CC BY
17
UDC 66.065.5
L. I. Matienko, L. A. Mosolova, V. I . Binyukov, E. M. Mil, G. E. Zaikov
APPLICATION OF AFM METHOD FOR RESEARCH OF STRUCTURAL SELF-ORGANIZATION OF COMPLEXES OF Ni (AND Fe) AS EFFECTIVE HOMOGENOUS CATALYSTS,
AND DIOXYGENASES MODELS
Keywords: homogeneous catalysis, oxidation, ethylbenzene, a-phenyl ethyl hydroperoxide, dioxygen, AFM method, nanostructures based on catalytic active complexes FeIIIx(acac)y18C6m(H2P)n, NixL1y(L1oJz(L2)n(H2O)m, (NiII(acac)2-L2 PhOH}, {LiStL2 PhOH} (L2
=MSt(M=Na, Li), MP, HMPA).
The role played by H-bonds and supramolecular macrostructures, in the mechanisms of homogeneous and enzymatic catalysis (nickel and iron complexes) is discussed. The AFM method has been used for research of possibility of the stable supramolecular nanostructures formation based on effective catalysts of ethylbenzene oxidations and Dioxygenases models: iron complexes FeIIIx(acac)y18C6m(H2O)n, and nickel complexes NixL1y(L1ox)z(L2)n(HP)m, {Ni"(acac)2L2PhOH} (L2 =MSt, MP, HMPA), and also complex of redox-inactive metal {LiStL2 PhOH} (L2 =HMPA) - with the assistance of intermolecular H-bonds; the assessing its role in mechanisms of catalysis.
Ключевые слова: гомогенный катализ, окисление, этилбензол, a-фенил этил гидропероксид, молекулярный кислород, метод АСМ, наноструктуры на основе каталитически активных комплексов Feшx(асас)y18C6m(Нfi)n> NixL1y(L1ox)z(L2)n(H2O)m, {NiП(асас)2 L2 PhOH}, {LiStL2PhOH} (L2 =MSt(M=Na, Li), MP, HMPA).
Обсуждается роль водородных связей и супрамолекулярных макроструктур в механизмах гомогенного и ферментативного катализа (комплексы никеля и железа). Методом АСМ исследовались возможности формирования стабильных супрамолекулярных наноструктур на основе эффективных катализаторов окисления этил-бензола и моделей диоксигеназ: комплексов железа FeIIIx(acac)y18C6m(H2O)n и никеля NixL1y(L1ca)z(L2)n(HP)m, {Ni"(acac)2L2PhOH} (L2 =MSt, MP, HMPA), а также комплекса редокс-неактивного металла {LiSt L2 PhOH} (L2 =HMPA) - при помощи межмолекулярных водородных связей; оценке его роли в механизмах катализа.
1 Introduction
In recent years, the studies in the field of homogeneous catalytic oxidation of hydrocarbons with molecular oxygen were developed in two directions, namely, the free-radical chain oxidation catalyzed by transition metal complexes and the catalysis by metal complexes that mimic enzymes. Low yields of oxidation products in relation to the consumed hydrocarbon (RH) caused by the fast catalyst deactivation are the main obstacle to the use of the majority of biomimetic systems on the industrial scale [1, 2].
However, the findings on the mechanism of action of enzymes, and, in particular, Dioxygenases and their models, are very useful in the treatment of the mechanism of catalysis by metal complexes in the processes of oxidation of hydrocarbons with molecular oxygen. Moreover, as one will see below, the investigation of the mechanism of catalysis by metal complexes can give the necessary material for the study of the mechanism of action of enzymes.
The problem of selective oxidation of alkylarens to hydro peroxides is economically sound. Hydro peroxides are used as intermediates in the large-scale production of important monomers. For instance, propylene oxide and styrene are synthesized from a-phenyl ethyl hydroperoxide, and cumyl hydro peroxide is the precursor in the synthesis of phenol and acetone [1, 2]. The method of modifying the Nin and Fen,m complexes used in the selective oxidation of alkylarens (ethylbenzene and cumene) with molecular oxygen to afford the corresponding hydro peroxides aimed at increasing their selectivity's has been first proposed by L.I. Matienko, and new efficient catalysts of selective
oxidation of ethylbenzene to a-phenyl ethyl hydroperoxide (PEH) were developed [1,2].
We used successfully at first the atomic force microscopy (AFM) to study the possibility of forming the supramolecular structures due to the intermolecular hydrogen bonds on the basis of heteroligand complexes of nickel, iron [3-5], ^L^L^L^^O),,, ("A"), FeIIIx(acac)y18C6m(H2O)n ("B"),
{NiII(acac)2 -L2 -PhOH} ("C") (L2 =MSt(M=Na, Li), MP, HMPA), and lithium {LiSt - L2 -PhOH} (L2=DMF, HMPA) ("D").
