# Synthesis and characterization of
[Fe(BPMEN)-ACC]SbF 6: a structural and functional
mimic of ACC-oxidase †

Y Roux, W Ghattas, F Avenier, R Guillot, Ariane Jalila Simaan, J.-P Mahy

► To cite this version:

Y Roux, W Ghattas, F Avenier, R Guillot, Ariane Jalila Simaan, et al.. Synthesis and characterization of [Fe(BPMEN)-ACC]SbF 6: a structural and functional mimic of ACC-oxidase †. Dalton Transactions, 2015, 44 (13), pp.5966-5968. 10.1039/c5dt00347d. hal-01258299

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Cite this: *Dalton Trans.*, 2015, **44**,
5966

Received 26th January 2015,
Accepted 24th February 2015

DOI: 10.1039/c5dt00347d

www.rsc.org/dalton

# Synthesis and characterization of [Fe(BPMEN)-ACC]SbF<sub>6</sub>: a structural and functional mimic of ACC-oxidase†

Y. Roux,‡ª W. Ghattas,‡ª F. Avenier,ª R. Guillot,ª A. J. Simaan*ᵇ and J.-P. Mahy*ª

A mononuclear Fe(II) complex bearing 1-aminocyclopropane-1-carboxylic acid (ACCH) was synthesized and characterized. X-ray crystallography demonstrated that ACC binds to the Fe(II) ion in a bidentate mode constituting the first structural mimic of the expected binding of ACC to the Fe(II) center of the ethylene forming enzyme ACC-oxidase (ACCO). [Fe(BPMEN)ACC]SbF₆ also constitutes a functional biomimetic complex of ACCO, as it reacts with hydrogen peroxide producing ethylene.

The final step in the biosynthesis of the phytohormone ethyl-
ene¹ is the oxidation of 1-aminocyclopropane-1-carboxylic acid
(ACCH)² catalyzed by ACC-oxidase (ACCO).³ The X-ray crystal
structure of substrate-free ACCO⁴ has confirmed the antici-
pated makeup of its active site i.e. a non-heme Fe(II) cation co-
ordinated by the classical N,N,O facial triad.⁵,⁶ In contrast, the
ambiguous binding of 1-aminocyclopropane-1-carboxylate
(ACC) in the active site of ACCO has only been probed by spec-
troscopic studies, which have nonetheless concluded that ACC
binds to the Fe(II) ion of the active site in a bidentate mode *via*
both its amine and carboxylate functions.⁷ Accordingly, the
structural characterization of a dinuclear ACC-containing
Fe(III) complex, [Fe₂(TACN)₂(µ-O)(µ-ACCH)₂]⁴⁺ (TACN = 1,4,7-tri-
azacyclononane) has shown that ACC can bind iron centres. In
this case however, two ACCH fragments are bridging two Fe(III)
cations by their carboxylate functions.⁸ Bidentate ACC has only
been reported in a few Cu(II)-ACC complexes,⁹ and has never
been observed in the case of mononuclear Fe(II) complexes.
Indeed, the good water solubility of amino acids is inappropri-
ate for the synthesis of Fe(II) complexes, which are best

obtained in aprotic poorly-coordinating solvents that conver-
sely do not dissolve amino acids.¹⁰ To the best of our knowl-
edge, structural characterization of amino acid-containing
Fe(II) complexes has only been reported in the case of a
proline-containing complex.¹¹ Proline is structurally distin-
guished from other natural α-amino acids by a secondary
amine function engaged in a 5-membered ring and therefore
is inappropriate to structurally mimic the coordination of
other natural α-amino acids. Furthermore, the synthesis of the
proline-containing Fe(II) complex relied on the solubilization
of proline in DMSO, which did not allow to get the structural
information when either phenylalanine, tryptophan or valine
was used instead of proline.¹¹

Here, in order to overcome the solubility limitations, we
treated an aqueous solution of ACCH with one equivalent of
tetra-*n*-butylammonium hydroxide (N(*n*-Bu)₄OH). Subsequent
water evaporation provided an ionic liquid fully miscible with
acetonitrile, which allowed its combination with an aceto-
nitrile solution of the previously described [Fe(BPMEN)-
(CH₃CN)₂](SbF₆)₂ complex (1) (BPMEN = N,N'-dimethyl-
N,N'-bis(pyridylmethyl)ethane-1,2-diamine) (Scheme 1).¹²

