porphyrins (BHP) allow us to vary the size of the pocket cavity by tuning the length of the baskethandle (BH) [4]. Four hybrid models have been designed [5] in which two pivalamido "pickets' (as in picket-fenee-porphyrins - PFP) [1] are present on each side of the amide-handle of variable length in a cross position allowing an increase of the central steric hindrance as shown in Fig. 1. The systematic decrease of the CO affinity constant in toluene solution, mainly due to a decrease of the

Abbreviations: BH, basket handle, BHP, basket-handle porphyrins, DCI, Dispositif de Collision dans l'Igloo, EXAFS, extended X-ray absorption fine structure, IR, infra-red, LURE, Laboratoire d'Utilisation du Rayonnement Electromagn6tique, PF, picket fence, PFP, picket fence porphyrins, Ply, pivaloyl, Poe, pocket, XANES, X-ray absorption near edge strueture, XAS, X-ray absorption spectroscopy, XRD, X-ray diffraction. Correspondence: A. Bianconi, Dipartimento di Fisica, Universit~ di Roma 'La Sapienza', P. Aldo Morn 2, 1-00185 Roma, Italy.

association rate constant, has been correlated with the length of the handle [4]. In order to find the correlation between the variation of the affinity constant and local structural changes these compounds have been investigated by several experimental methods. The FeC-O bond angle as a function of the length of the basket-handle was studied by X-ray absorption near edge structure (XANES) spectroscopy [6] and Raman spectroscopy of samples in solution [7] and by X-ray diffraction of single crystals [8]. The n = 10 polymethylene handle porphyrin was studied by XANES [6], and the Fe-C-O configuration was found to be linear in toluene solution as well as in solid samples in agreemen~. with the linear Fe-C-O configuration found in the same compound in the single crystal form by X-ray diffraction [8]. The infrared results have shown an evolution of the CO vibrational frequency in toluene solutions, from 1958 cm-n to 1948 cm-~ when going from the longer to the shorter basket-handle. However, the resonant Raman spectra of the basket-handle porphyrins in the energy range 450-600 cm-n have indicated a constant linear Fe-C-O configuration also for the shortest basket-handle length [7]. This is in agreement with the recent X-ray diffraction of a six membered (n = 6) polymethylene handle porphyrin [9]. This last work has

170 shown a relevant distortion of the tetrapyrrolic macrocycle for short basket-handle length. Here we have investigated the local structure of four different basket-handle porphyrins with different polymethylene basket-handles n = 10, 8, 7, 6 in solid state and in solution by XANES to probe the variations of the distortion of the porphyrin plane and the Fe-C-O angle induced by the change of the length of the handle. In fact, XANES spectroscopy [10-13] probes the stereochemistry of the cluster surrounding the absorbing Fe atom. XANES spectroscopy was recently applied to study synthetic tetrapyrrolic macrocycles [14-17], hemoproteins [18-27] in solution and the carbonyl-myoglobin single-crystal [28], It has been shown that XANES spectroscopy provides a reliable value of the average Fe-C-O bond angle by the intensity variation of the multiple scattering resonance at 16 eV. The effect of the distortion of the macrocycle plane is expected to affect the resonance at 22 eV which is determined by multiple scattering pathways of the excited photoelectron in the 43' plane of the porphyrin ring [28]. Materials and Methods

The Fe(Il) porphyrins were synthesized as previously described [7,8]. The fixation of 1-methyl-imidazole was insured by introducing, under argon, the base in the toluene solution of Fe(II) porphyrin (10 -2 M) in the stoichiometric ratio BHP/base = 1/10. Carbonyl derivatives were obtained by bubbling pure carbon monoxide in the resulting solutions. Just before the XANES measurements, the solutions were transferred under CO into a l cm thick aluminium cell through a latex septum, via a steel tube. The solid carbonyl samples were precipitated from the preceding solutions by addition of pure heptane. The finely ground powder was uniformly dispersed between two X-ray transparent adhesive tapes. The mass of the sample was kept small to avoid saturation effects. Four samples were prepared and studied: compound 1 (n = 10) is the carbonyl-l-methyl-imidazole Fe(II) derivative of the a-5,15-(2,2'-(dodecanediamido)-diphenyl): a,a-lO,20-bis-(o-pivalamidophenyl)porphyrin; compound 2 (n = 8) is the analogous derivative where the dodecane chain is replaced by a decane one; whereas compounds 3 (n = 7) and 4 (n = 6) retain the same structure with a nonane and octane chain, respectively. The XANES spectra were recorded at synchrotron radiation facility LURE at Orsay, France, by using the DCI storage ring operated at 1.85 GeV with an average intensity of about 150 mA. The XANES spectra of solid samples were measured by using a scanning twocrystals Si 311 monoehromator. Harmonics rejection was performed by slightly detuning the parallelism of

the crystals, which resulted in about 30% incident photon flux loss at 7 keV. The spectrum was recorded step by step, every 0.25 eV with 1 second accumulation time per point. The spectrum of a 6 # m thick iron metallic foil was recorded just after or just before an unknown XANES spectrum of a sample to check energy calibration. This insures an energy accuracy determination of 0.25 eV. The dispersive X-ray absorption spectrometer [29] combines a focusing dispersive X-ray optics and a position sensitive detector. The X-ray polychromator is made of a 23 cm long, 1 mm thick, triangle-shaped, bent Si 311 crystal. The crystal has been asymmetrically cut ( - 12°) to improve the energy resolution. The solid state detector is a 1024 silicon photodiodes array manufactured by Reticon. It is cooled and maintained at constant temperature (100 _ 0.1 K) to obtain low noise and increased life-time. Each photodJode, at 7 keV, is saturated with 6000 photons per second and therefore the 1024 pixels are able to face the available flux of 2.5. l0 ~° photons per second, in the minimum integration time of 2.7 ms. The sample was located at the polychromatic focus point of the X-ray beam. The detector position was optimized at the monochromatic focus point to achieve a resolution of ---1 eV. The monochromatic span coming from the 6 mm-wide source focused on the Rowland circle and the distance between the monochromatic focus point and the crystal is roughly two times the distance between the polychromatic focus point and the crystal. A processor (16 bits, 200 ns) can process the data immediately under the form of absorption spectrum. The acquisition time typically was 250" 8 ms for the solid samples and 18.8 s for the samples in toluene solution, in order to get each spectrum out of 10 I° photons per second roughly. An iron metallic foil was regularly used to calibrate the energy scale of the linear detector. Results and D~scuss-'.,on

The experimental XANES spectra of solid state samples measured by the step by step EXAFS mode are shown ~ Fig. 2. Fig. 3 shows the XANES spectra of the sam,: set of sample:: in toluene solution measured by the dispersive mode. We subtracted from the experimentaZ spectrum a linear background, determined by leas~-~quare fitting of the pre-edge experimental points. The absorption coefficient was normalized by taking the average of the EXAFS oscillation as unit absorbance. The spectra of the four compounds in Fig. 1 and 2 show the same absorption features classified following the notation of Ref. 28. The solid samples have been investigated also by dispersive mode and they show similar features. The spectra exhibits two pre-edge transitions, P~ and P2, the peaks C I and D, a shoulder

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X-ray absorption spectroscopy of carbonyl basket handle Fe(II) porphyrins: the distortion of the tetrapyrrolic macrocycle.

The distortion of the tetrapyrrolic macrocycle, the Fe-C-O bond angle and the Fe local electronic structure of carbonyl basket-handle Fe2+ porphyrins ...
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