The complexes A-D are effective catalysts of selective ethyl benzene oxidation to a-phenyl ethyl hydro peroxide (data about activity of complexes D are unpublished and reported here for the first time), but A and B - also are structure and functional models of Dioxygenases Ni(Fe)-ARD and FeII-Dke1. We assumed that the stability of the complexes A-D as the alkylarenes oxidation catalysts could be related to the stable supramolecular structures formation due to the intermolecular hydrogen bonds. And different activities of Ni(Fe)-ARD towards common substrates (Acireductone and dioxygen) as one of the reasons - with self-organization into various catalysts macrostructures due to intermolecular hydrogens bonds. These assumptions are supported by our AFM-research outlined in this article.
2 Experimental
Ethylbenzene (RH) was oxidized with dioxygen at 120°C in glass bubbling-type reactor [5] in the presence of iron complexes FeIIIx(acac)y18C6m(H2O)n, and nickel complexes NixL1y(L1ox)z(L2)n(H2O)m, {Ni"(acac)2-L2-PhOH} (L2
=MSt, MP, HMPA), and also redox-inactive metal complex {LiSt-L2-PhOH} (L2 = DMF, HMPA) [2,3].
Analysis of oxidation products. a-Phenylethylhydroperoxide (PEH) was analyzed by iodometry. By-products, including
methylphenylcarbinol (MPC), acetophenone (AP), and phenol (PhOH) as well as the RH content in the oxidation process were examined by GLC [3].
AFM SOLVER P47/SMENA/ with Silicon Cantilevers NSG11S (NT MDT) with curvature radius 10 nm, tip height: 10 - 15 ^m and cone angle < 22° in taping mode on resonant frequency 150 KHz was used [4-5].
As substrate the polished Silicone surface special chemically modified was used.
Waterproof modified Silicone surface was exploit for the self-assembly-driven growth due to H-bonding of complexes Femx(acac)y18C6m(H2O)n, NixL1y(L1ox)z(L2)n(H2O)m; {Nin(acac)2-L2-PhOH} (L2 =MSt, MP, HMPA), {LiSt-L2-PhOH} (L2 =HMPA) with Silicone surface. The saturated chloroform (CHCl3) or water solutions of complexes was put on a surface, maintained some time, and then solvent was deleted from a surface by means of special method -spin-coating process.
In the course of scanning of investigated samples it has been found, that the structures are fixed on a surface strongly enough due to H-bonding. The self-assembly-driven growth of the supramolecular structures on modified Silicone surface on the basis of researched complexes, due to H-bonds and perhaps the other non-covalent interactions was observed.
3 Results and Discussion
3.1 The role of Hydrogen-Bonds in mechanisms of homogeneous catalysis
As a rule, in the quest for axial modifying lig-ands L2 that control the activity and selectivity of homogeneous metal complex catalysts, attention of scientists is focused on their steric and electronic properties. The interactions of ligands L2 with L1 taking place in the outer coordination sphere are less studied; the same applies to the role of hydrogen bonds, which are usually difficult to control [6, 7].
Secondary interactions (hydrogen bonding, proton transfer) play an important role in the dioxygen activation and its binding to the active sites of metalloenzymes [8]. For example, respiration becomes impossible when the fragments responsible for the formation of H-bonds with the Fe-O2 metal site are removed from the haemoglobin active site [9]. Moreover, the O2-affinity of haemoglobin active sites is in a definite relationship with the network of H-bonds surrounding the Fe ion. The dysfunction of Cytochrome P450 observed upon the cleavage of its H-bonds formed with the Fe - O2 fragment demonstrated the important role of H-bonds that form the second coordination sphere around metal ions of many proteins [10].
In designing catalytic systems that mimic the enzymatic activity, special attention should be paid to the formation of H-bonds in the second coordination sphere of a metal ion.
Transition metal jff-diketonates are involved in various substitution reactions. Methine protons of che-
late rings in ^-diketonate complexes can be substituted by different electrophiles (E) (formally, these reactions are analogous to the Michael addition reactions) [1113]. This is a metal-controlled process of the C-C bond formation [13]. The complex NiII(acac)2 is the most efficient catalyst of such reactions. The rate-determining step of these reactions is the formation of a resonance-stabilized zwitter-ion [(MIIL1n)+E-] in which the proton transfer precedes the formation of reaction products [1,2]. The appearance of new absorption bands in electron absorption spectra of {Ni(acac)2+L2+E} mixtures, that can be ascribed to the charge transfer from electron-donating ligands of complexes L2 Ni(acac)2 to n-acceptors E (E is tetracyanethylene or chloranil) supports the formation of a charge-transfer complex L2 Ni(acac)2-E [1,2]. The outer-sphere reaction of the electrophile addition to y-C in an acetylacetonate ligand follows the formation of the charge-transfer complex.