When one equivalent of N(n-Bu)₄ACC was added to an
acetonitrile solution of complex 1, the solution turned from
purple to a pale yellow color. The monitoring of the UV-vis
absorbance as a function of the increasing amounts of

<figure><img src="image_3.png" /><figcaption>**Scheme 1** Preparation of the [Fe(BPMEN)ACC]SbF₆ complex (2) from [Fe(BPMEN)(CH₃CN)₂](SbF₆)₂ (1) under an inert atmosphere.</figcaption></figure>

<footnote><sup>a</sup>Institut de Chimie Moléculaire et des Matériaux d'Orsay, Université Paris Sud,
CNRS (UMR 8182), Orsay 91405 CEDEX, France.</footnote>

<footnote>E-mail: jean-pierre.mahy@u-psud.fr</footnote>

<footnote><sup>b</sup>Aix Marseille Université, Centrale Marseille, CNRS, iSm2 UMR 7313, 13397, Marseille, France. E-mail: jalila.simaan@univ-amu.fr</footnote>

<footnote>†Electronic supplementary information (ESI) available: Crystallographic data, UV-Vis measurements, magnetization measurements. CCDC 1029372 and 1029373. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5dt00347d</footnote>

<footnote>‡ Equally contributed to the work reported.</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1** Evolution of the UV-vis spectrum of a 0.5 mM acetonitrile solution of [Fe(BPMEN)(CH₃CN)₂](SbF₆)₂ (1) (bold black line) upon successive additions of upto 1 equiv. (red line) of N(n-Bu)₄ACC.</figcaption></figure>

N(n-Bu)₄ACC added to **1** (Fig. 1) showed the progressive evolution of the spectrum of **1** ($\lambda_{max}$ = 373 nm, $\varepsilon$ = 3340 M⁻¹ cm⁻¹, MLCT band)¹³ into a new spectrum ($\lambda_{max}$ = 395 nm, $\varepsilon$ = 860 M⁻¹ cm⁻¹). The occurrence of an isosbestic point at 410 nm clearly indicated a single transformation of the starting material into a new species. The transformation was optimal for one equivalent of N(n-Bu)₄ACC added. Further addition of N(n-Bu)₄ACC led to a decrease of the characteristic MLCT band at 395 nm that completely disappeared after the addition of three equivalents of N(n-Bu)₄ACC (Fig. S1†).

High resolution electrospray ionization mass spectrometry (HR ESI-MS) analysis was carried out on the pale yellow solution obtained after the addition of one equivalent of the amino acid (Fig. 2). The results revealed the formation of a single new compound characterized by a peak at *m/z* 426.1604, which is in agreement with the complexation of one ACC molecule to the Fe(II) ion of complex **1** in the place of two acetonitrile molecules observed in the X-ray crystal structure (Fig. S2†).

<figure><img src="image_5.png" /><figcaption>**Fig. 2** HR ESI-MS spectrum obtained upon additions of 1 equiv. of N(n-Bu)₄ACC onto [Fe(BPMEN)(CH₃CN)₂](SbF₆)₂ (1).</figcaption></figure>

The new pale yellow species was then precipitated by the addition of ether in the acetonitrile solution and recrystallized from slow ether diffusion in acetonitrile to afford monocrystals suitable for X-ray diffraction analysis. The resulting diffraction pattern was in agreement with a [Fe(BPMEN)ACC]SbF₆ molecular formula for complex **2** and a structure in which ACC is bound to the Fe(II) center in a bidentate mode *via* both its amine and carboxylate functions, as projected for the enzymatic active site (Fig. 3, Tables S1 and S2†).⁵ In addition, both the UV-vis spectrum and the mass spectrum of complex **2** obtained from the solid state matched those of the species formed in solution. In comparison with the X-ray structure of complex **1**, the distorted octahedral geometry is maintained in complex **2**, but the average metal-ligand distance has increased from 1.98 Å in **1** to 2.18 Å in **2**. It is noteworthy that average Fe-N distances below 2.0 Å in **1** and above 2.1 Å in **2** suggest a low spin to high spin transition upon complexation of the amino acid to the Fe(II) center.¹⁴

