2015 年 14 巻 2 号 p. 23-29
Some iron(III) complexes, including [Fe2(μ-O)(nta)2(H2O)2]2– (1), [Fe2(μ-O)(edda)2(H2O)2] (2), and [Fe2(μ-O)(ida)2(H2O)4] (3), are considered to be carcinogens causing renal injuries according to the results of previous animal experiments, where nitrilotriacetate [(nta)3–], ethylenediamine-N,N'-diacetate [(edda)2–], and iminodiacetate [(ida)2–] are the chelating ligands (chelators). On the other hand, a similar iron(III) complex [Fe2(μ-O)(pac)2(H2O)2] (4) is not considered to be a carcinogen, where N-(2-pyridylmethyl) iminodiacetate [(pac)2–] is the chelating ligand. In order to clarify the differences in carcinogenicity, the structures of complexes 1–4 were investigated based on the density functional theory (DFT) method, because the structures of complexes had not been clarified in solution. As a result, two-point interaction with hydrogen peroxide or α-helix was found to be possible for carcinogenic iron(III) complexes 1 and 3, whereas the interaction was found to be impossible for non-carcinogenic iron(III) complex 4.
Oxidative renal tubular injuries and carcinogenesis were reported to be caused to rodent kidneys by injection of a solution containing iron(III) ion and a chelating ligand (chalator), nitrilotriacetate [(nta)3–] or iminodiacetate [(ida)2–] [1,2,3,4](Figure 1). On the other hand, however, no injury was observed when N-(2-pyridylmethyl) iminodiacetate [(pac)2–] was used as the chelating ligand. From the spectroscopic studies, the iron(III) ions were suggested to exist as μ-oxo-diiron(III) complexes in solution [5]; however, detailed structures have not been clarified. Therefore, in this study, we investigated the diiron(III) structures, [Fe2(μ-O)(nta)2(H2O)2]2– (1), [Fe2(μ-O)(edda)2(H2O)2] (2), [Fe2(μ-O)(ida)2(H2O)4] (3), and [Fe2(μ-O)(pac)2(H2O)2] (4), based on DFT computations, where ethylenediamine-N,N’-diacetate [(edda)2–] is the chelating ligand. Since renal injuries were said to be caused with respect to hydrogen peroxide [5,6,7,8,9,10], the structures of H2O2-adducts were examined for 1–4.
Chemical structures of chelating ligands (chelators).
DFT computations were performed using GAMESS program [11,12] on FUJITSU PRIMERGY CX400 (TATARA computer) at Kyushu University. Structural optimizations were performed with LC-BOP/6-31G [13]. Molecular structures were drawn using Winmostar software [14].
Nitrilotriacetate trianion (nta)3– has a tripodal structure and works as a tetradentate ligand. In order to find the most stable structure of [Fe2(μ-O)(nta)2(H2O)2]2– complex anion (1), we considered six isomers described below as the initial structures in computation. If we assume a symmetrical dinuclear iron(III) complex with a linearly bridging oxo ion, there are two patterns in the orientations of amine nitrogens: a pattern with N-Fe-O(oxo) = 180° and a pattern with N-Fe-O(oxo) = 90°. For each pattern, concerning the orientation of two water molecules, we considered three isomers with the dihedral angles O(water)-Fe···Fe-O(water) = 0°, 90°, and 180°. The six isomers we considered can be described with the two angles N-Fe-O(oxo) and O(water)-Fe···Fe-O(water) as follows: N1 (180,0), N2(180,90), N3(180,180), N4(90,0), N5(90,90), and N6(90,180). As the result of structure optimization based on DFT, N1(180,0) and N4(90,0) converted into N2(180,90) and N5(90,90), respectively, and the four isomers were obtained as follows: N5(90,90) (A1), N2(180,90) (A2), N3(180,180) (A3), and N6(90,180) (A4) from the lower energy (Table 1). Since the energy difference between the most and the second most stable isomers was more than 11 kcal mol–1, only the most stable N5(90,90) (A1) is thought to exist.
| No | Isomer | Point Group | Energydifference a |
| A1 | N5 (90,90) | C2 | 0.0 |
| A2 | N2 (180,90) | C2 | 11.4 |
| A3 | N3 (180,180) | C1 | 19.8 |
| A4 | N6 (90,180) | C2 | 39.3 |
a kcal mol–1.