In our works we have modeled efficient cata-1 2 2 lytic systems {ML n + L } (M=Ni, Fe, L are crown
ethers or quaternary ammonium salts) for ethylbenzene oxidation to a-phenyl ethyl hydro peroxide, that was based on the established (for Ni complexes) and hypothetical (for Fe complexes) mechanisms of formation of cata-lytically active species and their operation [1,2]. Selectivity (SPEH)max, conversion, and yield of PEH in ethyl benzene oxidation catalyzed by these systems were substantially higher than those observed with conventional catalysts of ethyl benzene oxidation to PEH [1,2].
The high activity of systems {ML1n + L2} (L2 are crown ethers or quaternary ammonium salts) is associated with the fact that during the 2ethylbenzene oxidation, the active primary (M L 2)X(L )y complexes and heteroligand MIIxL1y(L1ox)z(L2)n complexes are formed to be involved in the oxidation process.
We established, that in complexes ML 2 L2, the axially coordinated electron-donating ligand L2 controls the formation of primary active complexes and the subsequent reactions of jff-diketonate ligands in the outer coordination sphere of these complexes. The coordination of an electron-donating extra-ligand L2 with an MIIL1
2 complex favorable for stabilization of the transient zwitter-ion L2[L1M(L1)+O2-] enhances the probability of regioselective O2 addition to the methine C-H bond of an acetylacetonate ligand activated by its coordination with metal ions. The outer-sphere reaction of O2 incorporation into the chelate ring depends on the nature of the metal and the modifying ligand L2 [1,2]. Thus for nickel complexes, the reaction of acac-ligand oxIyI genation follows a mechanism analogous to those of NiI-containing Acireductone Dioxygenase (ARD) [14] or Cu- and Fe-containing Quercetin 2,3-Dioxygenases [15,16]. Namely, incorporation of O2 into the chelate acac-ring was accompanied by the proton transfer and the redistribution of bonds in the transition complex leading to the scission of the cyclic system to form a chelate ligand OAc-, acetaldehyde and CO (in the Criegee rearrangement, Scheme 1).
In the effect of iron(II) acetylacetonate complexes, one can find an analogy with the action of FeII-ARD [14] or FeII-acetyl acetone Dioxygenase (Dke1) [17] (Scheme 2).
-□с
Scheme 1
f
<
Scheme 2
CH3
H + O=O -
H3C
л /
Fe
CH3
H3C
+ H3C
O о
H3C-y-CH3
о о
As the most effective catalytic systems of the ethylbenzene oxidation to the a-phenyl ethyl hydroperoxide are the triple systems [1-3]. Namely, the phenomenon of a substantial increase in the selectivity (S) and conversion (C) of the ethylbenzene oxidation to the a-phenyl ethyl hydroperoxide upon addition of
PhOH together with N-metylpyrrolidone-2 (MP), hexamethylphosphorotriamide (HMPA) or alkali metal stearate MSt (M = Li, Na) as modifying ligands to metal complex NiII(acac)2 was discovered in works L.I. Matienko and L.A. Mosolova [1-3]. In case of triple systems with additives of MSt the observed values of C [C >35% at SPEHmax = 90%, [ROOH]max (1.6-1.8 mol/l) far exceeded those obtained with the other ternary catalytic systems {NiII(acac)2+ L2 + PhOH} (L2 = MP, HMPA) and the majority of active binary systems. These results by L.I. Matienko and L.A. Mosolova are protected by the Russian Federation patent (2004) [3]. The feature of triple systems {Ni"(acach+ L2 + PhOH} (L2 =MSt, MP, HMPA) is that the in situ formed complexes NiII(acac)2^L2^PhOH are not transformed during oxidation, and have the long-term activity. Unlike binary systems, the acac- ligand in nickel complex does not undergo transformations in the course of ethyl benzene oxidation in this case. (The formation of triple complexes NiII(acac)2^L2^PhOH at vary early stages of oxidation was established with kinetic methods [3]). So the reaction rate remains practically the same during the oxidation process. In the course of the oxidation the rates of products accumulation unchanged during the long period t < 30 - 40 hours. At that unlike the catalysis with majority of binary systems, in ethyl benzene oxidation to PEH, catalyzed with triple systems, the byproducts AP and MPC formed parallel PEH during all ethyl benzene oxidation process.
3.2 Role of supramolecular nanostructures formation due to H-bonding in mechanism of catalysis
The high stability of effective catalytic complexes, which formed in the process of selective oxidation of ethylbenzene to PEH at catalysis with MIIxL1y(L1ox)z(L2)n, ( M=Ni, Fe, L1 =acac-, L1ox =OAc , L2 = crown ethers or quaternary ammonium
II 2
salts)) complexes and triple systems {Ni (acac)2+ L + PhOH} (L2 =N-metylpyrrolidone-2 (MP), hexamethylphosphorotriamide (HMPA) or alkali metal stearate MSt (M = Li, Na)) seems to be associated with the formation of supramolecular structures due to intermolecular H-bonds.