Bulk magnetization data were collected from the crystalline samples of complex **2**. The corresponding $\chi_m T$ vs. *T* plot (Fig. S3†) showed an initial sharp increase (upto ca. 50 K) followed by a slight monotonic increase of $\chi_m T$ with the increasing temperature. Both the overall shape of $\chi_m T$ vs. *T* and a $\chi_m T$ value reaching 3.48 cm³ K mol⁻¹ at 400 K concur with a high spin mononuclear Fe(II) center (*S* = 2, *g* = 2.1). Therefore, complex **2** is in the high spin state as suggested by the bond lengths obtained from the crystal structure. The coordination of the amino acid on the Fe(II) ion stabilizes the high spin state whereas, complex **1** is known to be in the low spin state at low temperature, in spin transition at room temperature and at the high spin state only above 400 K.¹⁵

Although the enzymatic system contains an Fe(II) ion in its active site, no functional mimic of ACCO reported so far involves Fe(II). Therefore, we tested complex **2** in the oxidation

<figure><img src="image_6.png" /><figcaption>**Fig. 3** X-ray crystal structure of the [Fe(BPMEN)ACC]SbF₆ complex (2) (thermal ellipsoids are set at 50% probability). The hydrogen atoms and the counter ion are omitted for the sake of clarity. Selected bond lengths: Fe–N1 2.203(2) Å, Fe–N2 2.232(2) Å, Fe–N3 2.242(3) Å, Fe–N4 2.203(2) Å, Fe–N5 2.211(2) Å, Fe–O1 2.0164(19) Å.</figcaption></figure>

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

of ACC into ethylene, first using O₂ and then in the presence of H₂O₂. The UV-vis spectrum of acetonitrile solutions of complex 2 did not change when O₂ was introduced. In contrast, when 10 equivalents of H₂O₂ were added to an acetonitrile solution of complex 2, its UV-vis spectrum changed drastically, however, no clean transformation with isosbestic points could be observed (Fig. S4†). The addition of up to 100 equivalents of H₂O₂ to complex 2 was then performed in sealed tubes and GC analysis of the resulting gas revealed that the formation of ethylene reached ca. 23% yield when 5 to 10 equivalents of H₂O₂ were added, compared to a 15% yield in the blank experiment using a 1:1 mixture of iron(II) triflate and N(n-Bu)₄ACC. The rather low ACC oxidation yield is not surprising considering the fact that complex 2 is hexacoordinated and thus, a direct interaction between the iron cation and hydrogen peroxide requires the de-coordination of one of the six ligands of iron. The formation of ethylene suggests that one of these six ligands is indeed labile enough to allow hydrogen peroxide activation at the metal center.

In summary, our work describes the synthesis, the reactivity and the characterization in solution and in the solid state of the first mononuclear Fe(II) complex bearing an ACC ligand. This complex demonstrates that ACC can bind to the Fe(II) ion in a bidentate mode, constituting a structural mimic of the binding of ACC to the Fe(II) center of ACCO.

We are grateful to Dr Eric Rivière for SQUID measurements and to the 'Agence Nationale de la Recherche' (ANR BIOXICAT and REBAR) and 'Région île de France' (DIM développement soutenable) for financial support.

## Notes and references

1. (a) G.-P. L. Girardin, *Fortschr. Gesamtgeb. Agrikulturchem.*, 1864, 7, 199–200; (b) D. Neljubow, *Beih. Bot. Zentralbl.*, 1901, **10**, 128–139.

2. (a) T. Boller, R. Herner and H. Kende, *Planta*, 1979, **145**, 293–303; (b) D. O. Adams and S. F. Yang, *Proc. Natl. Acad. Sci. U. S. A.*, 1979, **76**, 170–174.

3. (a) M. Costas, M. P. Mehn, M. P. Jensen and L. Que Jr., *Chem. Rev.*, 2004, **104**, 939–986; (b) S. F. Yang and N. E. Hoffman, *Annu. Rev. Plant Physiol.*, 1984, **35**, 155–189; (c) D. J. McGarvey and R. E. Christoffersen, *J. Biol. Chem.*, 1992, **267**, 5964–5967.