In all the cases, Fe-O-Fe angles are bent after the structure optimization (Figure 2). In the most stable isomer, Fe-O-Fe angle is 135.2°, and the two water molecules are in close proximity at cis positions [O(water)-Fe···Fe-O(water) = 68.8°, O(water)···O(water) = 2.96 Å].
Structures of optimized isomers A1-A4.
Ethylenediamine-N,N'-diacetate dianion (edda)2– has a linear structure and works as a tetradentate ligand. In order to find the most stable structure of [Fe2(μ-O)(edda)2(H2O)2] complex, we considered thirty two isomers as the initial structures in computation. For mononuclear edda complexes, there are three coordination modes: cis-α, cis-β, and trans modes (Figure 3). Since the ethylenediamine moiety forms a chiral chelating ring, we considered a δδ form and a δλ form for a dinuclear iron(III) complex. In the δδ form, both Fe(III) ions have the δ forms, whereas in the δλ form, one has the δ form and the other has the λ form. If we assume a symmetrical dinuclear iron(III) complex with a linearly bridging oxo ion, there are two patterns with respect to the orientations of two amine nitrogens on (edda)2–: a pattern with N-Fe-O(oxo) angles of (90°, 180°) (Pattern P) and a pattern with N-Fe-O(oxo) angles of (90°, 90°) (Pattern Q). For the cis-α mode, each of the δδ and δλ forms can take only Pattern P. For the pattern, concerning the orientation of two water molecules, we considered four isomers with the dihedral angles O(water)-Fe···Fe-O(water) = 0°, 90°, 180°, and 270°. Therefore, we considered eight initial structures for the cis-α mode. For the cis-β mode, each of the δδ and δλ forms can take two patterns, Patterns P and Q. For each pattern, concerning the orientation of two water molecules, we considered four isomers with the dihedral angles O(water)-Fe···Fe-O(water) = 0°, 90°, 180°, and 270°. Therefore, we considered sixteen initial structures for the cis-β mode. For the trans mode, each of the δδ and δλ form can take only Pattern Q. For the pattern, concerning the orientation of two (edda)2– ligands, we considered four isomers, rotating the ligands by 90°. In total, we considered eight initial structures for the trans mode. The result of the structure optimization is summarized in Table 2, and the obtained structures are shown in Figure 4. Among the initial thirty two isomers, fourteen isomers were finally obtained. Since the energy difference between the most and the second most stable isomers was more than 6 kcal mol–1, only the most stable isomer is thought to exist.
Three isomers for an octahedral edda complex (trans, cis-α, and cis-β) and two conformational isomers for ethylene diamine moiety.
| No | Isomer | Point Group | Energydifference a |
| B1 | cis-α,δδ,P,90° | C2 | 0.0 |
| B2 | cis-α,δδ,P,270° | C2 | 6.6 |
| B3 | cis-α,δλ,P,90° | C1 | 9.0 |
| B4 | cis-β,δδ,Q,90° | C2 | 12.6 |
| B5 | cis-β,δδ,P,270° | C2 | 14.7 |
| B6 | cis-β,δλ,P,180° | C1 | 22.4 |
| B7 | cis-β,δδ,Q,0° | C1 | 26.4 |
| B8 | cis-β,δλ,Q,0° | C1 | 29.1 |
| B9 | trans,δλ,Q | C1 | 32.8 |
| B10 | cis-β,δλ,Q,180° | C1 | 33.1 |
| B11 | cis-β,δδ,Q,270° | C2 | 34.6 |
| B12 | cis-β,δλ,Q,180° | C1 | 35.5 |
| B13 | trans,δδ,Q | C2 | 39.5 |
| B14 | cis-α,δλ,P,180° | Ci | 49.5 |
a kcal mol–1.