Hydrogen bonds vary enormously in bond energy from ~15-40 kcal/mol for the strongest interactions to less than 4 kcal/mol for the weakest. It is proposed, largely based on calculations, that strong hydrogen bonds have more covalent character, whereas electrostatics are more important for weak hydrogen bonds, but the precise contribution of electrostatics to hydrogen bonding is widely debated [18]. Hydrogen bonds are important in non-covalent aromatic interactions, where n- electrons play the role of the proton acceptor, which are a very common phenomenon in chemistry and biology. They play an important role in the structures of proteins and DNA, as well as in drug receptor binding and catalysis [19]. Proton-coupled bicarboxylates top the list as the earliest and still the best-studied systems suspected of forming low-barrier hydrogen bonds (LBHBs) in the vicinity of the active sites of enzymes
[20]. These hydrogen-bonded couples can be depicted o ' o
as R—c—0 ---H—o—c—R' and they can be abbre-
HC
O
O
HC
3
+
O
O
+
O
O
viated by the general formula X—HX. Proton-coupled bicarboxylates appear in 16% of all protein X-ray structures. There are at least five X-ray structures showing short (and therefore strong) hydrogen bonds between an enzyme carboxylate and a reaction intermediate or transition state analogue bound at the enzyme active site. The authors [20] consider these structures to be the best de facto evidence of the existence of low-barrier hydrogen bonds stabilizing high-energy reaction intermediates at enzyme active sites. Carboxylates figure prominently in the LBHB enzymatic story in part because all negative charges on proteins are carboxylates.
Nanostructure science and supramolecular chemistry are fast evolving fields that are concerned with manipulation of materials that have important structural features of nanometer size (1 nm to 1 ^m) [21]. Nature has been exploiting no covalent interactions for the construction of various cell components. For instance, microtubules, ribosomes, mitochondria, and chromosomes use mostly hydrogen bonding in conjunction with covalently formed peptide bonds to form specific structures.
H-bonds are commonly used for the fabrication of supramolecular assemblies because they are directional and have a wide range of interactions energies that are tunable by adjusting the number of H-bonds, their relative orientation, and their position in the overall structure. H-bonds in the center of protein helices can be 20 kcal/mol due to cooperative dipolar interactions [22, 23].
The porphyrin linkage through H-bonds is the binding type generally observed in nature. One of the simplest artificial self-assembling supramolecular porphyrin systems is the formation of a dimer based on carboxylic acid functionality [24].
3.2a The possible role of the self-assembling supramolecular macrostructures in mechanism of Acireductone Dioxygenases (ARDs) Ni(Fe)-ARD enzymes action involved in the methionine recycle pathway
The methionine salvage pathway (MSP) (Scheme 3) plays a critical role in regulating a number of important metabolites in prokaryotes and eukaryotes. Acireductone Dioxygenases (ARDs) Ni(Fe)-ARD are enzymes involved in the methionine recycle pathway, which regulates aspects of the cell cycle. The relatively subtle differences between the two metalloproteins complexes are amplified by the surrounding protein structure, giving two enzymes of different structures and activities from a single polypeptide (Scheme 3) [25]. Both enzymes Ni(Fe)-ARD are members of the structural super family, known as cupins, which also include Fe-Acetyl acetone Dioxygenase (Dke1) and Cysteine Dioxygenase. These enzymes that form structure super family of cupins use a triad of histidine-ligands (His), and also one or two oxygens from water and a carboxylate oxygen (Glu), for binding with Fe (Ni)-center [26]. Being the members of structural super family of cupins, the Ni(Fe)-ARDs present the unusual case of catalysis, as differ in the mechanism of action in relation to general substrates (1,2-dihydroxy-3-oxo-5 (methylthio) pent-1-ene (Acireductone) and dioxygen)).
Ni-ARD
MmARD
№1 + 14 J V Methi
jr fietK S-Aderiosylmethionine
(AdoMet) „ fayWM:p
mlriE, yugE SftB
^ /\ / rz
-7 if"" /
_ w
I
J-VHhylthioprapicnjlf ^t —
V jf foirfti
--Л— 0
S-Adenosylmeth ioninamine (dAdoMet)
l^-DihydiOxy-3-keIp-5-rnelhylthiOpentene
: гу- ^
Methylthioadenosine (MTA)
^ i *
b'
/Ithiope
A
2,3-Diketo-5-methylthiopentyl-1-P
Methylthioribulose-1-P
b
Scheme 3 - Acireductone Dioxygenases Ni-ARD and Fe-ARD' (a) [25] are involved in the methionine recycle pathway (b)
The two aci-reductone dioxygenase enzymes (ARD and ARD' share the same amino acid sequence, and only differ in the metal ions that they bind, which results in distinct catalytic activities. ARD has a bound Ni+2 atom while ARD' has a bound Fe+2 atom. The apo-protein, resulting from removal of the bound metal, is identical, and is catalytically inactive. ARD and ARD' can be interconverted by removing the bound metal and reconstituting the enzyme with the alternative metal.
a
ARD and ARD' act on the same substrate, the Acireductone, but they yield different products. ARD' catalyzes a 1,2-oxygenolytic reaction, yielding formate and 2-keto-4-methylthiobutryate, a precursor of methionine, and thereby part of the methionine salvage pathway, while Ni-ARD catalyzes a 1,3-oxygenolytic reaction, yielding formate, carbon monoxide, and methylthioproprionate, an off-pathway transformation of the aci-reductone. The role of the Ni-ARD catalyzed reaction is unclear.
