Biochimica et Biophysica Acta, 11! 9 (1992) 169-174 169 © 1992 Elsevier Science Publishers B.V. All rights reserved 0167-4838/92/$05.00 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 bas- ket-handle porphyrins (BHP) allow us to vary the size of the pocket cavity by tuning the length of the basket- handle (BH) [4]. Four hybrid models have been de- signed [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 hin- drance 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'Utili- sation 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 correla- tion between the variation of the affinity constant and local structural changes these compounds have been investigated by several experimental methods. The Fe- C-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 polymethy- lene 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 agree- men~. 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 bas- ket-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