Structures of optimized isomers B1-B14.
The most stable isomer has the cis-α mode, and the two-fold axis exists through the bridging oxo ion. The Fe-O-Fe angle is 135.7°, and the two water molecules are at cis positions [O(water)-Fe···Fe-O(water) = 104.2°]; however, the water molecules are not so close [O(water)···O(water) = 4.03 Å].
3.3 [Fe2(μ-O)(ida)2(H2O)4] complexIminodiacetate dianion (ida)2– has a linear structure, and works as a tridentate ligand. In order to find the most stable structure of [Fe2(μ-O)(ida)2(H2O)4] complex, we considered twenty isomers as the initial structures in computation. For mononuclear ida complexes, there are two coordination modes: fac and mer modes. If we assume a symmetrical dinuclear iron(III) complex with a linearly bridging oxo ion, there are two patterns with respect to the orientations of two amine nitrogens on (ida)2–: a pattern with the N-Fe-O(oxo) angle of 90° and a pattern with the N-Fe-O(oxo) angle of 180°. For the fac mode with the N-Fe-O(oxo) angle of 90°, there are two sub-patterns: a sub-pattern where two ligands in a dinuclear complex have the C2-related same structure and a sub-pattern where the two ligands are in the mirror image of each other. For each sub-pattern, we considered four conformers. For the fac mode with the N-Fe-O(oxo) angle of 180°, we considered three conformers. For the mer mode with the N-Fe-O(oxo) angle of 90°, there are two sub-patterns: a sub-pattern where two amine hydrogen atoms are in the anti positions and a sub-pattern where two amine hydrogen atoms are in the syn positions. For each sub-pattern, we considered three conformers. For the mer mode with the N-Fe-O(oxo) angle of 180°, we considered three conformers. In total, the number of isomers we considered as initial structures is twenty. The result of the structure optimization is summarized in Table 3, and the obtained structures are shown in Figure 5. Among the initial twenty isomers, twelve isomers were finally obtained. Since the energy difference between the most and the second most stable isomers was more than 11 kcal mol–1, only the most stable isomer is thought to exist.
| No | Isomer | Point Group | Energydifference a |
| C1 | fac, 90°,sab, | C2 | 0.0 |
| C2 | mer, 180°, | C1 | 11.4 |
| C3 | mer, 90°,anc, | C2 | 11.4 |
| C4 | mer, 180°, | C2 | 11.8 |
| C5 | mer, 90°,syd, | C2 | 11.9 |
| C6 | fac, 90°,mie, | C1 | 12.5 |
| C7 | mer, 180°, | C1 | 15.7 |
| C8 | fac, 90°,mie, | C1 | 16.1 |
| C9 | fac, 180°, | C1 | 18.8 |
| C10 | fac, 90°,sab, | C2 | 19.6 |
| C11 | mer, 90°,syd, | C1 | 23.2 |
| C12 | fac, 90°,sab, | C2 | 25.8 |
a kcal mol–1.
b Two ligands have the same structure.
c Two ligands' amine hydrogen atoms are in anti positions.
d Two ligands' amine hydrogen atoms are in syn positions.
e Two ligands are in the mirror image.
Structures of optimized isomers C1-C12.
The most stable isomer has the fac mode, and the two-fold axis exists through the bridging oxo ion. The Fe-O-Fe angle is 131.2°, and two of the water molecules are in close proximity at cis positions [O(water)-Fe···Fe–O(water) = 59.1°, O(water)···O(water) = 2.84 Å].