We assumed that one of the reasons for the different activity of NiII(FeII)-ARD in the functioning of enzymes in relation to the common substrates (Acireductone (1,2-Dihydroxy-3 -keto-5-
methylthiopentene-2) and O2) can be the association of catalyst in various macrostructure due to intermolecular H-bonds.
The FeIIARD operation seems to comprise the step of oxygen activation (Fe"+O2^ FeIII-O2-) (by analogy with Dkel action [14]). Specific structural organization of iron complexes may facilitate the following regioselective addition of activated oxygen to Acireductone ligand and the reactions leading to formation of methionine. Association of the catalyst in macrostructures with the assistance of the intermolecular H-bonds may be one of reasons of reducing NiIIARD activity in mechanisms of NiII(FeII)ARD operation [25]. Here for the first time we demonstrate the specific structures self-organization of functional model of iron (nickel) complexes.
The possibility of the formation of stable supramolecular nanostructures on the basis of iron (nickel) heteroligand complexes due to intermolecular H-bonds we have researched with the AFM method [4].
In the Fig. 1-2 three-dimensional and two-dimensional AFM image of the structures on the basis of iron complex with 18C6 Femx(acac)y18C6m(H2O)n, formed at putting a uterine solution on a hydrophobic surface of modified silicone are presented. It is visible that the generated structures are organized in certain way forming structures resembling the shape of tubule micro fiber cavity (Fig. 2c). The heights of particles are about 3-4 nm. In control experiments it was shown that for similar complexes of nickel NiII(acac)2-18C6(H2O)n (as well as complexes Ni2(OAc)3(acac)MP2H2O) this structures organization is not observed. It was established that these iron constructions are not formed in the absence of the aqueous environment. Earlier we showed the participation of H2O molecules in mechanism of FeIII,IIx(acac)y18C6m(H2O)n transformation by analogy with Dke1 action, and also the increase in catalytic activity of iron complexes (FeInx(acac)y18C6m(H2O)n, Fe\(acac)y18C6m(H2O)n and
FeIIxL1y(L1ox)z(18C6)n(H2O)m) in the ethyl benzene oxidation in the presence of small amounts of water [2, 27]. After our works in article [28] it was found that the possibility of decomposition of the /-diketone in iron complex by analogy with Fe-ARD' action increases in aquatic environment. That apparently is consistent with data, obtained in our previous works [2, 27].
As another example we researched the possibility of the supramolecular nanostructures formation on
the basis of Fex(acac)yCTABm at putting of solutions of Fex(acac)yCTABm in CHCl3 or H2O on the hydrophobic surface of modified silicon (CTAB=Me3(n-C1aH33)NBr). We used CTAB concentration in the 5-10 times less than Fe(acac)3 concentration to reduce the probability of formation of micelles in water. But formation of spherical micelles at these conditions cannot be excluded. Salts QX are known to form with metal compounds complexes of variable composition depending on the nature of the solvent [2]. So the formation of heteroligand complexes Fex(acac)yCTABm(CHCl3)p (and Fex(acac)y(OAc)z(CTAB)n(CHCl3)q also) seems to be probable.
b
Fig. 1 - The AFM two- (a) and three-dimensional (b) image of nanoparticles on the basis Fex(acac)y18C6m(H2O)n formed on the surface of modified silicone
Earlier we established outer-sphere complex formation between Fe(acac)3 and quaternary ammonium salt R4NBr with different structure of R - cation [2]. Unlike the action of 18C6, in the presence of salts Me4NBr, Me3(n-C16H33)NBr (CTAB), Et4NBr, Et3PhNCl, Bu4NI and Bu4NBr, a decrease in the maximum absorption of acetylacetonate ion (acac) , and its bathochromic shift (Ak ~ 10 nm) were observed (in CHCl3). Such changes in the UV-spectrum reflect the effect of R4NX on the conjugation in the (acac)" ligand in the case of the outer-sphere coordination of R4NX. A change in the conjugation in the chelate ring of the acetylacetonate complex could be due to the involvement of oxygen atoms of the acac ligand in the for-
a
mation of covalent bonds with the nitrogen atom or hydrogen bonds with CH groups of alkyl substituents [2].