4. Z. Zhang, J.-S. Ren, I. J. Clifton and C. J. Schofield, *Chem. Biol.*, 2004, **11**, 1383–1394.

5. (a) V. J. Lay, A. G. Prescott, P. G. Thomas and P. John, *Eur. J. Biochem.*, 1996, **242**, 228–234; (b) Z. Zhang, J. N. Barlow, J. E. Baldwin and C. J. Schofield, *Biochemistry*, 1997, **36**, 15999–16007.

6. (a) E. L. Hegg and L. Que Jr., *Eur. J. Biochem.*, 1997, **250**, 625–629; (b) K. Koehntop, J. Emerson and L. Que Jr., *J. Biol. Inorg. Chem.*, 2005, **10**, 87–93.

7. (a) A. M. Rocklin, D. L. Tierney, V. Kofman, N. M. W. Brunhuber, B. M. Hoffman, R. E. Christoffersen, N. O. Reich, J. D. Lipscomb and L. Que Jr., *Proc. Natl. Acad. Sci. U. S. A.*, 1999, **96**, 7905–7909; (b) A. Rocklin, K. Kato, H. Liu, L. Que Jr. and J. Lipscomb, *JBIC, J. Biol. Inorg. Chem.*, 2004, **9**, 171–182; (c) J. Zhou, A. M. Rocklin, J. D. Lipscomb, L. Que Jr. and E. I. Solomon, *J. Am. Chem. Soc.*, 2002, **124**, 4602–4609; (d) D. L. Tierney, A. M. Rocklin, J. D. Lipscomb, L. Que Jr. and B. M. Hoffman, *J. Am. Chem. Soc.*, 2005, **127**, 7005–7013.

8. W. Ghattas, Z. Serhan, N. El Bakkali-Taheri, M. Réglier, M. Kodera, Y. Hitomi and A. J. Simaan, *Inorg. Chem.*, 2009, **48**, 3910–3912.

9. (a) W. Ghattas, C. Gaudin, M. Giorgi, A. Rockenbauer, A. J. Simaan and M. Reglier, *Chem. Commun.*, 2006, 1027–1029; (b) W. Ghattas, M. Giorgi, C. Gaudin, A. Rockenbauer, M. Reglier, Y. Hitomi and A. J. Simaan, *Bioinorg. Chem. Appl.*, 2007, 43324; (c) W. Ghattas, M. Giorgi, Y. Mekmouche, T. Tanaka, A. Rockenbauer, M. Réglier, Y. Hitomi and A. J. Simaan, *Inorg. Chem.*, 2008, **47**, 4627–4638; (d) N. Judaš and N. Raos, *Inorg. Chem.*, 2006, **45**, 4892–4894.

10. (a) M. Costas, K. Chen and L. Que Jr., *Coord. Chem. Rev.*, 2000, **200–202**, 517–544; (b) E. P. Talsi and K. P. Bryliakov, *Coord. Chem. Rev.*, 2012, **256**, 1418–1434.

11. C. P. Magill, C. Floriani, A. Chiesi-Villa and C. Rizzoli, *Inorg. Chem.*, 1994, **33**, 1928–1933.

12. (a) K. Chen and L. Que Jr., *Chem. Commun.*, 1999, 1375–1376; (b) M. C. White, A. G. Doyle and E. N. Jacobsen, *J. Am. Chem. Soc.*, 2001, **123**, 7194–7195.

13. G. J. P. Britovsek, J. England and A. J. P. White, *Inorg. Chem.*, 2005, **44**, 8125–8134.

14. A. J. Simaan, S. Poussereau, G. Blondin, J.-J. Girerd, D. Defaye, C. Philouze, J. Guilhem and L. Tchertanov, *Inorg. Chim. Acta*, 2000, **299**, 221–230.

15. (a) K. P. Bryliakov, E. A. Duban and E. P. Talsi, *Eur. J. Inorg. Chem.*, 2005, **299**, 72–76; (b) G. J. P. Britovsek, J. England and A. J. P. White, *Inorg. Chem.*, 2005, **44**, 8125–8134.