3.4 [Fe2(μ-O)(pac)2(H2O)2] complexN(2-Pyridylmethyl) iminodiacetate (pac)2– works as a tetradentate ligand. In order to find the most stable structure of [Fe2(μ-O)(pac)2(H2O)2] complex, we considered twenty two isomers as the initial structures in computation. For mononuclear pac complexes, there are two coordination modes; with respect to the iminodiacetate moiety, one has a fac form and the other has a mer form. As in the case of nta complexes, N (amine)-Fe-O(oxo) angle and O(water)-Fe···Fe-O(water) angle were used to describe isomers, and in addition, N (pyridine)-Fe-O(water) angle was introduced to describe the isomers. For the mer isomers we considered six structures, and for fac structures we considered sixteen structures. The result of the structure optimization is summarized in Table 4, and the obtained structures are shown in Figure 6. Among the initial twenty two isomers, nine isomers were finally obtained. Since the energy difference between the most and the second most stable isomers was about 5 kcal mol–1, only the most stable isomer is thought to exist.
| No | Isomer | Point Group | Energydifference a |
| D1 | mer(180,0,180) | C2 | 0.0 |
| D2 | fac(90,180,90),sab | C2 | 5.0 |
| D3 | mer(180,180,180) | Ci | 5.8 |
| D4 | mer(90,0,90) | C2 | 11.4 |
| D5 | fac(180,0,90),sab | C2 | 13.9 |
| D6 | fac(180,90,90),mic | Ci | 14.5 |
| D7 | fac(90,90,90),mic | C1 | 20.7 |
| D8 | fac(90,90,90),sab | C2 | 25.8 |
| D9 | fac(90,180,90),mic | Ci | 35.9 |
a kcal mol–1.
b Two ligands have the same structure.
c Two ligands are in the mirror image.
Structures of optimized isomers D1-D9.
The most stable isomer has the mer mode with respect to the iminodiacetate moiety, and the two-fold axis exists through the bridging oxo ion. The Fe-O-Fe angle is 134.8°, and the two water molecules are in cis positions [O(water)-Fe···Fe-O(water) = 106.9°]; however, the water molecules are not so close [O(water)···O(water) = 3.95 Å].
3.5 Interaction with hydrogen peroxideSince the carcinogenicity is thought to be related with hydrogen peroxide [5,6,7,8,9,10], structural optimization has been done for H2O2-adducts of the complexes. In computation, the pair of adjacent water molecules is replaced with a hydrogen peroxide molecule for the most stable isomers (A1, B1, C1, D1). The H2O2-adducts were easily obtained only for A1 and C1 (Figure 7), and this result is consistent with the water···water distances in the optimized structures. In the most stable H2O2-adducts, A1-H2O2-1 and C1-H2O2-1, the bridging structures were very similar, and μ-H2O2-1κO,2κO' bridges were formed. In addition, hydrogen bonds were formed between carboxylato moieties and the hydrogen peroxide [15]. Concerning the carcinogenicity, the nta- and ida-systems are highly significant; the edda-system is less significant than the nta- and ida-systems; the pac-system is inactive. Therefore, the formation of H2O2-adduct seems to be necessary for the carcinogenicity.
Interaction of dinuclear iron(III) complexes with hydrogen peroxide. [Fe2(μ-O)(nta)2(μ-H2O2)]2– complex anions (A1-H2O2-1 and A1-H2O2-2) and [Fe2(μ-O)(ida)2(μ-H2O2)(H2O)2] complexes (C1-H2O2-1 and C1-H2O2-2).
The second most stable isomers of H2O2-adducts are also shown in Figure 7 (A1-H2O2-2 and C1-H2O2-2). A1-H2O2-2 is 14.6 kcal mol–1 less stable than A1-H2O2-1, so A1-H2O2-2 is not expected to be formed. C1-H2O2-2 is 3.9 kcal mol–1 less stable than C1-H2O2-1. Although the energy difference is much smaller than that of A1-H2O2-2, C1-H2O2-2 is expected to have little existence [less than 0.2% based on the Boltzmann's distribution law [16,17]]. The difference in the energy differences between A1-H2O2-2 and C1-H2O2-2 is presumably due to the flexibility of the ida ligand. That is, nta ligand is more inflexible because of the inter-chelating-chain repulsion. The flexibility of the ida ligand can be seen also in C-N-C bond angles. The C-N-C angles in the ida complexes (C1 and C1-H2O2-1) (113.6–113.9°) are slightly larger than the C-N-C angles at the facial positions in the nta complexes (A1 and A1-H2O2-1) (111.7–112.9°).