- '■ ____
^SetfwEP3»s
I
0,6
0,s
1,0
¡jm
1,2
\J
\ A
50 100 150
Plan*. nn1
Fig. 2 - The AFM two-dimensional image (a) of nanoparticles on the basis Fex(acac)y18C6m(H2O)n formed on the hydrophobic surface of modified silicone. The section of a circular shape with fixed length and orientation is about 50-80 nm (b), (c) The structure of the cell microtubules
0,1 0,2 0,3 Ch.-t 'Л О,G 0,7 Ofl 0.9 1.0
a
b
c
a
d
100 150 Plans, m
c
Fig. 3 - The AFM three - (a) two-dimensional image (b, d) of nanoparticles on the basis Fex(acac)yCTABm(CHCl3)p formed on the hydrophobic surface of modified silicone. The section of a circu-
lar shape with fixed length and orientation is about 50-70 nm (c)
In the Fig. 3 three-dimensional (a) and two-dimensional (b, d) AFM image of the structures on the basis of iron complex - Fex(acac)yCTABm(CHCl3)p, formed at putting a uterine solution on a surface of modified silicone are presented.
As one can see, these nanostructures are similar to macrostructures, observed previously, but with less explicit structures, presented in Fig. 1, 2, and which resemble the shape of tubule micro fiber cavity (Fig. 2c). The height of particles on the basis of Fex(acac)yCTABm(CHCl3)p is about 7-8 nm (Fig. 3c), that more appropriate parameter for particle with 18C6 Fex(acac)y18C6m(H2O)n.
We showed that complexes of nickel Ni"(acac)2^CTAB (1:1) (in CHCl3) do not form similar structures.
Unlike catalysis with iron-Dioxygenase, mechanism of catalysis by the NinARD does not include activation of O2, and oxygenation of Acireductone leads to the formation of products not being precursors of me-thionine [25]. Earlier we have showed that formation of multidimensional forms based on nickel complexes can be one of the ways of regulating the activity of two enzymes [4]. The association of complexes Ni2(AcO)3(acac)-MP-2H2O, which is functional and structure model of Ni-ARD, to supramolecular nanostructure due to intermolecular H-bonds (H2O -MP, H2O - (OAc-)(or (acac-)), is demonstrated on the next Fig.4. All structures (Fig.4) are various on heights from the minimal 3-4 nm to ~ 20-25 nm for maximal values (in the form reminding three almost merged spheres) [4].
E =
0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 fjm
pm
b
Fig. 4 - The AFM two- (a) and three-dimensional (b) image of nanoparticles on the basis Ni2(AcO)3(acac)L22H2O formed on the
hydrophobic surface of modified silicone
As one can see on Fig. 5 in case of binary complexes {Ni(acac)2 • MP} we also observed formation of nanostructures due to H-bonds. But these na-noparticles differ on form and are characterized with less height: h ~ 8 nm (Fig. 5) as compared with nanostructures on the basis of complexes Ni2(AcO)3(acac)L22H2O (Fig. 4).
Here we assume that it is necessary to take into account the role of the second coordination sphere, including Tyr-fragment, as one of possible mechanisms of reducing the activity of Ni''-ARD.
It is known that Tyrosine residues are located in different regions of protein by virtue of the relatively large phenol amphiphile side chain capable of (a) interacting with water and participating in hydrogen bond formation and (b) undergoing cation-rc and nonpolar interactions [29], typically both solvent-exposed and buried residues coexist with a protein.
^ 3<5 3,0 2 5 ?n ,.....
' 2,0 У 1,0 0,!
Fig. 5 - The AFM of three-dimensional image (5.0x5.0 (^m)) of nanoparticles on the basis {Ni(acac)2 ■ MP} formed on the surface of modified silicone (data presented on Fig. 4 are received and published at first)
3.2b Assumed effect of Tyr - fragment located in the second coordination sphere of metal complex
The versatile physicochemical properties of ty-rosine allow it to play a central role in conformation and molecular recognition [30]. Moreover, tyrosine has special role by virtue of the phenol functionality: for example, it can receive phosphate groups in target proteins by way of protein tyrosine kinases, and it participates in electron transfer processes with intermediate formation of Tyrosyl radical.
Tyrosine's can take part in different enzymatic reactions. Recently it has been researched role of Tyro-sine residues in mechanism of Heme oxygenase (HO) action. HO is responsible for the degradation of a histidine-ligated ferric protoporphyrin IX (Por) to biliverdin, CO, and the free ferrous ion. Tyrosyl radical formation reactions that occur after oxidizing Fe(III)(Por) to Fe(IV)=O(Por(+)) in human heme oxygenase isoform-1 (hHO-1) and the structurally homologous protein from Corynebacterium diphtheriae (cdHO) are described [31]. Site-directed mutagenesis on hHO-1 probes the reduction of Fe(IV)=O(Por(+)) by Tyrosine residues within 11 A of the prosthetic group (Fig.6). In hHO-1, radical Tyr58 is implicated as the most likely site of oxidation, based on the pH and pD dependent kinetics. The absence of solvent deuterium
a
isotope effects in basic solutions of hHO-1 and cdHO contrasts with the behavior of these proteins in the acidic solution, suggesting that long-range proton-coupled electron transfer predominates over electron transfer [31].