In the case of B1, formation of H2O2-adducts seemed to be impossible; however, after a large deformation, anadduct [[Fe2(μ-O)(edda)2(μ-H2O2)] (B1-H2O2)] was obtained(Figure 8). The bridging structure of B1-H2O2 is similar to those for A1-H2O2-1 and C1-H2O2-1. The deformation can be estimated using a change in dihedral angles O-Fe···Fe-O (Table 5). The changes in dihedral angles for A1 [33.5°] and C1 [26.7°] are smaller than that for B1 [70.2°]. Consequently, it seems to be more difficult to obtain B1-H2O2 than to obtain A1-H2O2-1 and C1-H2O2-1. This can be a reason why the carcinogenicity of the edda-systemis less significant than the nta- and ida-systems. In the case of D1, no H2O2-adduct has been obtained; most of the time an O-O bond cleavage occurs during the structure optimization procedure. The result can be a reason for the inactivity of the pac-system
Formation of [Fe2(μ-O)(edda)2(μ-H2O2)] (B1-H2O2) after a large deformation.
| No | Intact isomera | H2O2-adductb |
| A1 | 68.8° | 35.3° |
| B1 | 104.2° | 34.0° |
| C1 | 59.1° | 32.4° |
| D1 | 106.9° | ‒c |
a The O(water)-Fe···Fe-O(water) angle.
b The O(H2O2)-Fe···Fe-O(H2O2) angle for the most stable isomer.
c The structure was not obtained.
Related to the interaction between iron(III) complexes and glutathione cycle [8,9,10] and the iron deposition by the iron complexes [18], the interaction with tetraglycine (TG) in an α-helix form has been investigated. Since the α-helix is chiral, inverted structures of the iron complexes were also considered. In the most stable isomers, A1, A1*, C1, and C1* were possible to interact with TG at two positions (Figure 9) [A1:A1* = 97:3 and C1:C1* = 78:22 based on the Boltzmann's distribution law [16,17]], where rhw asterisk represents the inverted structure. In both A1-helix and A1*-helix (2.2 kcal mol–1 less stable), the helices were strongly deformed, breaking the initial intramolecular hydrogen bonds. On the other hand, in both C1-helix and C1*-helix (0.8 kcal mol–1 less stable), the helices were interacted with the diiron center, maintaining the intact helix structure. This difference can be explained by the flexibility and the inflexibility of the ida and nta ligands, discussed in section 3.5. That is, the ida complex is flexible enough to interact with an α-helix without breaking the intact helix structure, while the nta complex is so inflexible that the structure of α-helix was deformed to interact with the diiron center.
Interaction of dinuclear iron(III) complexes with tetraglycine (TG) in an α-helix form. [Fe2(μ-O)(nta)2(μ-TG)]2– complex anions (A1-helix and A1*-helix) and [Fe2(μ-O)(ida)2(μ-TG)(H2O)2] complexes (C1-helix and C1*-helix). Symbol * represents an inverted structure.
In this study, the most stable isomers were found for four diiron complexes 1–4 on the basis of the DFT method. The interaction of the diiron complexes with hydrogen peroxide was investigated, and carcinogenic 1 and 3 were found to interact with hydrogen peroxide, forming a μ-H2O2-1κO,2κO' bridge, while such an interaction was found to be impossible for non-carcinogenic 4.Comparing the structures of nta and ida complexes, the nta complexes were more inflexible due to inter-chelating-chain repulsion. In the interaction of 1 and 3 with tetraglycine (TG) in an α-helix form, nta complex 1 interacted with TG, breaking the initial intramolecular hydrogen bonds. On the other hand, ida complex 3 interacted with TG, maintaining the initial structure. This difference can be explained by the flexibility of complex 3.
Financial support by Yamagata University is gratefully acknowledged. The technical support on Tatara computer was provided by Dr. Yuichi Inadomi at Kyushu University.