Fig. 6 - Structure of hHO-1 showing five conserved Tyrosine residues [31]
Both MMOs oxidize a range of hydrocarbon substrates, but pMMO has a narrower substrate specificity. sMMO exhibits higher turnover numbers than pMMO both in cells and in isolated enzyme preparations, but neither enzyme achieves levels desirable for biological gas-to-liquid conversionprocesses. Mechanistic studies of pMMO are far behind those of sMMO, due to ongoing difficulties isolating highly active purified enzyme and uncertainty as to the atomic details of the active site (vide supra). In later studies, a (^-oxo)(^-hydroxo)CuIICuIII center was proposed to be the active species and was calculated to be more reactive toward methane than a di(^-oxo)CuIICuIII species. Formation of this (^-oxo)(^-hydroxo)CuIICuIII species is proposed to be facilitated by proton transfer involving residues Tyr374 and Glu35 (M. capsulatus (Bath) numbering, Tyr374 is not structurally conserved in other pMMOs) in the second coordination sphere of the dicopper center [32]. It is important to note that these intermediates have been calculated based on the moderate resolution crystal structures of pMMO and have not been detected experimentally [32].
More over Tyr-fragment may be involved in substrate H-binding in step of O2-activation by iron catalyst, and this can decrease the oxygenation rate of the substrate, as it is assumed in the case of Homoprotocatechuate 2.3-Dioxygenase [33].
Tyr-fragment is discussed as important in methyl group transfer from S-adenosylmethionine (AdoMet) to dopamine [34]. The experimental findings with the model of Methyltransferase and structural survey imply that methyl CH-O hydrogen bonding (with participation of Tyr-fragment) represents a convergent evolutionary feature of AdoMet-dependent Methyltransferases, mediating a universal mechanism for methyl transfer [35].
In the case of Ni-Dioxygenase ARD, Tyr-fragment, involved in the mechanism, can reduce the Ni'ARD-activity (Fig.7).
Really, we have found earlier [2, 3], that the inclusion of PhOH in complex Ni(acac)2 L2 (L2=N-methylpirrolidone-2), which is the primary model of
NiIIARD, leads to the stabilization of formed triple complex Ni(acac)2^L2^PhOH. In this case, as we have established, ligand (acac)- is not oxygenated with molecular O2. Also the stability of triple complexes Ni(acacVL»PhOH seems to be due to the formation of stable to oxidation of supramolecular macrostructures due to intra- and intermolecular H-bonds. Formation of supramolecular macrostructures due to intermolecular (phenol-carboxylate) H-bonds and, possible, the other non-covalent interactions [36-38], based on the triple complexes Ni(acac)2L2PhOH, established by us with the AFM-method [4,5,39] (in the case of L2=MP, HMPA, NaSt, LiSt) (Fig.8), is in favor of this hypothesis.
Fig. 7 - The structure of NiMARD with Tyr residue in the second coordination sphere [25]
Spontaneous organization process, i.e., self-organization, of triple complexes at the apartment of a uterine hydrocarbon solution of complexes on surfaces of modified silicon are driven by the balance between intermolecular, and molecule-surface interactions, which may be the consequence of hydrogen bonds and the other non-covalent interactions [40]. Data of structures on the basis of complexes {Ni(acac)2MPPhOH} (Fig. 9a) that self-organized on the surface of the modified silicon were got first.
Similar results were received also, when we used the triple complexes based on constant valence metal compound as catalysts. The association of triple complexes LiSt L2 PhOH (L2 =HMPA) (the triple complexes based on redox-inactive metal compound LiSt -L2, LiSt L2 PhOH (L2 =DMF, HMPA), are effective catalysts of the ethyl benzene oxidation with dioxygen into a-phenyl ethyl hydro peroxide [unpublished data]) in supramolecular structures due to intermolecular H-bonding may be followed from analysis of data, which we received with AFM-microscopy (Fig.9). Data of structures on the basis of complexes LiSt • HMPA -PhOH that self-organized on the surface of the modified silicon at the apartment of a uterine solution on a surface were got first.
As one can see (Fig. 9), nanoparticles on the basis of triple complexes {LiSt • HMPA -PhOH} (image on 4.0x4.0 (^m)) have clear cell-type structure and are characterized in height from 2 to 3.5 nm (Fig.9 a,b) and have a width of ~100 -120 nm (Fig.9 c).
«
до
2fl
ж
о о oj
1 fi 1-5
m « 3,s
I 4 b
d
Fig. 8 - a) The AFM three-dimensional image (5.0x5.0(^m)) of the structures (h ~ 80-100 nm) formed on a surface of modified silicone on the basis of triple complexes Ni"(acac)2-MP-PhOH; b) The AFM three-dimensional image (6.0x6.0(^m)) of the structures (h ~ 40 nm) formed on a surface of modified silicone on the basis of triple complexes {Ni"(acac)2-HMPA-PhOH}; c) The AFM three-dimensional image (30x30 (^m)) of the structures(h ~ 80 nm) formed on a surface of modified silicone on the basis of triple complexes NiII(acac)2-NaSt-PhOH; d) The AFM three-dimensional image (4.5x4.5(^m)) of the structures (h ~ 10 nm) formed on a surface of modified silicone on the basis of triple complexes NiII(acac)2-LiSt-PhOH
2,S 3.«
3,5 VI
20.J Holeht Crvp
0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 -1,0
Fig. 9 - The AFM three- (a) and two-dimensional image (b) (4.0x4.0 (^m)) of the structures received on a surface of modified silicone on the basis of triple complexes LiSt ■ HMPA -PhOH and profile of one of these
43
a
а
b
c
structures (c)
At the same time it is necessary to mean that important function of NiIIARD in cells is established now. Namely, carbon monoxide, CO, is formed as a result of action of nickel-containing Dioxygenase NiIIARD. It was established, that CO is a representative of the new class of neural messengers, and seems to be a signal transducer like nitrogen oxide, NO [l4, 25]
4 Conclusion
Usually in the quest for axial modifying lig-ands that control the activity and selectivity of homogeneous metal complex catalysts, the attention of scientists is focused on their steric and electronic properties. The interactions in the outer coordination sphere, the role of hydrogen bonds and also the other non covalent interactions are less studied.
We have assumed that the high stability of heteroligand MIIxL1y(L1ox)z(L2)n (M=Ni, Fe) complexes as selective catalysts of the ethylbenzene oxidation to PEH, formed during the ethylbenzene oxidation in the presence of {ML1n + L2} systems as a result of oxygenation of the primary complexes (MIIL12)x(L2)y, can be associated with the formation of the supramolecular structures due to the intermolecular H-bonds.
The supramolecular nanostructures on the basis of primary iron (FeIIIL1n)x(L2)y, (L1 =acac-, L2 =18C6) and nickel NiIIxL1y(L1ox)z(L2)n (L1=acac-, L1ox=OAc- , L2 =N-methylpirrolidone-2) complexes formed with assistance of intermolecular H-bonds, obtained with AFM method, indicate high probability of supramolecular structures formation due to H-bonds in the real systems, namely, in the processes of alkylarens oxidation. So the H-bonding seems to be one of the factors, responsible for the high activity and stability of these catalytic systems researched by us.
Since the investigated complexes are structural and functional models of NiII(FeII)ARD Dioxygenases, the received data could be useful in the interpretation of the action of these enzymes.
Specific structural organization of iron complexes may facilitate the first step in Fe" ARD operation: O2 activation and following regioselective addition of activated oxygen to Acireductone ligand (unlike mechanism of regioselective addition of no activated O2 to Acireductone ligand in the case of Ni" ARD action), and reactions leading to formation of methionine.
The formation of multidimensional forms (in the case of Ni ARD) may be one way of controlling NiII(FeII)ARD activity. The role of the second coordination sphere in mechanism of NiII(FeII)ARD operation, including Tyr-fragment as one of possib le mechanisms of reduce in enzymes activity in Ni (Fe )ARD enzymes operation, is discussed. Formation of supramolecular macrostructures due to intermolecular (phenol-carboxylate) H-bonds and, possible, the other non-covalent interactions, based on the triple complexes Ni(acac)2L2PhOH, established by us with the AFM-method (in the case of L2=MP, HMPA, NaSt, LiSt), is in favor of this hypothesis.
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© L. 1 Matienko - Doctor of Chemistry, Head of Laboratory, N. M. Emanuel Institute of Biochemical Physics RAS, Moscow, Russia, matienko@sky.chph.ras.ru, L. A. Mosolova - Ph.D., Senior researcher, N. M. Emanuel Institute of Biochemical Physics RAS, Moscow, Russia, V. 1 Binyukov - Ph.D., Leading researcher, N. M. Emanuel Institute of Biochemical Physics RAS, Moscow, Russia, bin707@mail.ru, E. M. Mil - Doctor of Biology, Leading researcher, N. M. Emanuel Institute of Biochemical Physics RAS, Moscow, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia.
© Л. И. Матиенко - доктор химических наук, заведующий лабораторией, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, matienko@sky.chph.ras.ru, Л. А. Мосолова - кандидат химических наук, старший научный сотрудник, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, В. И. Бинюков - кандидат биологических наук, ведущий научный сотрудник, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, bin707@mail.ru, Е. М Миль - доктор биологических наук, ведущий научный сотрудник, Институт биохимической физики им. Н.М. Эмануэля РАН, Москва, Россия, Г. Е. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия.
